SDO (Solar Dynamics Observatory)
SDO is a NASA satellite, considered to be a second-generation solar mission (also referred to as SOHO successor). SDO represents the first mission within NASA's LWS (Living With a Star) program, a space weather-focused and applications-driven research program. The goal of LWS is to understand the sun as a magnetic variable star and to measure its impact on life and society on Earth.
The overall SDO objective is to observe the dynamics of the solar interior, provide data on the sun's magnetic field structure, characterize the release of mass and energy from the sun into the heliosphere, and monitor variations in solar irradiance. The goal is to understand the dynamic state of the sun (its variability) on multiple temporal and spatial scales which influence life and technology on Earth - to enable the development of an operational capability for space weather prediction (the purpose of the LWS Program). 1) 2) 3) 4) 5) 6)
The SDO mission was assigned a number of mission objectives specifically designed to support the LWS goals of understanding the drivers of solar activity and variability that affect Earth and humanity. Specifically, SDO was designed to address seven science questions dealing with the sun's dynamic activity and its effect on the Earth: 7)
1) What mechanisms drive the quasi-periodic 11-year cycle of solar activity?
2) How is active region magnetic flux synthesized, concentrated, and dispersed across the solar surface?
3) How does magnetic reconnection on small scales reorganize the large-scale field topology and current systems and how significant is it in heating the corona and accelerating the solar wind?
4) Where do the observed variations in the Sun‘s EUV spectral irradiance arise, and how do they relate to the magnetic activity cycles?
5) What magnetic field configurations lead to the coronal mass ejections (CMEs), filament eruptions, and flares that produce energetic particles and radiation?
6) Can the structure and dynamics of the solar wind near Earth be determined from the magnetic field configuration and atmospheric structure near the solar surface?
7) When will activity occur, and is it possible to make accurate and reliable forecasts of space weather and climate?
• To provide nearly continuous coverage of solar activity
• To provide coverage of the regimes (interior, photosphere, corona) in which the activity occurs
• To provide sufficient data on the types of phenomena which impact Earth, near-Earth space and humanity
• To observe the solar variability over the relevant timescales (seconds to years).
Figure 1: Artist's rendition of the deployed SDO spacecraft (image credit: NASA)
Figure 2: Top view of the SDO spacecraft (image credit: NASA)
The spacecraft is being designed and built at NASA/GSFC. The SDO design consists of a bus module and an instrument module (Figure 5); the instrument module employs a graphite composite structure to minimize thermal distortions The spacecraft bus module contains the S/C and instrument electronics. Redundant HGAs (High Gain Antennas) are mounted at the end of rigid booms (must be rigid due to required waveguides).
The spacecraft is 3-axis stabilized. The ACS (Attitude Control System) is a single-fault tolerant design. Its fully redundant attitude sensor complement includes 16 coarse sun sensors, a digital sun sensor (DSS), 3 two-axis inertial reference units (IRU), 2 star trackers (ST), and 4 guide telescopes. Attitude actuation is performed using 4 reaction wheel assemblies (RWA) and 8 thrusters, and a single main engine nominally provides velocity-change thrust. - The attitude control software has five nominal control modes: 3 wheel-based modes and 2 thruster-based modes. A wheel-based safehold running in the attitude control electronics box improves the robustness of the system as a whole. All six modes are designed on the same basic proportional-integral-derivative attitude error structure, with more robust modes setting their integral gains to zero. 8)
The ST and DSS combine to provide two-out-of-three single-fault tolerance fine attitude determination. Any one of the 4 AIA guide telescopes may be selected as the ACS CGT (Controlling Guide Telescope). Control is actuated using reaction wheel assemblies (RWA) and attitude control thrusters. Orbit change maneuvers can be accomplished using either the thrusters or a main engine, i.e. the RCS (Reaction Control Subsystem); the main engine will be used nominally for all long maneuvers performed in achieving geosynchronous orbit from the launch orbit.
The ACS supports five operational modes. These are: sun acquisition, inertial, science, ΔH and ΔV. One mode, namely safehold, operates solely in the ACE (Attitude Control Electronics) software. The SDO remains sun-pointing throughout most of its mission for the instruments to take measurements of the sun.
Figure 3: Overview of ACS components in the spacecraft (image credit: NASA)
Figure 4: Block diagram of the SDO attitude control electronics (image credit: NASA)
Onboard Ephemeris: The SDO onboard ephemeris predicts the locations of the Sun, Moon, spacecraft, and ground station in geocentric inertial coordinates, referred to the mean-equator-and-equinox of J2000 (GCI, mean-of-J2000). Each object's velocity is derived from differencing successive position vectors and dividing the result by the ephemeris task sample time (nominally 1 second). The solar ephemeris accuracy is better than 2 arcseconds during the 10 year SDO mission lifetime and has been validated by the JPL DE405 ephemeris.
The following key spacecraft technologies are being introduced:
• Ethernet chipset
• Ka-band transmitter
• APS (Active Pixel Sensor) star tracker.
SDO uses a bi-propellant propulsion system, an AKM (Apogee Kick Motor), to boost the spacecraft from a GTO (Geosynchronous Transfer Orbit) into a GSO (Geosynchronous Orbit). The Spacecraft design life is 5 years (10 years for expendables). The launch mass of SDO is about 3,200 kg.
Figure 5: Illustration of the SDO spacecraft (image credit: NASA)
Table 1: Overview of spacecraft parameters
Figure 6: Photo of the integrated SDO spacecraft (image credit: NASA)
Figure 7: Overview of NASA's SDO Mission (video credit: NASA/GSFC)
Orbit: Inclined geosynchronous circular orbit (IGSO), altitude ~ 35,756 km, inclination = 28.5º, the spacecraft is positioned at a longitude of 102º W. The GSO permits nearly continuous observations of the sun and high data rates to the ground. Only two short eclipse periods per year are being encountered where the Earth's shadow grows to a maximum of about 72 minutes per day. Note: the inclined orbit will form a lemniscate, also referred to as analemma, (i.e., a figure 8 ground track) over the Earth during each day extending to ±28.5º in latitude (inclination) at the longitudinal position.
Figure 8: Illustration of SDO daily orbital trace of a figure 8 at the longitude of 102ºW with maximum latitude extensions of ±28.5º (image credit: NASA)
RF communications: Science data are downlinked in Ka-band (26.5 GHz) from its redundant onboard high-gain antennas at a data rate of 150 Mbit/s (includes data compression). There are no onboard recorders for the science data since the spacecraft is in continuous contact with the ground station. The TT&C data are in S-band (2215 MHz) using two onboard omni-directional antennas. - The continuous stream of science data from the SDO spacecraft will produce ~ 2 TByte of raw data every day.
• December 17, 2019: NASA's Solar Dynamics Observatory has observed a magnetic explosion the likes of which have never been seen before. In the scorching upper reaches of the Sun's atmosphere, a prominence — a large loop of material launched by an eruption on the solar surface — started falling back to the surface of the Sun. But before it could make it, the prominence ran into a snarl of magnetic field lines, sparking a magnetic explosion. 11)
- Scientists have previously seen the explosive snap and realignment of tangled magnetic field lines on the Sun — a process known as magnetic reconnection — but never one that had been triggered by a nearby eruption. The observation, which confirms a decade-old theory, may help scientists understand a key mystery about the Sun's atmosphere, better predict space weather, and may also lead to breakthroughs in the controlled fusion and lab plasma experiments.
- "This was the first observation of an external driver of magnetic reconnection," said Abhishek Srivastava, solar scientist at Indian Institute of Technology (BHU), in Varanasi, India. "This could be very useful for understanding other systems. For example, Earth's and planetary magnetospheres, other magnetized plasma sources, including experiments at laboratory scales where plasma is highly diffusive and very hard to control."
- Previously a type of magnetic reconnection known as spontaneous reconnection has been seen, both on the Sun and around Earth. But this new explosion-driven type — called forced reconnection — had never been seen directly, thought it was first theorized 15 years ago. The new observations have just been published in the Astrophysical Journal. 12)
- The previously-observed spontaneous reconnection requires a region with just the right conditions — such as having a thin sheet of ionized gas, or plasma, that only weakly conducts electric current — in order to occur. The new type, forced reconnection, can happen in a wider range of places, such as in plasma that has even lower resistance to conducting an electric current. However, it can only occur if there is some type of eruption to trigger it. The eruption squeezes the plasma and magnetic fields, causing them to reconnect.
- While the Sun's jumble of magnetic field lines are invisible, they nonetheless affect the material around them — a soup of ultra-hot charged particles known as plasma. The scientists were able to study this plasma using observations from NASA's SDO (Solar Dynamics Observatory), looking specifically at a wavelength of light showing particles heated 1-2 million kelvins (1.8-3.6 million F).
- The observations allowed them to directly see the forced reconnection event for the first time in the solar corona — the Sun's uppermost atmospheric layer. In a series of images taken over an hour, a prominence in the corona could be seen falling back into the photosphere. En route, the prominence ran into a snarl of magnetic field lines, causing them to reconnect in a distinct X shape.
Figure 9: Forced magnetic reconnection, caused by a prominence from the Sun, was seen for the first time in images from NASA's SDO ( video credit: NASA's Goddard Space Flight Center)
- Spontaneous reconnection offers one explanation for how hot the solar atmosphere is — mysteriously, the corona is millions of degrees hotter than lower atmospheric layers, a conundrum that has led solar scientists for decades to search for what mechanism is driving that heat. The scientists looked at multiple ultraviolet wavelengths to calculate the temperature of the plasma during and following the reconnection event. The data showed that the prominence, which was fairly cool relative to the blistering corona, gained heat after the event. This suggests forced reconnection might be one way the corona is heated locally. Spontaneous reconnection also can heat plasma, but forced reconnection seems to be a much more effective heater — raising the temperature of the plasma quicker, higher, and in a more controlled manner.
- While a prominence was the driver behind this reconnection event, other solar eruptions like flares and coronal mass ejections, could also cause forced reconnection. Since these eruptions drive space weather — the bursts of solar radiation that can damage satellites around Earth — understanding forced reconnection can help modelers better predict when disruptive high-energy charged particles might come speeding at Earth.
- Understanding how magnetic reconnection can be forced in a controlled way may also help plasma physicists reproduce reconnection in the lab. This is ultimately useful in the field of laboratory plasma to control and stabilize them.
- The scientists are continuing to look for more forced reconnection events. With more observations they can begin to understand the mechanics behind the reconnection and often it might happen.
Figure 10: Forced magnetic reconnection, caused by a prominence from the Sun, was seen for the first time in images from NASA's Solar Dynamics Observatory, or SDO. This image shows the Sun on May 3, 2012, with the inset showing a close-up of the reconnection event imaged by SDO's AIA (Atmospheric Imaging Assembly) instrument, where the signature X-shape is visible [image credit: NASA/SDO/Abhishek Srivastava/IIT(BHU)]
• July 24, 2019: In a pair of new papers, scientists paint a picture of how solar cycles suddenly die, potentially causing tsunamis of plasma to race through the Sun's interior and trigger the birth of the next sunspot cycle only a few short weeks later. 13)
- The new findings provide insight into the mysterious timing of sunspot cycles, which are marked by the waxing and waning of sunspot activity on the solar surface. While scientists have long known that these cycles last approximately 11 years, predicting when one cycle ends and the next begins has been challenging to pin down with any accuracy. The new research could change that.
Figure 11: This visualization of a computer model simulation shows a solar tsunami, which is initiated at the equator. As the tsunami travels toward the poles it buoys the toroidal magnetic fields (white lines) traveling deeper in the solar interior. As these bands are lifted to the surface, they erupt as sunspots on the solar surface (image credit: UCAR, Visualization: Mausumi Dikpati, NCAR)
- The new findings provide insight into the mysterious timing of sunspot cycles, which are marked by the waxing and waning of sunspot activity on the solar surface. While scientists have long known that these cycles last approximately 11 years, predicting when one cycle ends and the next begins has been challenging to pin down with any accuracy. The new research could change that.
- In one of the studies, which relies on nearly 140 years of solar observations from the ground and space, the scientists are able to identify "terminator" events that clearly mark the end of a sunspot cycle. With an understanding of what to look for in the run up to these terminators, the authors predict that the current solar cycle (Solar Cycle 24) will end in the first half of 2020, kicking off the growth of Solar Cycle 25 very shortly after.
- In a second study, motivated by the first, scientists explore the mechanism for how a terminator event could trigger the start of a new sunspot cycle using a sophisticated computer model. The resulting simulations show that "solar tsunamis" could provide the connection and explain the Sun's remarkably rapid transition from one cycle to the next.
- Both studies were led by the National Center for Atmospheric Research (NCAR).
- "The evidence for terminators has been hidden in the observational record for more than a century, but until now, we didn't know what we were looking for," said NCAR scientist Scott McIntosh, who directs the center's High Altitude Observatory and worked on both studies. "By combining such a wide variety of observations over so many years, we were able to piece together these events and provide an entirely new look at how the Sun's interior drives the solar cycle."
- The research was funded by the National Science Foundation, which is NCAR's sponsor, NASA's Living with a Star program, and the Indo-US Joint Networked R&D Center.
Flickers of light reveal mysteries
- Sunspot cycles are born after solar minimum, a period when the face of the Sun is quiet. As the cycle continues, more and more sunspots emerge, first appearing at about 35 degrees latitude in both hemispheres and slowly marching toward the equator over a decade before they fade again into the next solar minimum. The rough midpoint of this progression is solar maximum, when sunspots are the most abundant.
- Predicting the timing of sunspot evolution is a major scientific goal, in part because sunspot activity is tied to the solar storms that can disrupt Earth's upper atmosphere and affect GPS signals, power grids, and other critical technologies. But such predictions have proven challenging.
Figure 12: Images of the Sun from NASA's Solar Dynamics Observatory. The left image was taken last month during the current solar minimum. The image on the right was taken in April 2014 during the last solar maximum (image credit: NASA)
- For example, the Sun is currently in a solar minimum. Scientist know the relative peace means that the current solar cycle is wrapping up, but it has been difficult to say whether the new cycle will begin in a few months or a few years. McIntosh and his colleagues think their studies can provide more clarity, both into the timing of cycles and also into what drives the cycles themselves.
- The researchers began by studying the movement of coronal bright points – ephemeral flickers of extreme ultraviolet light in the solar atmosphere. By observing bright points, which occur even in the relative calm of a solar minimum, the scientists think they have gained a more complete view of the solar cycle than if they focused only on sunspot activity.
- The bright points first appear at higher latitudes than sunspots (around 55º) and migrate toward the equator at approximately 3º latitude per year, reaching the equator after a couple decades. The paths traced by the bright points overlap with sunspot activity in the mid-latitudes (around 35º) until they both reach the equator and disappear. This disappearance, which the researchers call a terminator event, is followed very shortly after with a large burst of bright point activity at the mid-latitudes, marking the beginning of the next sunspot cycle.
- In the new study that identifies terminator events, published in the journal Solar Physics, the scientists corroborate the bright point observations with a number of other observations from a variety of spacecraft- and ground-observing facilities stretching back over 13 solar cycles. 14)
- "We were able to identify these terminators by looking at data from a whole range of different measures of solar activity – magnetic fields, spectral irradiance, radio flux – in addition to the bright points," said University of Maryland scientist Bob Leamon, a co-author of the paper who is also a researcher at NASA's Goddard Space Flight Center. "The results demonstrate that you really need to be able to step back and use all the available data to appreciate how things work – not just one spacecraft or one observation or one model."
- McIntosh and his team have identified that coronal bright points allow them to better "see" the solar cycle unfolding. But why does the sunspot cycle start surging in the midlatitudes a few weeks after the terminator?
- The paper on solar tsunamis, led by NCAR scientist Mausumi Dikpati and published in Scientific Reports, explores the possible mechanisms behind the observations. It suggests that coronal bright points are markers for the movement of the Sun's "toroidal magnetic fields," which wrap around the Sun like rubber bands stretching in the east-west direction and migrate slowly toward the equator over the same two decades. 15)
- When these toroidal magnetic fields bob to the surface, they create sunspots along with the bright points they were already producing. As they travel, they also act as magnetic dams, trapping plasma behind them. When the toroidal magnetic fields from the Sun's northern and southern hemispheres touch in the middle, their opposing charges cause their mutual annihilation, releasing the pent-up fluid behind them in a tsunami. This fluid rushes forward, collides, and then ripples backward, traveling toward the poles at a rate of about 300 meters per second.
- As the solar tsunami reaches the Sun's mid-latitudes, it encounters the toroidal magnetic fields of the next cycle, which are already marching toward the equator (this progression is marked by the path of coronal bright points) but traveling deeper within the Sun's interior. The tsunami buoys those magnetic fields, lifting them toward the surface and producing the remarkable surge of bright points – and accompanying sunspot activity – that marks the beginning of the new sunspot cycle.
- "We have observed the sunspot cycle for hundreds of years, but it's been a mystery what mechanism could transport a signal from the equator, where the cycle ends, to the Sun's mid-latitudes, where the next cycle begins, in such a relatively short amount of time," said Dikpati.
- As a body, the research provides a new way of thinking about the workings of the solar interior that challenges some of the conventional thinking about processes on the Sun. Whether or not the research is on the right track – and could improve our predictive capabilities – will soon get its first test.
- There are a number of instruments that are ideally suited to observe the inevitable end of the current solar cycle and the start of the next, according to the authors. These include the Parker Solar Probe, which launched last August, the STEREO-A spacecraft, the Solar Dynamics Observatory, the Daniel K. Inouye Solar Telescope, and other assets.
- "In the next year, we should have a unique opportunity to extensively observe a terminator event as it unfolds and then to watch the launch of Sunspot Cycle 25," McIntosh said. "We believe the results, especially if the terminator arrives when predicted, could revolutionize our understanding of the solar interior and the processes that create sunspots and shape the sunspot cycle."
• April 5, 2019: For five months in mid 2017, Emily Mason did the same thing every day. Arriving to her office at NASA's Goddard Space Flight Center in Greenbelt, Maryland, she sat at her desk, opened up her computer, and stared at images of the Sun — all day, every day. "I probably looked through three or five years' worth of data," Mason estimated. Then, in October 2017, she stopped. She realized she had been looking at the wrong thing all along. 16)
- Mason, a graduate student at The Catholic University of America in Washington, D.C., was searching for coronal rain: giant globs of plasma, or electrified gas, that drip from the Sun's outer atmosphere back to its surface. But she expected to find it in helmet streamers, the million-mile tall magnetic loops — named for their resemblance to a knight's pointy helmet — that can be seen protruding from the Sun during a solar eclipse. Computer simulations predicted the coronal rain could be found there. Observations of the solar wind, the gas escaping from the Sun and out into space, hinted that the rain might be happening. And if she could just find it, the underlying rain-making physics would have major implications for the 70-year-old mystery of why the Sun's outer atmosphere, known as the corona, is so much hotter than its surface. But after nearly half a year of searching, Mason just couldn't find it. "It was a lot of looking," Mason said, "for something that never ultimately happened."
- The problem, it turned out, wasn't what she was looking for, but where. In a paper published today in the Astrophysical Journal Letters, Mason and her coauthors describe the first observations of coronal rain in a smaller, previously overlooked kind of magnetic loop on the Sun. After a long, winding search in the wrong direction, the findings forge a new link between the anomalous heating of the corona and the source of the slow solar wind — two of the biggest mysteries facing solar science today. 17)
Figure 13: Mason searched for coronal rain in helmet streamers like the one that appears on the left side of this image, taken during the 1994 eclipse as viewed from South America. A smaller pseudostreamer appears on the western limb (right side of image). Named for their resemblance to a knight's pointy helmet, helmet streamers extend far into the Sun's faint corona and are most readily seen when the light from the Sun's bright surface is occluded (image credit: © 1994 Úpice observatory and Vojtech Rušin, © 2007 Miloslav Druckmüller)
How It Rains on the Sun
- Observed through the high-resolution telescopes mounted on NASA's SDO spacecraft, the Sun – a hot ball of plasma, teeming with magnetic field lines traced by giant, fiery loops — seems to have few physical similarities with Earth. But our home planet provides a few useful guides in parsing the Sun's chaotic tumult: among them, rain.
- On Earth, rain is just one part of the larger water cycle, an endless tug-of-war between the push of heat and pull of gravity. It begins when liquid water, pooled on the planet's surface in oceans, lakes, or streams, is heated by the Sun. Some of it evaporates and rises into the atmosphere, where it cools and condenses into clouds. Eventually, those clouds become heavy enough that gravity's pull becomes irresistible and the water falls back to Earth as rain, before the process starts anew.
- On the Sun, Mason said, coronal rain works similarly, "but instead of 60-degree water you're dealing with a million-degree plasma." Plasma, an electrically-charged gas, doesn't pool like water, but instead traces the magnetic loops that emerge from the Sun's surface like a rollercoaster on tracks. At the loop's foot points, where it attaches to the Sun's surface, the plasma is superheated from a few thousand to over 1.8 million degrees Fahrenheit. It then expands up the loop and gathers at its peak, far from the heat source. As the plasma cools, it condenses and gravity lures it down the loop's legs as coronal rain.
Figure 14: Coronal rain, like that shown in this movie from NASA's SDO in 2012, is sometimes observed after solar eruptions, when the intense heating associated with a solar flare abruptly cuts off after the eruption and the remaining plasma cools and falls back to the solar surface. Mason was searching for coronal rain not associated with eruptions, but instead caused by a cyclical process of heating and cooling similar to the water cycle on Earth (image credit: NASA's Solar Dynamics Observatory/Scientific Visualization Studio/Tom Bridgman, Lead Animator)
- Mason was looking for coronal rain in helmet streamers, but her motivation for looking there had more to do with this underlying heating and cooling cycle than the rain itself. Since at least the mid-1990s, scientists have known that helmet streamers are one source of the slow solar wind, a comparatively slow, dense stream of gas that escapes the Sun separately from its fast-moving counterpart. But measurements of the slow solar wind gas revealed that it had once been heated to an extreme degree before cooling and escaping the Sun. The cyclical process of heating and cooling behind coronal rain, if it was happening inside the helmet streamers, would be one piece of the puzzle.
- The other reason connects to the coronal heating problem — the mystery of how and why the Sun's outer atmosphere is some 300 times hotter than its surface. Strikingly, simulations have shown that coronal rain only forms when heat is applied to the very bottom of the loop. "If a loop has coronal rain on it, that means that the bottom 10% of it, or less, is where coronal heating is happening," said Mason. Raining loops provide a measuring rod, a cutoff point to determine where the corona gets heated. Starting their search in the largest loops they could find — giant helmet streamers — seemed like a modest goal, and one that would maximize their chances of success.
- She had the best data for the job: Images taken by NASA's SDO (Solar Dynamics Observatory), a spacecraft that has photographed the Sun every twelve seconds since its launch in 2010. But nearly half a year into the search, Mason still hadn't observed a single drop of rain in a helmet streamer. She had, however, noticed a slew of tiny magnetic structures, ones she wasn't familiar with. "They were really bright and they kept drawing my eye," said Mason. "When I finally took a look at them, sure enough they had tens of hours of rain at a time."
- At first, Mason was so focused on her helmet streamer quest that she made nothing of the observations. "She came to group meeting and said, ‘I never found it — I see it all the time in these other structures, but they're not helmet streamers,'" said Nicholeen Viall, a solar scientist at Goddard, and a coauthor of the paper. "And I said, ‘Wait ...hold on. Where do you see it? I don't think anybody's ever seen that before!'"
A Measuring Rod for Heating
- These structures differed from helmet streamers in several ways. But the most striking thing about them was their size.
- "These loops were much smaller than what we were looking for," said Spiro Antiochos, who is also a solar physicist at Goddard and a coauthor of the paper. "So that tells you that the heating of the corona is much more localized than we were thinking."
- While the findings don't say exactly how the corona is heated, "they do push down the floor of where coronal heating could happen," said Mason. She had found raining loops that were some 30,000 miles high, a mere two percent the height of some of the helmet streamers she was originally looking for. And the rain condenses the region where the key coronal heating can be happening. "We still don't know exactly what's heating the corona, but we know it has to happen in this layer," said Mason.
Figure 15: Mason's article analyzed three observations of RNTPs (Raining Null-Point Topologies), a previously overlooked magnetic structure shown here in two wavelengths of extreme ultraviolet light. The coronal rain observed in these comparatively small magnetic loops suggests that the corona may be heated within a far more restricted region than previously expected (image credit: NASA's Solar Dynamics Observatory/Emily Mason)
A New Source for the Slow Solar Wind
- But one part of the observations didn't jibe with previous theories. According to the current understanding, coronal rain only forms on closed loops, where the plasma can gather and cool without any means of escape. But as Mason sifted through the data, she found cases where rain was forming on open magnetic field lines. Anchored to the Sun at only one end, the other end of these open field lines fed out into space, and plasma there could escape into the solar wind. To explain the anomaly, Mason and the team developed an alternative explanation — one that connected rain on these tiny magnetic structures to the origins of the slow solar wind.
- In the new explanation, the raining plasma begins its journey on a closed loop, but switches — through a process known as magnetic reconnection — to an open one. The phenomenon happens frequently on the Sun, when a closed loop bumps into an open field line and the system rewires itself. Suddenly, the superheated plasma on the closed loop finds itself on an open field line, like a train that has switched tracks. Some of that plasma will rapidly expand, cool down, and fall back to the Sun as coronal rain. But other parts of it will escape – forming, they suspect, one part of the slow solar wind.
- Mason is currently working on a computer simulation of the new explanation, but she also hopes that soon-to-come observational evidence may confirm it. Now that Parker Solar Probe, launched in 2018, is traveling closer to the Sun than any spacecraft before it, it can fly through bursts of slow solar wind that can be traced back to the Sun — potentially, to one of Mason's coronal rain events. After observing coronal rain on an open field line, the outgoing plasma, escaping to the solar wind, would normally be lost to posterity. But no longer. "Potentially we can make that connection with Parker Solar Probe and say, that was it," said Viall.
Digging Through the Data
- As for finding coronal rain in helmet streamers? The search continues. The simulations are clear: the rain should be there. "Maybe it's so small you can't see it?" said Antiochos. "We really don't know."
- But then again, if Mason had found what she was looking for she might not have made the discovery — or have spent all that time learning the ins and outs of solar data.
- "It sounds like a slog, but honestly it's my favorite thing," said Mason. "I mean that's why we built something that takes that many images of the Sun: So we can look at them and figure it out."
• On the evening of March 6, 2019, the Moon started to transit the Sun, then doubled back and retraced its steps in the other direction — at least, that's what it looked like from the perspective of NASA's SDO (Solar Dynamics Observatory) mission in orbit around Earth. 18)
Figure 16: The relative speeds and positions of the Moon, the Sun and NASA's SDO resulted in this unusual lunar transit where the Moon appears to pause and reverse course (image credits: NASA/Goddard/SDO)
- SDO sees lunar transits regularly, when the Moon passes in front of its view of the Sun. The Moon's unusual apparent behavior during this particular transit is a phenomenon similar to retrograde motion: When a celestial object appears to move backwards because of the way that different objects move at different speeds at different points in their orbits. In this case, the first part of the transit — when the Moon moves left to right — appears to be "reverse" motion. SDO overtakes the Moon, moving at about 1.9 miles per second perpendicular to the Sun-Earth line compared to the Moon's 0.6 miles per second, making the Moon appear to move in the opposite direction you would see if you were standing still on Earth.
- The second part of the transit — when the Moon appears to pause and rewind — happens as SDO enters the dusk part of its orbit and begins moving away from the Moon, nearly parallel to the shadow it's casting through space. At that point, the Moon once again moves faster than SDO – when compared to the Sun-Earth line – overtaking it. So the spacecraft now sees it move in the other direction — the same direction that a stationary observer on Earth would see.
Figure 17: NASA's SDO spacecraft spotted a lunar transit just as it began the transition to the dusk phase of its orbit, leading to the Moon's apparent pause and change of direction during the transit. This animation (with orbits to scale) illustrates the movement of the Moon, its shadow and SDO (image credit: NASA/SDO)
- This isn't the first time that SDO has seen the Moon seem to move in two different directions during a lunar transit. This time, the Moon just happened to remain in SDO's sight as it began the dusk part of its orbit, leading to the freeze-frame effect.
- This lunar transit lasted about four hours, from 5 p.m. to 9:07 p.m. EST, and, at peak, the Moon covered 82 percent of the Sun's face. The Moon's edge appears sharp because the Moon has no atmosphere. On the other hand, Earth eclipses of the Sun have a blurry edge when seen by SDO, because the gases in Earth's atmosphere let through only part of the Sun's light.
• February 7, 2019: New research undertaken at Northumbria University, Newcastle (UK) shows that the Sun's magnetic waves behave differently than currently believed. Their findings have been reported in the latest edition of the prominent journal, Nature Astronomy.19)
- After examining data gathered over a 10-year period, the team from Northumbria's Department of Mathematics, Physics and Electrical Engineering found that magnetic waves in the Sun's corona - its outermost layer of atmosphere - react to sound waves escaping from the inside of the Sun.
- These magnetic waves, known as Alfvénic waves, play a crucial role in transporting energy around the Sun and the solar system. The waves were previously thought to originate at the Sun's surface, where boiling hydrogen reaches temperatures of 6,000 K and churns the Sun's magnetic field.
- However, the researchers have found evidence that the magnetic waves also react — or are excited — higher in the atmosphere by sound waves leaking out from the inside of the Sun.
- The team discovered that the sound waves leave a distinctive marker on the magnetic waves. The presence of this marker means that the Sun's entire corona is shaking in a collective manner in response to the sound waves. This is causing it to vibrate over a very clear range of frequencies.
- This newly-discovered marker is found throughout the corona and was consistently present over the 10-year time-span examined. This suggests that it is a fundamental constant of the Sun — and could potentially be a fundamental constant of other stars.
- The findings could therefore have significant implications for our current ideas about how magnetic energy is transferred and used in stellar atmospheres.
- Dr Richard Morton, the lead author of the report and a senior lecturer at Northumbria University, said: "The discovery of such a distinctive marker — potentially a new constant of the Sun — is very exciting. We have previously always thought that the magnetic waves were excited by the hydrogen at the surface, but now we have shown that they are excited by these sound waves. This could lead to a new way to examine and classify the behavior of all stars under this unique signature. Now we know the signature is there, we can go looking for it on other stars. 20)
- "The Sun's corona is over one hundred times hotter than its surface and energy stemming from the Alfvénic waves is believed to be responsible for heating the corona to a temperature of around one million degrees. The Alfvénic waves are also responsible for heating and accelerating powerful solar wind from the Sun which travels through the solar system. These winds travel at speeds of around a million miles per hour. They also affect the atmosphere of stars and planets, impacting on their own magnetic fields, and cause phenomena such as aurora."
- Dr Morton added: "Our evidence shows that the Sun's internal acoustic oscillations play a significant role in exciting the magnetic Alfvénic waves. This can give the waves different properties and suggests that they are more susceptible to an instability, which could lead to hotter and faster solar winds."
- The research was funded by the UK Science and Technology Facilities Council and the US Air Force Office of Scientific Research. It was undertaken by Dr Morton and Professor James McLaughlin from Northumbria's Solar Physics research group, together with Dr Micah Weberg, who recently moved from Northumbria to Washington DC's Naval Research Laboratory.
- The study of the Alfvénic wave motions is supported by extreme ultraviolet images of the corona from the 17.1 nm (Fe IX) channel of the AIA (Atmospheric Imaging Assembly) instrument on-board NASA's SDO (Solar Dynamics Observatory), which enables direct measurement of the transverse oscillatory displacements of the corona's fine-scale magnetic structure.
Figure 18: April 21, 2011 marks the one-year anniversary of the Solar Dynamics Observatory (SDO) First Light press conference, where NASA revealed the first images taken by the spacecraft. In the last year, the sun has gone from its quietest period in years to the activity marking the beginning of solar cycle 24. SDO has captured every moment with a level of detail never-before possible. The mission has returned unprecedented images of solar flares, eruptions of prominences, and the early stages of coronal mass ejections (CMEs). In this video are some of the most beautiful, interesting, and mesmerizing events seen by SDO during its first year (image credit: NASA/Goddard Space Flight Center/SDO, Music courtesy of Moby Gratis) 21)
• April 9, 2018: Despite their appearance solar tornadoes are not rotating after all, according to a European team of scientists. A new analysis of these gigantic structures, each one several times the size of the Earth, indicates that they may have been misnamed because scientists have so far only been able to observe them using 2-dimensional images. Dr. Nicolas Labrosse will present the work, carried out by researchers at the University of Glasgow, Paris Observatory, University of Toulouse, and Czech Academy of Sciences, at the European Week of Astronomy and Space Science (EWASS) in Liverpool on Friday 6 April. 22)
- Solar tornadoes were first observed in the early 20th century, and the term was re-popularized a few years ago when scientists looked at movies obtained by the AIA instrument on the NASA SDO (Solar Dynamics Observatory). These show hot plasma in extreme ultraviolet light apparently rotating to form a giant structure taking the shape of a tornado (as we know them on Earth).
- Now, using the Doppler effect to add a third dimension to their data, the scientists have been able to measure the speed of the moving plasma, as well as its direction, temperature and density. Using several years' worth of observations, they were able to build up a more complete picture of the magnetic field structure that supports the plasma, in structures known as prominences.
- Nicolas Labrosse, lead scientist in the study, explains: "We found that despite how prominences and tornadoes appear in images, the magnetic field is not vertical, and the plasma mostly moves horizontally along magnetic field lines. However we see tornado-like shapes in the images because of projection effects, where the line of sight information is compressed onto the plane of the sky."
- Dr. Arturo López Ariste, another member of the team, adds: "The overall effect is similar to the trail of an airplane in our skies: the airplane travels horizontally at a fixed height, but we see that the trail starts above our heads and ends up on the horizon. This doesn't mean that it has crashed!"
- Giant solar tornadoes – formally called tornado prominences – have been observed on the Sun for around a hundred years. They are so called because of their striking shape and apparent resemblance to tornadoes on Earth, but that is where the comparison ends.
- Whereas terrestrial tornadoes are formed from intense winds and are very mobile, solar tornadoes are instead magnetized gas. They seem to be rooted somewhere further down the solar surface, and so stay fixed in place.
- "They are associated with the legs of solar prominences – these are beautiful concentrations of cool plasma in the very hot solar corona that can easily be seen as pink structures during total solar eclipses," adds Lacrosse.
- "Perhaps for once the reality is less complicated than what we see!" comments Dr. Brigitte Schneider, another scientist involved in the work. She continues: "Solar tornadoes sound scary but in fact they normally have no noticeable consequences for us. However, when a tornado prominence erupts, it can cause what's known as space weather, potentially damaging power, satellite and communication networks on Earth."
Figure 19: Composite image of an erupting solar prominence observed by SDO on Aug. 31, 2012 (image credit: NASA / SDO / GSFC)
• From March 20-23, 2018, NASA's SDO (Solar Dynamics Observatory) captured three sequences of our Sun in three different extreme ultraviolet wavelengths. The resulting images illustrate how different features that appear in one sequence are difficult, if not impossible, to see in the others. 23)
- In the red sequence (304 Å), we can see very small spicules - jets of solar material - and some small prominences at the Sun's edge, which are not easy to see in the other two sequences. In the second sequence (193 Å), we can readily observe the large and dark coronal hole, though it is difficult to make out in the others. In the third (171 wavelengths), we can see strands of plasma waving above the surface, especially above the one small, but bright, active region near the right edge. These are just three of the ten extreme ultraviolet wavelengths in which SDO images the Sun every 12 seconds, every day.
Figure 20: SDO/AIA image of the sun in three wavelengths (image credit: NASA)
• March 5, 2018: Magnetohydrodynamic (MHD) Alfvén waves have been a focus of laboratory plasma physics and astrophysics for over half a century. Their unique nature makes them ideal energy transporters, and while the solar atmosphere provides preferential conditions for their existence, direct detection has proved difficult as a result of their evolving and dynamic observational signatures. 24)
- The study used advanced high-resolution observations from the Dunn Solar Telescope in New Mexico (USA) alongside complementary observations from NASA's SDO (Solar Dynamics Observatory), to analyze the strongest magnetic fields that appear in sunspots. These sunspots have intense fields similar to modern MRI (Magnetic Resonance Imaging) machines in hospitals and are much bigger than our own planet.
- Scientists at Queen's University Belfast have led an international team to the ground-breaking discovery that magnetic waves crashing through the Sun may be the key to heating its atmosphere and propelling the solar wind. The international team included Queen's University Belfast, Belfast, UK; the Space Research Institute, AAS (Austrian Academy of Sciences), Graz, Austria; Abastumani Astrophysical Observatory at Ilia State University, Tbilisi, Georgia; NSO (National Solar Observatory), Boulder, CO, USA; IAC (Instituto de Astrofisica de Canarias), Spain; Lockheed Martin, Solar and Astrophysics Laboratory, Palo Alto, CA, USA, and the California State University Northridge, USA. 25)
- In 1942, a Swedish engineer and physicist by the name of Hannes Alfvén predicted the existence of a new type of wave – one that could exist purely as a result of magnetism acting on plasma. The important nature of these revolutionary waves earned Hannes Alfvén the Nobel Prize for Physics in 1970. Since his prediction, Alfvén waves have been associated with a variety of sources, including nuclear reactors, the coma of comets, laboratory experiments, medical MRI imaging, and in the atmosphere of our nearest star – the Sun. It is here, in the turbulent multi-million-degree atmosphere of the Sun, that Alfvén waves have been hypothesized to play an important role in maintaining such elevated temperatures.
- The solar surface hosts a web of diverse magnetic fields, from sunspots exhibiting sizes that dwarf the Earth, to dynamic bright grains only a few hundred kilometers across. The magnetic nature of the Sun's atmosphere supports the plethora of MHD wave activity observed in recent years. Such wave motion is predominantly generated near the surface of the Sun, with the creation of upwardly propagating MHD waves providing a conduit for the transportation of heat, from the vast energy reservoir of the solar photosphere, to the outermost extremities of the multimillion-degree corona.
- In comparison to other MHD modes, Alfvén waves are the preferred candidates for energy transport since they do not reflect or dissipate energy freely. Observational studies have been limited by the challenging requirements on instrumentation needed to identify the Doppler line-of-sight (LOS) velocity perturbations and non-thermal broadening associated with Alfvén waves, thus there is only tentative evidence of their existence within the Sun's magnetized plasma. Given the difficulties associated with resolving the intrinsic wave signatures, to date there has been no observational evidence brought forward to verify the dissipative processes associated with Alfvén waves.
Intensity scans of the chromospheric Ca II 8,542 Å spectral line at high spatial (71 km per pixel) and temporal (5.8 s) resolution were conducted for a 70 x 70 Mm2 region centered around a large sunspot on 24 August 2014 using the IBIS (Interferometric BIdimensional Spectrometer) at the Dunn Solar Telescope. Three-dimensional values for the vector magnetic field are derived through nonlinear force-free extrapolations applied to simultaneous magnetograms obtained by the HMI (Helioseismic and Magnetic Imager) aboard the SDO ( Solar Dynamics Observatory) spacecraft. The resulting images (Figure 21) highlight the connectivity between the magnetic fields and the structuring of the sunspot atmosphere. Intensity thresholding of running-mean-subtracted images of the chromospheric umbra allowed for the identification of 554,792 individual shock signals across the 135 minute duration of the dataset.
Figure 21: The building blocks of the magnetized solar atmosphere observed on 24 August 2014 (image credit: Queen's Study Team)
Legend to Figure 21: Co-spatial images revealing the structure of the sunspot at 13:00 UT on 24 August 2014. The lower image shows the magnitude of the photospheric magnetic field from HMI, revealing high umbral field strengths (color bar relates to the field strengths in gauss). The image above is taken from the blue wing of the Ca II 8,542 Å spectral line, displaying the photospheric representation of the sunspot. Above this is the photospheric plasma temperature of the region derived from CAISAR (CAlcium Inversion using a Spectral ARchive) at log(τ 500 nm) ~-2 (or ~ 250 km above the photosphere), showing the clear temperature distinction between the umbra, penumbra and surrounding quiet Sun (color bar in units of Kelvin). The upper image shows the chromospheric core of the Ca II 8,542 Å spectral line, highlighting the strong intensity gradient between the umbra and penumbra at these heights. In each of these images, the red contours represent the inner and outer boundaries of the plasma-β = 1 region at the height where shocks first begin to manifest (~ 250 km), where magneto-acoustic and Alfvén waves can readily convert.
- The sunspot displays a classical structure, providing two preferential locations for plasma shock formation in the umbra: first, in the vicinity of strong, vertical magnetic fields near the umbral center of mass, and second, in the presence of weaker, inclined fields towards the outer boundary of the umbra (Figure 22). The first population is attributed to UFs (Umbral Flashes), whereby the near-vertical propagation of magneto-acoustic waves across multiple density scale heights promotes their efficient steepening into shocks. The second population, whose intensity excursions are formed along inclined magnetic fields that channel waves almost horizontally (inclinations of approximately 70–80 degrees), provides the first indication of Alfvén wave shock formation. Alfvén shocks are predicted to form in regions with high negative Alfvén speed gradients, which is fulfilled by the volume expansion of the magnetic field lines and the highly inclined environment in which they exist, hence minimizing the effects of density stratification and promoting a negative gradient in the associated Alfvén speed.
- The extrapolated vector magnetic fields are employed with the complementary CAISAR code, producing maps of magnetic and thermal pressure that are used to establish the physical locations where the ratio of these parameters, the plasma-β, equals unity (Figure 21). Here, the plasma-β = 1 region is analogous to the locations where the characteristic velocities of slow and fast waves are equal. Thus, ubiquitous magneto-acoustic waves throughout the umbra are capable of converting into Alfvén modes through the process of mode conversion. This provides a bulk generation of Alfvén waves that can form both Alfvén and resonantly driven shocks under the correct atmospheric conditions. The magnetic topologies representative of the outermost boundary of the sunspot umbra provide such an environment where a steep negative Alfvén speed gradient is encountered by the propagating waves. UF shock formation in this regime is drastically suppressed due to the heavily inclined magnetic field lines providing substantially reduced density stratification along the paths of wave propagation.
- Examination of the Doppler LOS velocities of the shocked plasma further distinguishes between UFs near the sunspot core and the Alfvén shocks at the periphery of the umbra. The LOS plasma velocities associated with the UF population are blue-shifted and display characteristic ‘sawtooth' spectral profiles throughout their temporal evolution, consistent with the established morphology of UFs (Umbral Flashes). The second population, which dominates the outer umbral perimeter where the plasma-β equals unity, displays an intermingling of red- and blue-shifted plasma moving perpendicularly to the wavevector (Figure 22) during the onset of plasma shock events. This is in stark contrast to conventional UFs, and provides further direct evidence of Alfvén shocks, since large velocity excursions perpendicular to the vector magnetic field are representative of Alfvén waves undergoing nonlinear processes during the creation of shocks (Figure 23). Furthermore, the observed red-shifts indicate that approximately 70% of the observed shocks are due to the direct steepening of Alfvén waves, which is depicted in the right-hand side of Figure 23.
Figure 22: A statistical insight into the magnetic, velocity and occurrence relationships between shock phenomena in a sunspot umbra. a,b, (image credit: Queen's Study Team)
Legend to Figure 22: The inclination of the magnetic field as a function of the total magnetic field strength for shock pixels at their point of formation, where an inclination of 0° represents an upwardly orientated vertical field. The color scheme of the data points denotes their distance from the center of the umbral core in Mm (a) and the LOS velocities of the resultant shock emission profiles, where positive values represent red-shifted plasma motion away from the observer, while negative values indicate blue-shifted plasma towards the observer (b). The diameters of the data points in b represent a visualization of the relative temperature increases of shocked plasma above their local quiescent background temperatures, where the largest circles represent significant temperature enhancements (up to a maximum of ~ 20%) that are synonymous with higher magnetic field strengths when compared to the smaller temperature enhancements associated with lower magnetic field strengths. The data points are over-plotted with the probability density function of shock occurrence as a function of the total magnetic field strength.
Figure 23: A cartoon representation of a sunspot umbral atmosphere demonstrating a variety of shock phenomena (image credit: Queen's Study Team)
Legend to Figure 23: A side-on perspective of a typical sunspot atmosphere, showing magnetic field lines (orange cylinders) anchored into the photospheric umbra (bottom of image) and expanding laterally as a function of atmospheric height, into the upper atmospheric regions of the TR (Transition Region) and corona. The light blue annuli highlight the lower and upper extents of the mode-conversion region for the atmospheric heights of interest. The mode-conversion region on the left-hand side shows a schematic of nonlinear Alfvén waves resonantly amplifying magneto-acoustic waves, increasing the shock formation efficiency in this location. The mode-conversion region on the right-hand side demonstrates the coupling of upwardly propagating magneto-acoustic oscillations (the sinusoidal motions) into Alfvén waves (the elliptical structures), which subsequently develop tangential blue- and red-shifted plasma during the creation of Alfvén shocks. The central portion represents the traditional creation of UFs that result from the steepening of magnetoacoustic waves as they traverse multiple density scale heights in the lower solar atmosphere. Image not to scale.
• February 26, 2018: A dramatic magnetic power struggle at the Sun's surface lies at the heart of solar eruptions, new research using NASA data shows. The work highlights the role of the Sun's magnetic landscape, or topology, in the development of solar eruptions that can trigger space weather events around Earth. 26)
- The scientists, led by Tahar Amari, an astrophysicist at the Center for Theoretical Physics at the École Polytechnique in Palaiseau Cedex, France, considered solar flares, which are intense bursts of radiation and light. Many strong solar flares are followed by a coronal mass ejection, or CME, a massive, bubble-shaped eruption of solar material and magnetic field, but some are not — what differentiates the two situations is not clearly understood.
- Using data from NASA's SDO (Solar Dynamics Observatory), the scientists examined an October 2014 Jupiter-sized sunspot group, an area of complex magnetic fields, often the site of solar activity. This was the biggest group in the past two solar cycles and a highly active region. Though conditions seemed ripe for an eruption, the region never produced a major CME on its journey across the Sun. It did, however, emit a powerful X-class flare, the most intense class of flares. What determines, the scientists wondered, whether a flare is associated with a CME?
Figure 24: On Oct. 24, 2014, NASA's SDO observed an X-class solar flare erupt from a Jupiter-sized sunspot group (image credit: Tahar Amari et al./Center for Theoretical Physics/École Polytechnique/NASA Goddard/Joy Ng)
- The team of scientists included SDO's observations of magnetic fields at the Sun's surface in powerful models that calculate the magnetic field of the Sun's corona, or upper atmosphere, and examined how it evolved in the time just before the flare. The model reveals a battle between two key magnetic structures: a twisted magnetic rope — known to be associated with the onset of CMEs — and a dense cage of magnetic fields overlying the rope.
- The scientists found that this magnetic cage physically prevented a CME from erupting that day. Just hours before the flare, the sunspot's natural rotation contorted the magnetic rope and it grew increasingly twisted and unstable, like a tightly coiled rubber band. But the rope never erupted from the surface: Their model demonstrates it didn't have enough energy to break through the cage. It was, however, volatile enough that it lashed through part of the cage, triggering the strong solar flare.
- By changing the conditions of the cage in their model, the scientists found that if the cage was weaker that day, a major CME would have erupted on Oct. 24, 2014. The group is interested in further developing their model to study how the conflict between the magnetic cage and rope plays out in other eruptions. Their findings are summarized in a paper published in Nature on Feb. 8, 2018. 27)
- "We were able to follow the evolution of an active region, predict how likely it was to erupt, and calculate the maximum amount of energy the eruption can release," Amari said. "This is a practical method that could become important in space weather forecasting as computational capabilities increase."
Figure 25: In this series of images, the magnetic rope, in blue, grows increasingly twisted and unstable. But it never erupts from the Sun's surface: The model demonstrates the rope didn't have enough energy to break through the magnetic cage, in yellow (image credit: Tahar Amari et al./Center for Theoretical Physics/École Polytechnique/NASA Goddard/Joy Ng)
• On 11 February 2018, the SDO spacecraft was 8 years on orbit.
• November 1, 2017: This sequence of images of Figure 26 shows the Sun from its surface to its upper atmosphere, all taken at about the same time on Oct. 27, 2017. The first shows the surface of the Sun in filtered white light; the other seven images were taken in different wavelengths of extreme ultraviolet light. Note that each wavelength reveals somewhat different features. They are shown in order of temperature from the first one at 6,000 degree C. surface, out to about 10 million degrees C. in the upper atmosphere. Yes, the Sun's outer atmosphere is much, much hotter than the surface. Scientists are getting closer to solving the processes that generate this phenomenon. 28)
Figure 26: On 27 Oct. 2017, the AIA (Atmospheric Imaging Assembly) instrument on SDO captured this image sequence of the Sun from its surface (left) to its upper atmosphere (right) in different wavelengths (image credit: NASA/GSFC/SDO)
• September 6, 2017: The sun emitted two significant solar flares on the morning of Sept. 6, 2017. The first peaked at 9:10 UTC and the second, larger flare, peaked at 12:02 UTC. NASA's SDO (Solar Dynamics Observatory), which watches the sun constantly, captured images of both events. Solar flares are powerful bursts of radiation. Harmful radiation from a flare cannot pass through Earth's atmosphere to physically affect humans on the ground, however — when intense enough — they can disturb the atmosphere in the layer where GPS and communications signals travel.
Figure 27: This animation shows both the X2.2 and the X9.3 flares that the Sun emitted on Sept. 6, 2017. The imagery was captured by the AIA instrument of NASA's SDO and shows light in the 131 angstrom wavelength (image credit: NASA/Goddard/SDO)
- The first flare is classified as an X2.2 flare and the second is an X9.3 flare. X-class denotes the most intense flares, while the number provides more information about its strength. An X2 is twice as intense as an X1, an X3 is three times as intense, etc.
- Both flares erupted from an active region labeled AR 2673, which also produced a mid-level solar flare on Sept. 4, 2017. The X9.3 flare was the largest flare so far in the current solar cycle, the approximately 11-year-cycle during which the sun's activity waxes and wanes. The current solar cycle began in December 2008, and is now decreasing in intensity and heading toward solar minimum. This is a phase when such eruptions on the sun are increasingly rare, but history has shown that they can nonetheless be intense. The radio black out from this particular flare is already passed.
• On May 25 2017, NASA's SDO mission saw a partial solar eclipse in space when it caught the moon passing in front of the sun. The lunar transit lasted almost an hour, between 6:24 and 7:17 UTC, with the moon covering about 89 percent of the sun at the peak of its journey across the sun's face. The moon's crisp horizon can be seen from this view because the moon has no atmosphere to distort the sunlight. 29)
- While the moon's edge appears smooth in these images, it's actually quite uneven. The surface of the moon is rugged, sprinkled with craters, valleys and mountains. Peer closely at the image, and you may notice the subtle, bumpy outline of these topographical features.
- Later this summer on Aug. 21, 2017, SDO will witness another lunar transit, but the moon will only barely hide part of the sun. However, on the same day, a total eclipse will be observable from the ground. A total solar eclipse — in which the moon completely obscures the sun — will cross the United States on a 110 km wide ribbon of land stretching from Oregon to South Carolina. Throughout the rest of North America — and even in parts of South America, Africa, Europe and Asia — a partial eclipse will be visible.
- The moon's rough, craggy terrain influences what we see on Earth during a total solar eclipse. Light rays stream through lunar valleys along the moon's horizon and form Baily's beads, bright points of light that signal the beginning and end of totality.
- The moon's surface also shapes the shadow, called the umbra, that races across the path of totality: Sunlight peeks through valleys and around mountains, adding edges to the umbra. These edges warp even more as they pass over Earth's own mountain ranges. Visualizers used data from NASA's LRO (Lunar Reconnaissance Orbiter), coupled with NASA topographical data of Earth, to precisely map the upcoming eclipse in unprecedented detail. This work shows the umbral shape varies with time, and is not simply an ellipse, but an irregular polygon with slightly curved edges.
Figure 28: Illustration of the lunar transit observed by NASA's SDO (image credit: NASA/GSFC, SDO/Joy Ng, producer)
• April 10, 2017: New research on solar storms finds that they not only can cause regions of excessive electrical charge in the upper atmosphere above Earth's poles, they also can do the exact opposite: cause regions that are nearly depleted of electrically charged particles. The finding adds to our knowledge of how solar storms affect Earth and could possibly lead to improved radio communication and navigation systems for the Arctic. 30)
- A team of researchers from Denmark, the United States and Canada made the discovery while studying a solar storm that reached Earth on Feb. 19, 2014. The storm was observed to affect the ionosphere in all of Earth's northern latitudes. Its effects on Greenland were documented by a network of global navigation satellite system, or GNSS, stations as well as geomagnetic observatories and other resources. Attila Komjathy of NASA/JPL (Jet Propulsion Laboratory), Pasadena, California, developed software to process the GNSS data and helped with the data processing. The results were published in the journal Radio Science. 31)
- Solar storms often include an eruption on the sun called a CME (Coronal Mass Ejection). This is a vast cloud of electrically charged particles hurled into space that disturbs the interplanetary magnetic field in our solar system. When these particles and the magnetic disturbances encounter Earth's magnetic field, they interact in a series of complex physical processes, and trigger perturbations in the Earth's magnetic field. Those perturbations are called geomagnetic storms. The interactions may cause unstable patches of excess electrons in the ionosphere, an atmospheric region starting about 80 km above Earth's surface that already contains ions and electrons.
- The 2014 geomagnetic storm was a result of two powerful Earth-directed CMEs. The storm initially produced patches of extra electrons in the ionosphere over northern Greenland, as usual. But just south of these patches, the scientists were surprised to find broad areas extending 500 to 1,000 km where the electrons were "almost vacuumed out," in the words of Per Høeg of the National Space Research Institute at the Technical University of Denmark, Lyngby. These areas remained depleted of electrons for several days.
- The electrons in the ionosphere normally reflect radio waves back to ground level, enabling long-distance radio communications. Both electron depletion and electron increases in this layer can possibly cause radio communications to fail, reduce the accuracy of GPS systems, damage satellites and harm electrical grids.
- "We don't know exactly what causes the depletion," Komjathy said. "One possible explanation is that electrons are recombining with positively charged ions until there are no excess electrons. There could also be redistribution — electrons being displaced and pushed away from the region, not only horizontally but vertically."
Figure 29: A solar eruption on Sept. 26, 2014, seen by NASA's Solar Dynamics Observatory. If erupted solar material reaches Earth, it can deplete the electrons in the upper atmosphere in some locations while adding electrons in others, disrupting communications either way (image credit: NASA)
• March 22, 2017: For 15 days starting on March 7, 2017, NASA's SDO (Solar Dynamics Observatory) returned visible light images of a yolk-like spotless sun. This is the longest stretch of spotlessness since the last solar minimum in April 2010, indicating the solar cycle is marching on toward the next minimum, which scientists predict will occur between 2019—2020. 32)
- The sun goes through a natural 11-year cycle marked by two extremes: solar maximum and solar minimum. Sunspots are dark regions of complex magnetic activity on the sun's surface, so the number of sunspots at any given time is used as an index for solar activity. Solar maximum is characterized by intense solar activity and the greatest sunspot number. Conversely, during solar minimum, the sun is least active and sunspot number is at its lowest. Solar activity, however, does not end as sunspot number decreases toward minimum. There are other sources of solar activity such as high-speed streams of solar material from coronal holes, which spark aurora and other space weather effects.
Figure 30: The SDO images here compare the sun on March 20, 2017, and February 27, 2014, during the last solar maximum when the sun sported numerous spots (image credit: NASA)
• November 3, 2016: On Oct. 19, 2016, operators instructed NASA's SDO spacecraft to look up and down and then side to side over the course of six hours, as if tracing a great plus sign in space. During this time, SDO produced some unusual data. Taken every 12 seconds, SDO images show the sun dodging in and out of the frame. SDO captured these images in extreme ultraviolet light, a type of light that is invisible to our eyes. Here, they are colorized in red. 33)
- SDO operators schedule this maneuver, one of a series of maneuvers that SDO completed on Oct. 27, 2016, twice a year to calibrate the spacecraft's instruments. Veering motions allow scientists to assess how light travels through SDO's instruments – whether light is reflected inside the instrument, for example – and how these instruments are changing over time.
- This particular maneuver is the EVE cruciform maneuver, designed to help SDO's Extreme ultraviolet Variability Experiment, or EVE, take accurate measurements of the sun's extreme UV emissions. EVE studies these emissions over time, so that we may better understand their role in influencing Earth's climate and local space environment.
Figure 31: Images from NASA's SDO during a routine EVE cruciform maneuver show the sun dodging in and out of the frame (image credit: NASA/GSFC, SDO, Joy Ng)
• October 20, 2016: While it often seems unvarying from our viewpoint on Earth, the sun is constantly changing. Material courses through not only the star itself, but throughout its expansive atmosphere. Understanding the dance of this charged gas is a key part of better understanding our sun – how it heats up its atmosphere, how it creates a steady flow of solar wind streaming outward in all directions, and how magnetic fields twist and turn to create regions that can explode in giant eruptions. Now, for the first time, researchers have tracked a particular kind of solar wave as it swept upward from the sun's surface through its atmosphere, adding to our understanding of how solar material travels throughout the sun. 34)
- Tracking solar waves like this provides a novel tool for scientists to study the atmosphere of the sun. The imagery of the journey also confirms existing ideas, helping to nail down the existence of a mechanism that moves energy – and therefore heat – into the sun's mysteriously-hot upper atmosphere, called the corona. A study on these results was published Oct. 11, 2016, in The Astrophysical Journal Letters. 35)
- "We see certain kinds of solar seismic waves channeling upwards into the lower atmosphere, called the chromosphere, and from there, into the corona," said Junwei Zhao, a solar scientist at Stanford University in Stanford, California, and lead author on the study. "This research gives us a new viewpoint to look at waves that can contribute to the energy of the atmosphere."
- The study makes use of the wealth of data captured by NASA's SDO (Solar Dynamics Observatory), NASA's IRIS (Interface Region Imaging Spectrograph), and the BBSO (Big Bear Solar Observatory) in Big Bear Lake, California. Together, these observatories watch the sun in 16 wavelengths of light that show the sun's surface and lower atmosphere. SDO alone captures 11 of these.
- "SDO takes images of the sun in many different wavelengths at a high time resolution," said Dean Pesnell, SDO project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "That lets you see the frequencies of these waves – if you didn't have such rapid-fire images, you'd lose track of the waves from one image to the next."
- Though scientists have long suspected that the waves they spot in the sun's surface, called the photosphere, are linked to those seen in the lowest reaches of the sun's atmosphere, called the chromosphere, this new analysis is the first time that scientists have managed to actually watch the wave travel up through the various layers into the sun's atmosphere.
- When material is heated to high temperatures, it releases energy in the form of light. The type, or wavelength, of that light is determined by what the material is, as well as its temperature. That means different wavelengths from the sun can be mapped to different temperatures of solar material. Since we know how the sun's temperature changes throughout the layers of its atmosphere, we can then order these wavelengths according to their height above the surface – and essentially watch solar waves as they travel upwards.
- The implications of this study are twofold – first, this technique for watching the waves itself gives scientists a new tool to understand the sun's lower atmosphere. "Watching the waves move upwards tells us a lot about the properties of the atmosphere above sunspots – like temperature, pressure, and density," said Ruizhu Chen, a graduate student scientist at Stanford who is an author on the study. "More importantly, we can figure out the magnetic field strength and direction."
- The effect of the magnetic field on these waves is pronounced. Instead of traveling straight upwards through the sun, the waves veer off, taking a curved path through the atmosphere. "The magnetic field is acting like railroad tracks, guiding the waves as they move up through the atmosphere," said Pesnell, who was not involved in this study.
- The second implication of this new research is for a long-standing question in solar physics – the coronal heating problem. -The sun produces energy by fusing hydrogen at its core, so the simplest models suggest that each layer of the sun should be cooler as you move outward. However, the sun's atmosphere, called the corona, is about a hundred times hotter than the region below – counter to what you would expect.
- No one has as-yet been able to definitively pinpoint the source of all the extra heat in the corona, but these waves may play a small role. "When a wave travels upwards, a number of different things can happen," said Zhao. "Some may reflect back downwards, or contribute to heating – but by how much, we don't yet know."
Figure 32: Scientists analyzed sunspot images from a trio of observatories — including the Big Bear Solar Observatory, which captured this footage — to make the first-ever observations of a solar wave traveling up into the sun's atmosphere from a sunspot (image credit: BBSO/Zhao et al)
Figure 33: Scientists used data from NASA's SDO, NASA's IRIS, and the BBSO to track a solar wave as it channeled upwards from the sun's surface into the atmosphere (image credit: Zhao et al/NASA/SDO/IRIS/BBSO)
• April 19, 2016: Solar flares are intense bursts of light from the sun. They are created when complicated magnetic fields suddenly and explosively rearrange themselves, converting magnetic energy into light through a process called magnetic reconnection – at least, that's the theory, because the signatures of this process are hard to detect. But during a December 2013 solar flare, three solar observatories captured the most comprehensive observations of an electromagnetic phenomenon called a current sheet, strengthening the evidence that this understanding of solar flares is correct. 36)
- These eruptions on the sun eject radiation in all directions. The strongest solar flares can impact the ionized part of Earth's atmosphere – the ionosphere – and interfere with our communications systems, like radio and GPS, and also disrupt onboard satellite electronics. Additionally, high-energy particles – including electrons, protons and heavier ions – are accelerated by solar flares.
- Unlike other space weather events, solar flares travel at the speed of light, meaning we get no warning that they're coming. So scientists want to pin down the processes that create solar flares – and even some day predict them before our communications can be interrupted.
- "The existence of a current sheet is crucial in all our models of solar flares," said James McAteer, an astrophysicist at New Mexico State University in Las Cruces and an author of a study on the December 2013 event, published on April 19, 2016, in the Astrophysical Journal Letters. "So these observations make us much more comfortable that our models are good."
- And better models lead to better forecasting, said Michael Kirk, a space scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the study. "These complementary observations allowed unprecedented measurements of magnetic reconnection in three dimensions," Kirk said. "This will help refine how we model and predict the evolution of solar flares."
Looking at Current Sheets:
- A current sheet is a very fast, very flat flow of electrically-charged material, defined in part by its extreme thinness compared to its length and width. Current sheets form when two oppositely-aligned magnetic fields come in close contact, creating very high magnetic pressure. Electric current flowing through this high-pressure area is squeezed, compressing it down to a very fast and thin sheet. It's a bit like putting your thumb over the opening of a water hose – the water, or, in this case, the electrical current, is forced out of a tiny opening much, much faster. This configuration of magnetic fields is unstable, meaning that the same conditions that create current sheets are also ripe for magnetic reconnection.
- "Magnetic reconnection happens at the interface of oppositely-aligned magnetic fields," said Chunming Zhu, a space scientist at New Mexico State University and lead author on the study. "The magnetic fields break and reconnect, leading to a transformation of the magnetic energy into heat and light, producing a solar flare."
- Because current sheets are so closely associated with magnetic reconnection, observing a current sheet in such detail backs up the idea that magnetic reconnection is the force behind solar flares. "You have to be watching at the right time, at the right angle, with the right instruments to see a current sheet," said McAteer. "It's hard to get all those ducks in a row."
- This isn't the first time scientists have observed a current sheet during a solar flare, but this study is unique in that several measurements of the current sheet – such as speed, temperature, density and size – were observed from more than one angle or derived from more than method.
- The multi-faceted view of the December 2013 flare was made possible by three solar-watching missions: NASA's SDO (Solar Dynamics Observatory) and STEREO (Solar and Terrestrial Relations Observatory) and Hinode (Solar-B), a collaboration between JAXA, NASA, UKSA and ESA.
- Even when scientists think they've spotted something that might be a current sheet in solar data, they can't be certain without ticking off a long list of attributes. Since this current sheet was so well-observed, the team was able to confirm that its temperature, density, and size over the course of the event were consistent with a current sheet.
Figure 34: During a December 2013 solar flare, three NASA missions observed a current sheet form – a strong clue for explaining what initiates the flares. This animation shows four views of the flare from NASA's Solar Dynamics Observatory, NASA's Solar and Terrestrial Relations Observatory, and JAXA/NASA's Hinode, allowing scientists to make unprecedented measurements of its characteristics. The current sheet is a long, thin structure, especially visible in the views on the left. Those two animations depict light emitted by material with higher temperatures, so they better show the extremely hot current sheet (image credit: NASA/JAXA/SDO/STEREO/Hinode, courtesy Zhu, et al.)
• Nov. 18, 2015: An international team of researchers, led by Queen's University Belfast (Northern Ireland), has devised a high-precision method of examining magnetic fields in the Sun's atmosphere, representing a significant leap forward in the investigation of solar flares and potentially catastrophic 'space weather'. 37)
- The technique pioneered by the Queen's-led team, published in the journal Nature Physics, will feed into the DKIST (Daniel K Inouye Solar Telescope) project (which will be the largest solar telescope in the world when construction is finished in 2019 on the Pacific island of Maui), as well as allowing greater advance warning of potentially devastating space storms. The new technique allows changes in the Sun's magnetic fields, which drive the initiation of solar flares, to be monitored up to ten times faster than previous methods. 38)
- The Queen's-led team, who span universities in Europe, the Asia-Pacific region and the USA, harnessed data from both NASA's premier space-based telescope, SDO (Solar Dynamics Observatory), and the ROSA (Rapid Oscillations in the Solar Atmosphere) multi-camera system (ground-based), which was designed at Queen's University Belfast, using detectors made by Northern Ireland company Andor Technology.
- Lead researcher David Jess from Queen's Astrophysics Research Center said: "Continual outbursts from our Sun, in the form of solar flares and associated space weather, represent the potentially destructive nature of our nearest star. Our new techniques demonstrate a novel way of probing the Sun's outermost magnetic fields, providing scientists worldwide with a new approach to examine, and ultimately understand, the precursors responsible for destructive space weather.
Specific research results include:
1) The datasets used provided unprecedented images of all layers of the Sun's tenuous atmosphere, allowing the team to piece the jigsaw puzzle together of how magnetic fields permeate the dynamic atmosphere. Images captured by NASA's SDO and STEREO spacecrafts provided million-degree vantage points of how these magnetic fields stretch far out into the Sun's corona (the region of the Sun's atmosphere visible during total solar eclipses).
2) Waves propagated along magnetic fields, similar to how sound waves travel through the air on Earth. The speed at which these waves can travel is governed by the characteristics of the Sun's atmosphere, including its temperature and the strength of its magnetic field. The waves were found to propagate with speeds approaching half a million (500,000) mph, and when coupled with temperatures of around 1,000,000 degrees in the Sun's outer atmosphere, the researchers were able to determine the magnetic field strengths to a high degree of precision.
3) The strength of the magnetic fields decreases by a factor of 100 as they travel from the surface of the Sun out into the tenuous, hot corona. While the magnetic fields have decreased in strength, they still possess immense energy that can twist and shear, ultimately releasing huge blasts towards Earth in the form of solar flares. The team's methods provide a much faster way of examining magnetic field changes in the lead up to solar flares, which can ultimately be used to provide advanced warning against such violent space weather.
Figure 35: Left: An image of our sun taken by NASA's SDO, showing million degree plasma being channeled into loop-like shapes by the immense magnetic fields. Right: A zoom-in of the highly magnetic region of the sun's corona studied by David Jess and colleagues from Queen's University Belfast, Northern Ireland (image credit: Queen's University Belfast)
• On Sept. 13, 2015, as NASA's SDO kept up its constant watch on the sun, its view was photobombed not once, but twice (Figure 36). Just as the moon came into SDO's field of view on a path to cross the sun, Earth entered the picture, blocking SDO's view completely. When SDO's view of the sun emerged from Earth's shadow, the moon was just completing its journey across the sun's face. 39)
- Though SDO sees dozens of Earth eclipses and several lunar transits each year, this is the first time ever that the two have coincided. This alignment of the sun, moon and Earth also resulted in a partial solar eclipse on Sept. 13, visible only from parts of Africa and Antarctica.
- SDO's orbit usually gives us unobstructed views of the sun, but Earth's revolution around the sun means that SDO's orbit passes behind Earth twice each year, for two to three weeks at a time. During these phases, Earth blocks SDO's view of the sun for anywhere from a few minutes to over an hour once each day.
Figure 36: SDO captured this image of Earth and the moon transiting the sun together on Sept. 13, 2015. The edge of Earth, visible near the top of the frame, appears fuzzy because Earth's atmosphere blocks different amounts of light at different altitudes. On the left, the moon's edge is perfectly crisp, because it has no atmosphere. This image was taken in extreme ultraviolet wavelengths of 171Å. Though this light is invisible to our eyes, it is typically colorized in gold (image credit: NASA, SDO)
• August 31, 2015: As navigators know, Earth's magnetic axis is tilted with respect to its rotation axis. Such misalignment had not been expected in the Sun — but now it's been seen. NASA's SDO (Solar Dynamics Observatory) has been trained on the Sun for half of the current 11-year cycle of solar activity. Using SDO's HMI (Helioseismic and Magnetic Imager), Adur Pastor Yabar of the Institute of Astrophysics of the Canary Islands and his colleagues created daily maps of the line-of-sight strength and polarity of the Sun's magnetic field for each of the mission's first 1700 days. The Sun's rotation period varies with latitude. It's 25.5 days at the equator and 34.4 days at the poles. 40) 41)
- To look for variations not associated with differential rotation, Pastor Yabar and his colleagues averaged each map over all longitudes in 1º-wide latitudinal belts. When the researchers Fourier-transformed the entire sequence of binned maps, they discovered that a more-or-less monthly oscillation showed up at every latitude on every day. Random sprouting of active regions that rotate in and out of view could conceivably account for the oscillation, but when the researchers excluded active regions, the oscillation persisted. Their latitudinal belts did not sample the same parts of the Sun, as they had assumed, but wobbled up and down over the solar disk—hence the oscillation. Based on that and other lines of evidence, Pastor Yabar and his colleagues concluded that the Sun's magnetic and rotational axes must be misaligned. Dynamo models that presume alignment will require modification.
• July 2015: The SDO (Solar Dynamics Observatory) mission is approved by NASA for its first extended mission. NASA will work with the mission to address the findings that were raised by the Senior Review panel, to allow the mission to continue its successful operations. The SDO mission will be invited to the 2017 Heliophysics Senior Review. 42)
• June 2015: The SDO instruments provide a combination of imaging and spectral data that sample the Sun from its interior to the outer corona. These instruments provide a cadence and uniform quality of data that well characterize the photospheric magnetic fields (both longitudinal and vector), represent the thermal structure in the chromosphere and corona, allow the probing of different depths in the solar convection zone and summarize the broad UV spectral irradiance changes due to both long-term and very short-term solar activity variation. High data quality and uniformity combined with physically-appropriate time cadence and full disk coverage make the SDO data indispensable for studies of large-scale connectivity in the solar atmosphere. The general availability of many SDO data products with minimal latency allows for complementary usage in the context of space weather situational awareness and forecasting efforts. Some SDO data will also be used to develop important synergy between solar and stellar astrophysics. The science impact from this complex mission has been major. Recent science results cited from the SDO prime mission phase include breakthroughs in coronal seismology, developing a better understanding of evolution of magnetic fields in solar upper atmosphere decoding the importance of helicity and global connectivity to the eruption of magnetic systems, and establishing the hierarchy of organized flows in the convection zone. 43)
- SDO spacecraft / instrument health and status: With one exception, all instruments are in a good health and expected continue taking a good quality of data. EVE/MEGS-A has failed, and there are no expectations for this instrument to provide further solar irradiance data in the 5–38 nm band. HMI (Helioseismic Magnetic Imager) reboots of the flight software and anomalous upsets have not affected the science data significantly due to their low occurrence rate. All SDO spacecraft subsystems are operating nominally with two exceptions, neither of which interferes with SDO's ability to continue normal operations for at least five more years.
- SDO is a valuable asset to the HSO (Heliophysics System Observatory). It provides a breadth of observational information that will enable cutting-edge investigations; the data are presently used by many NASA Heliophysics missions, as well as for near-real-time space-weather forecasting by US and international agencies.
• May 6, 2015: The sun emitted a significant solar flare, an X2.7 class flare, peaking at 22:11 UTC on May 5, 2015. NASA's SDO (Solar Dynamics Observatory), which watches the sun constantly, captured an image of the event. This flare was also accompanied by a coronal mass ejection that blasted a mass of plasma into space. Both are triggered by disruptions of areas of intense magnetic fields. Solar flares are powerful bursts of radiation. Harmful radiation from a flare cannot pass through Earth's atmosphere to physically affect humans on the ground, however — when intense enough — they can disturb the atmosphere in the layer where GPS and communications signals travel. 44)
Figure 37: NASA's Solar Dynamics Observatory captured these images of a solar flare – as seen in the bright flash on the left – on May 5, 2015 (image credit: NASA, SDO, Wiesinger)
Legend to Figure 37: Each image shows a different wavelength of extreme ultraviolet light that highlights a different temperature of material on the sun. By comparing different images, scientists can better understand the movement of solar matter and energy during a flare. From left to right, the wavelengths are: visible light, 171 Å, 304 Å, 193 Å and 131 Å. Each wavelength has been colorized.
• In early May 2015, the SDO mission is providing 5 years of science data to the scientists and the public. The project has been watching Solar Cycle 24 rise to solar maximum, storing about 7 PByte of data, releasing almost 200 million images, and having about 1900 scientific papers published. 45)
• On March 11, 2015, an X2-class fare erupted. The bright flash of the flare was followed by streams of dark ejecta that move across the sun to the left. The flare did cause some radio blackouts on Earth when it disturbed our ionosphere. The flare was also associated with a coronal mass ejection. The images were taken in extreme ultraviolet light. 46)
Figure 38: A strong flare X2-class flare erupted on March 11, 2015 into space from an active region that was roughly facing towards Earth (image credit: NASA)
• February 11, 2015 marked the 5th anniversary of SDO on orbit. The mission is providing very detailed images of the Earth-facing side of the sun 24 hours a day. NASA released two videos on this occasion (links in Ref. 47) showcasing highlights from the last five years of sun watching. The first is a time lapse of the past five years. Different colors represent different wavelengths of extreme ultraviolet light, ultraviolet light, and visible light, which in turn correspond to solar material at different temperatures. Additionally SDO returns solar magnetic field data that helps scientists study solar activity. 47)
- The second video showcases highlights from the last five years. Watch the movie to see giant clouds of solar material hurled out into space, the dance of giant loops hovering in the corona, and huge sunspots growing and shrinking on the sun's surface.
- Five years into its mission, SDO continues to send back tantalizing imagery to incite scientists' curiosity. For example, in late 2014, SDO captured imagery of the largest sun spots seen since 1995 as well as a torrent of intense solar flares. Solar flares are bursts of light, energy and X-rays. They can occur by themselves or can be accompanied by what's called a coronal mass ejection, or CME, in which a giant cloud of solar material erupts off the sun, achieves escape velocity and heads off into space. In this case, the sun produced only flares and no CMEs, which, while not unheard of, is somewhat unusual for flares of that size. Scientists are looking at that data now to see if they can determine what circumstances might have led to flares eruptions alone.
• On January 19, 2015, the AIA (Atmospheric Imaging Assembly) instrument on SDO captured its 100 millionth image of the sun. AIA uses four telescopes working in parallel to gather eight images of the sun – cycling through 10 different wavelengths — every 12 seconds. The SDO downloads ~1.5 TB of data per day. AIA is responsible for about half of that. Every day it provides 57,600 detailed images of the sun that show the dance of how solar material sways and sometimes erupts in the solar atmosphere, the corona. 48)
Figure 39: The 100 millionth image of the sun acquired by AIA on Jan. 19, 2015 (image credit: NASA)
Legend to Figure 39: The dark areas at the bottom and the top of the image are coronal holes — areas of less dense gas, where solar material has flowed away from the sun.
• June 10, 2014: The sun emitted a significant solar flare, peaking at 11:42 UTC on June 10, 2014 (X 2.2 flare). NASA's SDO (Solar Dynamics Observatory) – which typically observes the entire sun 24 hours a day - captured images of the flare. The sun released a second X-class flare (X 1.5), peaking at 12:52 UTC. 49) On June 11, 2014, the active region (AR2087) blazed again with a third flare at 9:05 UTC (X 1.0 class flare).
Figure 40: A solar flare bursts off the left limb of the sun in this image captured by SDO on June 10, 2014, at 11.42 UTC. This is classified as an X 2.2 flare, shown in a blend of two wavelengths of light: 171 and 131 Ä, colorized in gold and red, respectively (image credit: NASA, SDO)
• April 2014: New research that uses data from SDO, to track bright points in the solar atmosphere and magnetic signatures on the sun's surface, offers a way to probe the star's depths faster than ever before. The technique opens the door for near real-time mapping of the sun's roiling interior – movement that affects a wide range of events on the sun from its 22-year sunspot cycle to its frequent bursts of X-ray radiation called solar flares. 50) 51)
Figure 41: Brightpoints in the sun's atmosphere, left, correspond to magnetic parcels on the sun's surface, seen in the processed data on the right. Green spots show smaller parcels, red and yellow much bigger ones. Images based on data from NASA's SDO captured at 15:00 GMT on May 15, 2010 (image credit: NASA)
- One of the most common ways to probe the sun's interior is through a technique called helioseismology in which scientists track the time it takes for waves – not unlike seismic waves on Earth — to travel from one side of the sun to the other. From helioseismology solar scientists have some sense of what's happening inside the sun, which they believe to be made up of granules and super-granules of moving solar material. The material is constantly overturning like boiling water in a pot, but on a much grander scale: A granule is approximately the distance from Los Angeles to New York City; a super-granule is about twice the diameter of Earth.
- Instead of tracking seismic waves, the new research probes the solar interior using the HMI (Helioseismic Magnetic Imager) on SDO, which can map the dynamic magnetic fields that thread through and around the sun. Since 2010, Scott McIntosh has tracked the size of different magnetically-balanced areas on the sun, that is, areas where there are an even number of magnetic fields pointing down in toward the sun as pointing out. Think of it like looking down at a city from above with a technology that observed people, but not walls, and recording areas that have an even number of men and women. Even without seeing the buildings, you'd naturally get a sense for the size of rooms, houses, buildings, and whole city blocks – the structures in which people naturally group.
- The team found that the magnetic parcels they mapped corresponded to the size of granules and supergranules, but they also spotted areas much larger than those previously noted — about the diameter of Jupiter. It's as if when searching for those pairs of men and women, one suddenly realized that the city itself and the sprawling suburbs was another scale worth paying attention to. The scientists believe these areas correlate to even larger cells of flowing material inside the sun.
- The researchers also looked at these regions in SDO imagery of the sun's atmosphere, the corona, using the AIA (Atmospheric Imaging Assembly) instrument. They noticed that ubiquitous spots of extreme ultraviolet and X-ray light, known as brightpoints, prefer to hover around the vertices of these large areas, dubbed g-nodes.
- By opening up a way to peer inside the sun quickly, these techniques could provide a straightforward way to map the sun's interior and perhaps even improve our ability to forecast changes in magnetic fields that can lead to solar eruptions.
• On March 29, 2014, an X-class flare (CME) erupted from the right side of the sun... and vaulted into history as the best-observed flare of all time. The flare was witnessed by four different NASA spacecraft and one ground-based observatory — three of which had been fortuitously focused in on the correct spot as programmed into their viewing schedule a full day in advance. 52) 53)
Figure 42: This combined image shows the March 29, 2014, X-class flare as seen through the eyes of different observatories. SDO is on the bottom/left, which helps show the position of the flare on the sun. The darker orange square is IRIS data. The red rectangular inset is from Sacramento Peak. The violet spots show the flare's footpoints from RHESSI (image credit: NASA/GSFC)
The telescopes involved were:
- NASA's IRIS (Interface Region Imaging Spectrograph)
- NASA's SDO (Solar Dynamics Observatory)
- NASA's RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager)
- JAXA's Solar-B/Hinode Observatory
- National Solar Observatory's Dunn Solar Telescope located at Sacramento Peak in New Mexico.
Numerous other spacecraft provided additional data about what was happening on the sun during the event and what the effects were at Earth.
- NASA's STEREO mission and the SOHO mission of ESA and NASA both watched the great cloud of solar material that erupted off the sun with the flare.
- NOAA's GOES satellite tracked X-rays from the flare, and other spacecraft measured the effects of the flare as it came toward Earth.
This event was particularly exciting for the IRIS team, as this was the first X-class flare ever observed by IRIS (launched on June 28, 2013). IRIS provided scientists with the first detailed view of what happens in this region during a flare.
Figure 43: A closeup of the sunspot at the root of the March 29, 2014, X-class flare taken by the HMI instrument of SDO (image credit: NASA)
Figure 44: EUV radiation streams out of an X-class solar flare as seen in this image captured on March 29, 2014 by the AIA instrument of SDO (image credit: NASA, Ref. 53)
• On January 30, 2014, the SDO got its own private solar eclipse showing from its geosynchronous orbital perch. Twice a year during new phase, the moon glides in front of the sun from the observatory's perspective. The events are called lunar transits rather than eclipses since they're seen from outer space. Transits typically last about a half hour, but at 2.5 hours, today's was one of the longest ever recorded. The next one occurs on July 26, 2014. 54)
Figure 45: A lunar transit across the sun as seen by the SDO in six different color-coded wavelengths on January 30, 2014 (image credit: NASA, Universe Today)
Legend to Figure 45: The times of each photo are given in CST (Central Standard Time). In the last frame, the moon is silhouetted against the solar corona. At maximum about 90% of the sun was covered.
• January 07, 2014: An enormous sunspot, labeled AR1944 (Active Region 1944), slipped into view over the sun's left horizon late on Jan. 1, 2014. The sunspot steadily moved toward the right, along with the rotation of the sun, and now sits almost dead center, as seen in the image (Figure 46) from NASA's Solar Dynamics Observatory.
Sunspots are dark areas on the sun's surface that contain complex arrangements of strong magnetic fields that are constantly shifting. The largest dark spot in this configuration is approximately two Earths wide, and the entire sunspot group is some seven Earths across. 55)
Sunspots are part of what's known as active regions, which also include regions of the sun's atmosphere, the corona, hovering above the sunspots. Active regions can be the source of some of the sun's great explosions: solar flares that send out giant bursts of light and radiation due to the release of magnetic energy, or coronal mass ejections that send huge clouds of solar material out into space. As the sunspot group continues its journey across the face of the sun, scientists will watch how it changes and evolves to learn more about how these convoluted magnetic fields can cause space weather events that can affect spaceborne systems and technological infrastructure on Earth.
- On January 7, 2014 (18:18:34 UTC), as the giant AR1944 sunspot turned toward Earth, it erupted with a powerful X1.2-class flare.
Figure 46: One of the largest sunspots in the last nine years, labeled AR1944, was seen in early January 2014, as captured by NASA's SDO. An image of Earth has been added for scale (image credit: NASA)
• Nov. 9, 2013: The Sun is currently acting like it's in solar maximum. Our Sun has emitted dozens of solar flares in since Oct. 23, 2013, with at least six big X-class flares.
Figure 47: The AIA instrument of SDO captured this image of the sun showing an X1.1 class flare on Nov. 8, 2013 (image credit: NASA)
• On October 21, 2013, NASA successfully launched a Black Brant IX sounding rocket at 10:00 hours UTC from the White Sands Missile Range, N.M., carrying instrumentation to support the calibration of the EVE (EUV Variability Experiment) aboard SDO. EVE measures the total extreme ultraviolet output of the sun, called its irradiance. 56)
As part of the planned SDO/EVE program, the rocket calibration flight occurs about once a year to accurately determine the long-term variations of the solar extreme ultraviolet irradiance. This kind of calibration is known as an under-flight. It uses a near-replica of the SDO/EVE instrument to gather a calibrated sounding rocket observation in coordination with the orbital satellite's observations.
Comparison of the two data sets then validates the accuracy of the SDO/EVE data, providing crucial calibration of any long-term changes in the orbital instrumentation. This was the fourth under-flight calibration for the EVE instrument. The previous flight was successfully conducted on June 23, 2012.
• May 15, 2013: According to "SpaceWeather.com," AR1748 (sunspot active region 1748) has produced "the strongest flares of the year so far, and they signal a significant increase in solar activity." In only two days, sunspot AR1748 has produced four X-flares. The latest X-flare from this active sunspot occured on May 15th at 0152 UT. NASA's SDO (Solar Dynamics Observatory) captured the extreme ultraviolet flash. 57)
The AR1748 has produced an X1.7-class flare (0217 UT on May 13), an X2.8-class flare (1609 UT on May 13), an X3.2-class flare (0117 UT on May 14), and an X1-class flare (0152 on May 15). These are the strongest flares of the year, and they signal a significant increase in solar activity. 58)
Figure 48: An X3.2-class flare observed by SDO's AIA instrument at 0114 UT, 14 May 2013 (NASA/SDO/AIA)
Legend to Figure 49: SID events make themselves known by the effect they have on low-frequency radio signals. When a SID passes by, the atmosphere overhead becomes a good reflector for radio waves, allowing signals to be received from distant transmitters. Battaiola monitored a faraway 21.75 kHz radio station to receive the SIDs over his location.
• On April 3, 2013 the SDO project performed an HMI roll maneuver. The entire SDO spacecraft is spun around so that HMI can verify its operation and measure some calibration data. 59)
- Twice a year, SDO performs a 360º roll maneuver about the axis on which it points toward the Sun. This produces some unique views; the rolls are necessary to help calibrate the instruments, particularly the HMI instrument, which is making precise measurements of the solar limb to study the shape of the Sun. The rolls also help the science teams to know how accurately the images are aligned with solar north. 60)
• The SDO mission and its payload are operating nominally in 2013. On Feb. 11, 2013, SDO was 3 years on orbit providing an enornous amount of information. 61)
Legend to Figure 50: These six images from SDO, chosen to show a representative image about every six months, track the rising level of solar activity since the mission first began to produce consistent images in May, 2010. The period of solar maximum is expected in 2013. The images were taken in the 171 Angstrom wavelength of extreme ultraviolet light.
• On April 4, 2012, the SDO spacecraft performed a 360º spin. It rolled completely about its axis– something it does twice a year . This maneuver helps the HMI (Helioseismic and Magnetic Imager ) instrument, one of three instruments onboard SDO, take measurements of the solar limb to study the shape of the sun. The roll helps scientists remove optical distortions from the images and to precisely determine the boundaries of the sun's horizon, or "limb". Accumulated over time, such data shows whether the sun's sphere changes in concert with the 11-year solar cycle, during which the sun moves through periods of greater and lesser activity as evidenced by the changing frequency of giant solar eruptions. 63)
• In 2012, the spacecraft and its payload are operating nominally. On Feb. 11, 2012, the SDO spacecraft was 2 years on orbit.
The SDO mission serves as a clear example of the importance of the systems engineering role across all phases of the mission development lifecycle and its contribution to mission success. The best metric to evaluate the success of this approach is in the successful launch and the on-orbit performance of the SDO mission itself, which, despite challenging technical drivers and development obstacles along way, is currently enabling ground-breaking science after only two years into its five-year mission life (Ref. 7).
Figure 51: AIA instrument image observed at 171 Ä showing the current conditions of the quiet corona and upper transition region of the Sun (image credit: NASA) 64)
• Sept. 2011 (the late phase of solar flares): Over the course of a year, the science team used the EVE (Extreme ultraviolet Variability Experiment) instrument on SDO to record data from many flares. EVE doesn't snap conventional images. T. Woods is the principal investigator for the EVE instrument and he explains that it collects all the light from the sun at once and then precisely separates each wavelength of light and measures its intensity. This doesn't produce pretty pictures the way other instruments on SDO do, but it provides graphs that map out how each wavelength of light gets stronger, peaks, and diminishes over time. EVE collects this data every 10 seconds, a rate guaranteed to provide brand new information about how the sun changes, given that previous instruments only measured such information every hour and a half or didn't look at all the wavelengths simultaneously – not nearly enough information to get a complete picture of the heating and cooling of the flare.
Recording extreme ultraviolet light, the EVE spectra showed four phases in an average flare's lifetime (Figure 52). The first three have been observed and are well established (though EVE was able to measure and quantify them over a wide range of light wavelengths better than has ever been done).
- The first phase is the hard X-ray impulsive phase, in which highly energetic particles in the sun's atmosphere rain down toward the sun's surface after an explosive event in the atmosphere known as magnetic reconnection. They fall freely for some seconds to minutes until they hit the denser lower atmosphere, and then the second phase, the gradual phase, begins.
- Second phase: Over the course of minutes to hours, the solar material, called plasma, is heated and explodes back up, tracing its way along giant magnetic loops, filling the loops with plasma. This process sends off so much light and radiation that it can be compared to millions of hydrogen bombs.
- The third phase is characterized by the sun's atmosphere — the corona — losing brightness, and so is known as the coronal dimming phase. This is often associated with what's known as a CME (Coronal Mass Ejection), in which a great cloud of plasma erupts off the surface of the sun.
But the fourth phase, the late phase flare, spotted by EVE was new. Anywhere from one to five hours later for several of the flares, they saw a second peak of warm coronal material that didn't correspond to another X-ray burst. The late phase turns out to be different, the emissions happen substantially later, and it happens after the main flare exhibits that initial peak. 65) 66)
To try to understand what was happening, the team looked at the images collected from SDO's AIA (Advanced Imaging Assembly) as well. They could see the main phase flare eruption in the images and also noticed a second set of coronal loops far above the original flare site. These extra loops were longer and become brighter later than the original set (or the post-flare loops that appeared just minutes after that). These loops were also physically set apart from those earlier ones.
Figure 52: Graph of the EVE spectra showing the total intensity of any given EUV wavelength of light coming off of the sun (image credit: NASA)
Legend of Figure 52: This image shows a single moment from May 5, 2010. Instead of a conventional picture, the EVE produces graphs like this, called spectra, that show the total intensity of any given EUV wavelength of light coming off of the sun.
The intensity, the project recorded in those late phase flares, is usually dimmer than the X-ray intensity. But the late phase goes on much longer, sometimes for multiple hours, so it's putting out just as much total energy as the main flare that typically only lasts for a few minutes. Because this previously unrealized extra source of energy from the flare is equally important to impacting Earth's atmosphere, Woods and his colleagues are now studying how the late phase flares can influence space weather (Ref. 65).
• In 2011, the spacecraft and its payload are operating nominally.
Figure 53: SDO image of the sun taken on Jan. 10, 2011 with the AIA instrument in the EUV range (image credit: NASA)
Legend to Figure 53: The image captures a dark coronal hole just about at sun center. Coronal holes are areas of the sun's surface that are the source of open magnetic field lines that head way out into space. They are also the source regions of the fast solar wind, which is characterized by a relatively steady speed of approximately 800 km/s. As the sun continues to rotate, the high speed solar wind particles blowing from this hole will likely reach Earth in a few days and may spark some auroral activity. 67)
• On Aug.1, 2010, a most unusual solar event occurred. Nearly the entire Earth-facing side of the Sun erupted in a tumult of activity, comprising a large solar flare, a solar tsunami, multiple filaments of magnetism lifting off the solar surface, radio bursts and half a dozen coronal mass ejections (CMEs). At the same time, NASA's three solar spacecraft, SDO and the two STEREO spacecraft, were ideally positioned to capture both the action on the Earth-facing side of the Sun, and most activity around the backside, leaving a wedge of only 30 degrees of the solar surface unobserved. 68) 69)
Explosions on the sun are not localized or isolated events, according to Karel Schrijver and Alan Title of LMSAL (Lockheed Martin, Solar & Astrophysics Laboratory). Instead, solar activity is interconnected by magnetism over breathtaking distances. Solar flares, tsunamis, coronal mass ejections--they can go off all at once, hundreds of thousands of miles apart, in a dizzyingly-complex concert of mayhem.
For several decades, scientists studying the sun have observed solar flares that appear to occur almost simultaneously but originated in completely different areas on the Sun. Solar physicists called them "sympathetic" flares, but it was thought these near-synchronous explosions in the solar atmosphere were too far apart – sometimes millions of kilometers distant – to be related. But now, with the continuous high-resolution and multi-wavelength observations with the SDO, combined with views from the twin STEREO spacecraft, the scientists are seeing how these sympathetic eruptions — sometimes on opposite sides of the sun — can connect through looping lines of the Sun's magnetic field. 70)
Figure 54: Locations of key events are labeled in this extreme ultraviolet image of the sun, obtained by the Solar Dynamics Observatory during the Great Eruption of Aug. 1, 2010. White lines trace the sun's magnetic field (image credit: Karel Schrijver and Alan Title of LMSAL)
• On May 14, 2010, SDO passed a major milestone when it completed its post-launch check out (end of commissioning phase) and officially began its five-year science mission to study the sun (phase E). The project at NASA/GSFC declared SDO an operational mission. All of the instruments and the spacecraft are performing extremely well. SDO is now sending 1.5 TB of data/day to Earth, and will continue to do so at least until the end of the prime phase of the mission in 2015. 71) 72)
The SDO has allowed scientists for the first time to comprehensively view the dynamic nature of storms on the sun. Solar storms have been recognized as a cause of technological problems on Earth since the invention of the telegraph in the 19th century. 73)
• On April 19, 2010, SDO observed a massive eruption on the sun — one of the biggest in years. Astronomers have seen eruptions like this before, but rarely so large and never in such fluid detail. Coronal rain has long been a mystery. It's not surprising that plasma should fall back to the sun. After all, the sun's gravity is powerful. The puzzle of coronal rain is how slowly it seems to fall. The rain appears to be buoyed by a 'cushion' of hot gas.
Using the AIA (Atmospheric Imaging Assembly) instrument with an array of ultraviolet telescopes, SDO can remotely measure the temperature of gas in the sun's atmosphere. Coronal rain turns out to be relatively cool—"only" 60,000 K. When the rains falls, it is supported, in part, by an underlying cushion of much hotter material, between 1,000,000 and 2,200,000 K. 74)
Figure 55: Coronal rain. Encircled are two plasma streamers, one hitting the sun's surface and another incoming behind it (image credit: NASA)
Figure 56: A full-disk multiwavelength extreme ultraviolet image of the sun taken by the AIA instrument on March 30, 2010 (image credit: NASA)
Legend to Figure 56: False colors trace different gas temperatures. Reds are relatively cool (~60,000 K); blues and greens are hotter (> 1,000,000 K). SDO is able to monitor not just one small patch of sun, but rather the whole thing--full disk, atmosphere, surface, and even interior. 75)
Figure 57: An erupting prominence observed by the AIA instrument on March 30, 2010 (image credit: NASA)
• Following several precise propulsion burns to circularize its orbit, SDO arrived "on station" on March 16, 2010. All systems are operating nominally.
Sensor complement: (HMI, AIA, EVE)
The SDO sensor complement consists of three instruments which are pointed toward the sun to provide continuous, high cadence (cyclic) observations of the full solar disk and coronal imaging in multiple wavelengths to improve the understanding and forecasting of the sun's impact on our terrestrial environment. 76) 77)
• HMI (Helioseismic and Magnetic Imager) measures the surface magnetic fields and the flows that distribute it on global and local solar scales. A study of the origins of solar variability using solar oscillations and the longitudinal photospheric magnetic field to characterize and understand the sun's interior and the various components of magnetic activity.
• AIA (Atmospheric Imaging Assembly) images the solar outer atmosphere. A study of coronal energy storage and release evidenced in rapidly evolving coronal structures over a broad temperature range that are intrinsically tied to the Sun's magnetic field and irradiance variations.
• EVE (EUV Variability Experiment), a spectrometer/spectrograph providing the solar full-disk distribution of the spectral irradiance in the EUV and UV ranges that cause variations in composition, density, and temperature of the Earth's ionosphere and thermosphere. A study of the sun's transient and steady state coronal plasma emissions that are driven by variations in the solar magnetic field.
HMI (Helioseismic and Magnetic Imager)
The HMI instrument is being developed at LMSAL (Lockheed Martin, Solar & Astrophysics Laboratory) in Palo Alto, CA (PI: P. Scherrer of Stanford University). HMI is a joint project of the Stanford University, Hansen Experimental Physics Laboratory, and LMSAL, with key contributions from the High Altitude Observatory of NCAR, and the HMI Science Team. The overall objective of HMI is to extend the capabilities of the SOHO/MDI (Michelson Doppler Imager) instrument with continuous full-disk coverage at considerably higher spatial and temporal resolution line-of-sight magnetograms with the optional channel for full Stokes polarization measurements [I = (I; Q; U; V)] and hence vector magnetogram determination (3-D imagery of the sun's interior employing a technique known as helioseismology, which maps the inside of the sun by measuring the velocity of low-frequency sound waves that ricochet below its surface). 78) 79) 80) 81) 82)
Note: Since the two instruments, HMI and AIA, are both being developed at LMSAL, there is a lot of organizational synergism and cooperation between the two instruments on all levels.
HMI makes interference measurements of the motion of the solar photosphere to study solar oscillations and measurements of the polarization in a spectral line to study all three components of the photospheric magnetic field. HMI produces data to determine the interior sources and mechanisms of solar variability and how the physical processes inside the sun are related to surface magnetic field and activity. It also produces data to enable estimates of the coronal magnetic field for studies of variability in the extended solar atmosphere. HMI observations will enable establishing the relationships between the internal dynamics and magnetic activity in order to understand solar variability and its effects, leading to reliable predictive capability, one of the key elements of the LWS (Living With a Star) program.
The HMI observation goals are being addressed in a coordinated investigation in a number of parallel studies:
• Convection-zone dynamics and the solar dynamo
• Origin and evolution of sunspots, active regions and complexes of activity
• Sources and drivers of solar activity and disturbances
• Links between the internal processes and dynamics of the corona and heliosphere
• Precursors of solar disturbances for space-weather forecasts.
HMI will observe the full solar disk in the Fe I absorption line at 6173 Å (goal of 1 arcsecond resolution). The HMI instrument will produce measurements in the form of filtergrams in a set of polarizations and spectral line positions at a regular cadence for the duration of the mission that meet these basic requirements:
4) Full-disk Doppler velocity and line-of-sight magnetic flux images with 1.5 arcsec resolution at least every 50 seconds
5) Full-disk vector magnetic images of the solar magnetic field with 1.5 arc-sec resolution at least every 10 minutes.
The primary observables (Dopplergrams, longitudinal and vector magnetograms, and continuum intensity images) will be constructed from the raw filtergrams and will be made available at full resolution and cadence. Other derived products such as subsurface flow maps, far-side activity maps, and coronal and solar wind models that require longer sequences of observations shall be produced and made available.
In effect the solar turbulence is analogous to earthquakes. In manner similar to how seismologists can learn about the interior of the Earth by studying the waves generated in an earthquake. HMI's helioseismologists learn about the structure, temperature and flows in the solar interior.
The HMI instrument consists of a refracting telescope, a polarization selector, an image stabilization system (ISS), a narrow-band tunable filter. In addition, there are two 4096 x 4096 pixel CCD cameras with mechanical shutters and control electronics. The twin cameras of HMI operate independently. One is referred to as the "Doppler camera"; the objective is to measure the line-of-sight component of the magnetic field and velocity vectors. The second camera is referred to as "Magnetic camera"; the objective is to measure the vector magnetic field and line of sight velocities. 83)
The optics package consists of the following elements:
- Telescope section
- Polarization selectors - 3 rotating waveplates for redundancy
- Focus blocks
- ISS (Image Stabilization System)
- 5 element Lyot filter. One element tuned by rotating waveplate
- 2 tunable Michelson interferometers. 2 waveplates and 1 polarizer for redundancy
- Reimaging optics and beam distribution system
- 2 functionally identical CCD cameras - "Doppler" and "Magnetic"
The combined Lyot-Michelson filter system in HMI produces a transmission profile with a FWHM of 76 mÅ. The tuning positions are 69 mÅ apart from each other.
Figure 58: Principal optics package components of the HMI instrument (image credit: Stanford University)
Figure 59: Photo of the HMI instrument (image credit: NASA)
Figure 60: Optical layout of the HMI instrument (image credit: Stanford University)
Table 2: Overview of HMI observation requirements
PCU (Polarization Calibration Unit):
HMI polarization calibration requires the input of fixed polarization states into the instrument and the measurement of the observed parameters with the HMI. The PCU creates the polarization states by using a linear polarizer and retarder (wave plate) that can be inserted into the optical path and rotated independently. The PCU consists of a TCP/IP control interface (Newport XPS-C4) and two mechanical units (size: 787 mm x 508 mm x 203 mm), with 175 mm clear apertures that house the polarization optics. Each mechanical unit contains a linear and a rotational stage. The linear stages (Newport IMS300CC) move the polarization optics into and out of the optical path with a linear position resolution of 1.25 microns. The rotational stages (Newport RV240CC) move the calibration optics to any given angle with a resolution of 0.001º. 84)
Figure 61: HMI accommodation on SDO (image credit: Stanford University)
Figure 62: Functional block diagram of the HMI (image credit: Stanford University)
AIA (Atmospheric Imaging Assembly):
The AIA instrument is being designed and developed at LMSAL (Lockheed Martin Solar and Astrophysics Laboratory), Palo Alto, CA; (PI: Alan Title, LMSAL). The AIA science team includes scientists and engineers from many national and international institutions. The SAO (Smithsonian Astrophysical Observatory) has a major role in the AIA program.
The objective is to provide an unprecedented view of the solar corona, taking images that span at least 1.3 solar diameters in multiple wavelengths nearly simultaneously, at a resolution of about 1 arcsec and at a cadence of 10 seconds or better. The primary goal of the AIA science investigation is to use these data, together with data from other SDO instruments, as well as from other observatories, to significantly improve our understanding of the physics behind the activity displayed by the sun's atmosphere, which drives space weather in the heliosphere and in planetary environments. 85) 86)
Themes of the AIA Investigation
1) Energy input, storage, and release: the 3-D dynamic coronal structure.
2) Coronal heating and irradiance:thermal structure and emission.
3) Transients: sources of radiation and energetic particles
4) Connections to geospace: material and magnetic field output of the sun
5) Coronal seismology: a new diagnostic to access coronal physics
Table 3: Overview of AIA observation requirements for various science themes
AIA instrument design overview:
• Four ST (Science Telescopes), each with 8 science channels
- 7 EUV channels in a sequence of Fe line and He 304 Å
- 1 UV channel with CTN, 1600 Å, 1700 Å filters
• Active secondaries for image stabilization. Each ST is equipped with an ISS (Image Stabilization System)
• Four GT (Guide Telescopes)
• Four 4096 x 4096 pixel thinned back-illuminated CCDs (the sampling of 0.6 arcsec requires a 4096 x 4096 pixel detector). Note, the AIA and HMI CCDs: a 4096 x 4096 pixel science-grade CCD with 12 µm pixel pitch developed by ev2 and RAL, are currently the largest CCD to have ever flown in a space mission (Ref. 83).
• Full CCD readout in 2.5 seconds
• Reconfiguration of all mechanisms in 1 second (filter wheels, sector shutter, focal plane shutters)
• Onboard data compression via several lookup tables
Figure 63: Photo of the CCD device (image credit: ev2, LMSAL)
The AIA design provides the following instrument capabilities:
• Seven EUV (Extreme Ultraviolet) and three UV/visible channels. Four of the EUV wavelength bands open new perspectives on the solar corona, having never been imaged or imaged only during brief rocket flights. The set of six EUV channels that observe ionized iron allow the construction of relatively narrow-band temperature maps of the solar corona from below 1 MK to above 20 MK.
• A field of view (FOV) exceeding 41 arcmin (or 1.28 solar radii in the EW and NS directions), with 0.6 arcsec pixels
• A detector full well > 150,000 electrons and ~ 15 e/photon, with a camera readout noise of ≤ 25 electrons
• A sustained 10 second cadence during most of the mission
• A capability to adjust the observing program to changing solar conditions in order to implement observing programs that are optimized to meet the requirements of specific scientific objectives. This allows, for example, a 2 second cadence in a reduced field of view for flare studies.
• Provision of images in multiple EUV and UV pass bands. The basic observables are full-sun intensities at a range of wavelengths. Together, these will comprise the data archive, which is freely accessible to the research community and, with limitations dictated by resources, to other interested parties.
Derived data products, such as coronal thermal charts, maps of variability, and comparisons to HMI magnetograms and to (non-)potential field extrapolations will be made available regularly through the data-processing pipeline for a subset of the data for use in evaluation of the data and to aid the discovery of phenomena and cataloging of events. Software will be made available to researchers to create these data products for other datasets; a core library of easy-to-use, publicly-available software will be developed as part of the SolarSoft IDL environment to enable and support the investigations that are required to meet the primary AIA science goals
Table 4: Definition of AIA instrument spectral bands
Table 5: AIA instrument design characteristics
Figure 64: Illustration of a single AIA science telescope with quad selector (image credit: LMSAL)
Figure 65: AIA science telescope assembly (image: credit: LMSAL)
Figure 66: Optical layout of the AIA science telescope (image credit: LMSAL)
Figure 67: AIA telescope array mounted on IM (Instrument Module), image credit: LMSAL
AIA camera systems:
• The camera systems with CCD detectors are key elements of HMI & AIA. The HMI and AIA instrument use identical cameras and CCDs except that the AIA CCDs are back-side thinned.
• Each CCD detector array has a size of 4096 x 4096 pixels with 12 µm pixels (they were provided by e2v technologies ltd., Chelmsford, Essex, UK)
• CEB (Camera Electronics Box): 8 Mpixel/s via 2 Mpixel/s from 4 ports simultaneously
The AIA instrument has a data rate allocation of 67 Mbit/s (max, using data compression). The data is communicated over the IEEE 1355 high-rate science data bus (SpaceWire).
Camera readout electronics: Each AIA and HMI CCD (Figure 63) is driven and read out through its own dedicated CEB (Camera Electronics Box). It has dimensions of 152 mm x 131 mm x 95 mm and a mass of 2.9 kg. The enclosure walls are 5 mm thick aluminum to ensure sufficient attenuation of space radiation over mission life. During exposures the CCD and CEB consumes 12 W rising to 17 W during readout. The CEB contains four electronics cards mounted above a separately screened input filter and DC-DC power converter. A photo of the assembled unit, minus front panel and lid, is reproduced in Figure 68 (Ref. 83).
The upper-most card carries four video processing and digitization ASICs operating in parallel at 2 Mpixel/s and each connected to one of the CCD's quadrant readout amplifiers. The second card in the stack provides all of the CCD's low-noise DC bias voltages. Supplies to each of the CCD's output amplifiers are buffered separately to minimize crosstalk between channels. An 8-channel 10-bit DAC ASIC enables software programming and fine adjustment of the bias supplies. Telemetry circuitry internal to the CEB allows monitoring of the CEB's secondary power rails, CCD bias voltages and the CCD and CEB operating temperatures. The third card carries a waveform generator and sequencer ASIC and sufficient clock driver buffers to enable CCD readout through any or all of its quadrant readout amplifiers. The final card provides a SpaceWire communications interface with the main AIA or HMI control electronics. A single link is used for programming the CEB's ASICs and registers, commanding a CCD readout and the return of the CCD's digitized video data at 200 Mbit/s.
Figure 68: Photo of the CEB (Camera Electronics Box), image credit: RAL
A key component of the camera electronics is a custom-designed and space-qualified CCD video signal processing and digitization ASIC. It provides 2 Mpixel/s video amplification, CDS processing and 16 bit digitization of a 1 V input signal. The design is fully-differential to aid rejection of common-mode noise. A 10-bit DAC enables ± 500 mV of programmable DC offset to be introduced into the video signal and a 7-bit programmable x1-x3 gain amplifier enables the ADC to be matched to the required CCD signal swing. The ADC is a 16 bit fully-differential pipelined converter using feedback capacitor switching in the amplifier stages, and over-ranging at intervals in order to minimize differential non-linearity due to capacitor mismatching and amplifier gain errors. Triple-voting logic is used to enhance the single-event upset tolerance of the logic and registers. The ASIC was manufactured on a 0.35 µm 3.3 V CMOS process known for its excellent tolerance to ionizing radiation. With its inputs grounded, the ASIC's noise is 3.5 ADU rms in 16 bits or 53 µV rms. The CCD provides ~ 4.5 µV/ e- and so the equivalent noise is ~ 12 e- rms. The combined noise floor of the CCD and electronics is ~ 4 ADU rms or ~ 16 e- rms. The power consumption from a 3.3 V supply is 400 mW (Ref. 83).
Figure 69: Photo of the AIA telescope array (image credit: NASA)
EVE (EUV Variability Experiment)
The Extreme ultraviolet Variability Experiment (EVE) has been designed and developed at LASP (Laboratory for Atmospheric and Space Physics) of the University of Colorado (CU) at Boulder, CO (PI T. Woods). The science team consists of members from: CU/LASP, USC (university of Southern California), NRL (Naval Research Laboratory), MIT/LL (Massachusetts Institute of Technology/ Lincoln Laboratory), NOAA, and the University of Alaska, Utah State University. The objective is to measure the solar extreme ultraviolet (EUV) irradiance with unprecedented spectral resolution, temporal cadence, accuracy, and precision. Use of physics-based models of the solar EUV irradiance to advance the understanding of the solar EUV irradiance variations based on the activity of the solar magnetic features. 87) 88) 89) 90)
Specific EVE science objectives are:
1) Specify the solar EUV spectral irradiance and its variability on multiple time scales.
- EUV: 0-105 nm (0.1 nm resolution at >10 nm) and H I Lyman-á(121.6 nm)
- Time Scales: < 20 s cadence, continuous sequence
2) Advance current understanding of how and why the solar EUV spectral irradiance varies.
- Use AIA & HMI solar images to understand the interactions of the solar magnetic fields and the evolution of the solar features (e.g., plage, active network) and how these affect the solar EUV variations
3) Improve the capability to predict the EUV spectral irradiance variability
- Develop new forecast and nowcast models of the solar EUV irradiance for use in the NOAA space weather operations
4) Understand the response of the geospace environment to variations in the solar EUV spectral irradiance and the impact on human endeavors
- Use solar EUV irradiances with thermosphere and ionosphere models to better define the solar influences on Earth's atmosphere
- Input EVE solar data near real-time into NOAA operational atmospheric models to improve accuracy of solar storm warnings and satellite drag calculations and to predict better communication disruptions
The EVE measurement approach is to observe simultaneously the solar EUV irradiance with different instrument types (multiple subsystems and technology) to meet the wavelength, resolution, and accuracy requirements.
Table 6: Overview of EVE instrument modules and measurements
Table 7: EVE instrument parameters
Figure 70: Overview of the EVE instrument (image credit: CU/LASP)
The EVE instrument consists of the following elements/modules: MEGS, ESP, and EEB.
• MEGS (Multiple EUV Grating Spectrograph). A set of 2 Rowland-circle grating spectrographs that measure the 5-105 nm spectral irradiance with 0.1 nm spectral resolution and with 10 second cadence. The MEGS have laminar groove profile (50% duty cycle of grooves) to suppress even orders.
- MEGS-A uses single, holographic, spherical grating at 80º grazing incidence
- MEGS-B uses dual, holographic, spherical grating, used near normal incidence
- CCD array type of size: 1024 x 2048 pixels (CCID-28 devices of MIT/LL, heritage: flown on Chandra and XMM/Newton)
- Back-thinned, back-illuminated
- Passively cooled to -100º C
Figure 71: Cross-section of the MEGS optics system (image credit: CU/LASP)
- MEGS-A has two slits and two filters: Slit 1: Mo/C, 5-20 nm; Slit 2: Si, 17.0 -37.0 nm
- MEGS-B has one slit and no primary filter. Additional removable filters for higher order checks.
Table 8: Overview of MEGS-A parameters
Figure 72: Schematic view of the MEGS-A device (image credit: CU/LASP)
Table 9: Overview of MEGS-B parameters
Figure 73: Schematic view of the MEGS-B device (image credit: CU/LASP)
• MEGS-SAM (Multiple EUV Grating Spectrograph-Solar Aspect Monitor). The objective is to provide pulse height analysis of X-ray photons. The device provides also MEGS pointing information with precision of 9 arcseconds. MEGS-SAM has a wavelength coverage of 0.1 -7 nm with a spectral resolution of 0.01-1 nm, and a spatial resolution with 10 arcsec/pixel. Detector: pinhole illuminates the MEGS-A CCD.
Table 10: Overview of MEGS-SAM parameters
Figure 74: Schematic of the MEGS-SAM device (image credit: CU/LASP)
• MEGS-P: Photometer for Lyman-α H I 121.6 nm and He I 58.4 nm emissions.
- Technique: grating + filter photometer
- MEGS-P channels are located in MEGS-B entrance baffles, providing a resolution of 5 nm
- Detector: IRD Si photodiode
- Filter: Acton Lyman-α filter and Al/Sn foil filter
Table 11: Parameters of the MEGS-P device
Figure 75: Schematic of the MEGS-P device (image credit: CU/LASP)
• ESP (EUV Spectrophotometer): A transmission grating spectrograph with stable Si photodiodes to provide solar X-ray measurement short of 5 nm, calibrations for MEGS sensitivity changes and higher time cadence (0.25 s). The ESP is very similar to the SOHO SEM instrument. ESP is of SEM instrument heritage flown on SOHO and also of TIMED heritage.
Table 12: Parameters of the ESP device
Figure 76: Optical layout of the ESP instrument (image credit: CU/LASP)
• EEB (EVE Electronics Box): Electronics that control the MEGS and ESP instruments and provides an interface to/from the SDO spacecraft.
EVE data products:
• Near real-time space weather data product of the solar EUV irradiance for NOAA SEC operations
• High quality solar EUV irradiances on 10 s cadence and averaged over 1 day provided daily to EVE's archive and FTP distribution center.
SDO ground system:
Data reception and spacecraft commanding will be conducted via a dedicated and newly implemented ground station at White Sands, NM. The SDO ground system consists of five major elements: 91)
1) SDOGS (SDO Ground Station), located at White Sands, NM and co-located with the WSGT (White Sands Ground Terminal) for TDRS service support. Two dual-feed antennas of 18 m diameter (S-band and Ka-band) are being allocated for SDO science data acquisition and TT&C operations support. A major function of the DDS is to continuously receive the high-rate science telemetry from the SDOGS Ka-band system and to deliver the science data to the SOCs in near real-time.
2) DDS (Data Distribution System), located at White Sands, NM.
• Receives the science telemetry data, processes it into files and distributes them to the instrument teams in near-real-time
• Provides a short-term (30 day) storage capability and supports data retransmissions as needed
• Provides the remote monitor and control capabilities of the DDS and SDOGS, from the MOC
3) MOC (Missions Operations Center), located at GSFC
• Supports the conventional real-time TT&C functions, which allows the Flight Operations Team (FOT) to monitor the health and status of the observatory and to control its operations
• Provides mission planning, trending and analysis, remote control and monitoring of DDS and ground station functions, and flight dynamics functions, including attitude determination and control and orbit maneuver computations and execution.
4) SOC (Science Operations Center). The 3 SOCs are located at the PI home institutions:
• They provide real-time health and safety monitoring as well as the command function for the science instrument
• Provision of science mission planning
• Science data processing, analysis, archiving, and distribution to the user community
5) GRN (Ground Communications Network)
• Provides connectivity between each of the ground system elements supporting all levels of data exchange and voice communications for SDO mission operations.
- One Optical Carrier Level 3 (OC3) network to AIA (67 Mbit/s) from DDS
- One Optical Carrier Level 3 (OC3) network to HMI (55 Mbit/s) from DDS
- One T3 circuit to EVE (7 Mbit/s) from DDS
- TT&C data: Four T1 circuits from MOC to/from SDOGS for S-band housekeeping telemetry and commands, two per SDOGS antenna site (restore time is < 1minute).
Figure 77: Ka-band end-to-end data flow configuration (image credit: NASA)
Figure 78: S-band end-to-end data flow configuration (image credit: NASA)
Figure 79: Overview of the SDO ground system (image credit: NASA)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).