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)
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.
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.
As of July 2020, the previously single large SDO file has been split into two files, to make the file handling manageable for all parties concerned, in particular for the user community.
• This article covers the SDO mission and its imagery in the period 2020-2019, in addition to some of the mission milestones.
Mission status for the period of 2020-2019
• June 24, 2020: As of June 2020, NASA’s Solar Dynamics Observatory – SDO – has now been watching the Sun non-stop for over a full decade. From its orbit in space around Earth, SDO has gathered 425 million high-resolution images of the Sun, amassing 20 million gigabytes of data over the past 10 years. This information has enabled countless new discoveries about the workings of our closest star and how it influences the solar system. 11)
Figure 9: This 10-year time lapse of the Sun at 17.1 nanometers (an extreme ultraviolet wavelength that shows the Sun’s outermost atmospheric layer – the corona) shows the rise and fall of the solar cycle and notable events, like transiting planets and solar eruptions (video credits: NASA’s Goddard Space Flight Center/SDO)
- With a triad of instruments, SDO captures an image of the Sun every 0.75 seconds. The Atmospheric Imaging Assembly (AIA) instrument alone captures images every 12 seconds at 10 different wavelengths of light. This 10-year time lapse showcases photos taken at a wavelength of 17.1 nanometers, which is an extreme ultraviolet wavelength that shows the Sun’s outermost atmospheric layer – the corona. Compiling one photo every hour, the movie condenses a decade of the Sun into 61 minutes. The video shows the rise and fall in activity that occurs as part of the Sun’s 11-year solar cycle and notable events, like transiting planets and eruptions. The custom music, titled “Solar Observer,” was composed by musician Lars Leonhard.
- While SDO has kept an unblinking eye pointed toward the Sun, there have been a few moments it missed. The dark frames in the video are caused by Earth or the Moon eclipsing SDO as they pass between the spacecraft and the Sun. A longer blackout in 2016 was caused by a temporary issue with the AIA instrument that was successfully resolved after a week. The images where the Sun is off-center were observed when SDO was calibrating its instruments.
- SDO and other NASA missions will continue to watch our Sun in the years to come, providing further insights about our place in space and information to keep our astronauts and assets safe.
• 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. 12)
- 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. 13)
- 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 10: 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 11: 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. 14)
- 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 12: 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 13: 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. 15)
- “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. 16)
- 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. 17)
- 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. 18)
Figure 14: 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 15: 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 16: 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. 19)
Figure 17: 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 18: 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. 20)
- 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. 21)
- "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 19: 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) 22)
Figure 20: 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)
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. 23) 24)
• 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). 25) 26) 27) 28) 29)
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:
8) Full-disk Doppler velocity and line-of-sight magnetic flux images with 1.5 arcsec resolution at least every 50 seconds
9) 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. 30)
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 21: Principal optics package components of the HMI instrument (image credit: Stanford University)
Figure 22: Photo of the HMI instrument (image credit: NASA)
Figure 23: 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. 31)
Figure 24: HMI accommodation on SDO (image credit: Stanford University)
Figure 25: 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. 32) 33)
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. 30).
• 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
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 27: Illustration of a single AIA science telescope with quad selector (image credit: LMSAL)
Figure 28: AIA science telescope assembly (image: credit: LMSAL)
Figure 29: Optical layout of the AIA science telescope (image credit: LMSAL)
Figure 30: 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 26) 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 31 (Ref. 30).
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.
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. 30).
Figure 32: 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. 34) 35) 36) 37)
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 33: 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 34: 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 35: Schematic view of the MEGS-A device (image credit: CU/LASP)
Table 9: Overview of MEGS-B parameters
Figure 36: 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 37: 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 38: 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 39: 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: 38)
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 40: Ka-band end-to-end data flow configuration (image credit: NASA)
Figure 41: S-band end-to-end data flow configuration (image credit: NASA)
Figure 42: 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 (email@example.com).