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+2021, in addition to some of the mission milestones.
Mission status for the period of 2020-2019
• March 10, 2021: Zipping through space at close to the speed of light, Solar Energetic Particles, or SEPs, are one of the main challenges for the future of human spaceflight. Clouds of these tiny solar projectiles can make it to Earth – a 93 million mile journey – in under an hour. They can fry sensitive spacecraft electronics and pose serious risks to human astronauts. But their onset is extraordinarily hard to predict, in part because we still don’t know exactly where on the Sun they come from. 11)
Figure 9: A solar flare from AR 11944 emitted on January 7th, 2014 seen in several different wavelengths of light from NASA’s Solar Dynamics Observatory. From right to left, the artificially-colored images show plasma at approximately 1 million degrees Fahrenheit (600,000 degrees Celsius), 4.5 million degrees Fahrenheit (2.5 million degrees Celsius), and 12.7 million degrees Fahrenheit (7.1 million degrees Celsius), image credits: NASA/SDO
- A new study tracing three SEP bursts back to the Sun has provided the first answer.
- “We have for the first time been able to pinpoint the specific sources of these energetic particles,” said Stephanie Yardley, space physicist at the University College London and coauthor of the paper. “Understanding the source regions and physical processes that produce SEPs could lead to improved forecasting of these events.” Study authors David Brooks, space physicist at George Mason University in Washington, D.C., and Yardley published their findings in Science Advances on March 3, 2021. 12)
- SEPs can shoot out from the Sun in any direction; catching one in the vastness of space is no small feat. NASA’s Heliophysics System Observatory – a growing fleet of Sun-studying spacecraft, strategically placed throughout the solar system – was designed in part to increase the chances of those lucky encounters.
- Scientists have divided SEP events into two major types: impulsive and gradual. Impulsive SEP events usually happen after solar flares, the bright flashes on the Sun produced by abrupt magnetic eruptions.
- “There's this really sharp spike, and then an exponential decay with time,” said Lynn Wilson, project scientist for the Wind spacecraft at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Figure 10: A close-up view of one of the flares from AR 11944 emitted on January 7th, 2014. This flare may be how the SEPs detected by Wind were released from the Sun (image credit: NASA/SDO)
- Gradual SEPs last longer, sometimes for days. They come in large swarms, making the blasts a bigger risk to astronauts and satellites. Gradual SEPs are pushed along from behind by CMEs (Coronal Mass Ejections) large plumes of solar material that billow through space like a tidal wave. The SEPs act like surfers, caught by that wave and propelled to incredible speeds.
- The greatest mystery about gradual SEPs is not what speeds them up, but where they come from in the first place. For reasons still not fully understood, SEPs contain a different mix of particles than the other solar material streaming off the Sun in the solar wind – fewer carbon, sulfur, and phosphorous ions, for instance. Some scientists suspect they’re cut from an entirely different cloth, forming in a different feature or layer of the Sun than the rest of the solar wind.
- To find out where SEPs come from, Brooks and Yardley traced gradual SEP events from January 2014 back to their origin on the Sun.
- They started with NASA’s Wind spacecraft, which orbits at the L1 Lagrange point about 1 million miles closer to the Sun than we are. One of Wind’s eight instruments is the EPACT (Energetic Particles: Acceleration, Composition, and Transport) instrument, which specializes in detecting SEPs. EPACT captured three strong SEP blasts on January 4th, 6th and 8th.
- Wind’s data showed that these SEP events indeed had a specific “fingerprint” – a different mix of particles than is typically found in the solar wind.
- “There is often less sulfur in SEPs compared to the solar wind, sometimes a lot less” said Brooks, lead author of the paper. “This is a unique fingerprint of SEPs that allows us to search for places in the Sun's atmosphere where sulfur is also lacking.”
- They turned to JAXA/NASA’s Sun-watching Hinode spacecraft, an observatory in which Brooks serves a critical operational role for NASA from Japan. Hinode was watching Active Region 11944, a bright area of strong magnetic field with a large dark sunspot visible from Earth. AR 11944 had produced several large flares and CMEs in early January that released and accelerated the SEPs Wind observed.
- Hinode’s EIS (Extreme Ultraviolet Imaging Spectrometer) instrument scanned the active region, breaking the light into spectral lines used to identify specific elements. They looked for places in the active region with a matching fingerprint, where the specific mix of elements agreed with what they saw in Wind’s data.
- “This type of research is exactly what Hinode was designed to pursue,” said Sabrina Savage, the U.S. project scientist for Hinode. “Complex system science cannot be done in a bubble with only one mission.”
- Hinode’s data revealed the source of the SEP events – but it wasn’t what either Brooks or Yardley expected.
- As a rule, the solar wind can escape more easily by finding open magnetic field lines – field lines anchored to the Sun at one end but streaming out into space on the other.
- “I really thought we were going to find it at the edges of the active region where the magnetic field is already open and material can escape directly,” Brooks said. “But the fingerprint matched only in regions where the magnetic field is still closed.”
- The SEPs had somehow broken free from strong magnetic loops connected to the Sun at both ends. These loops trap material near the top of the chromosphere, one layer below where solar flares and coronal mass ejections erupt.
- “People have already been thinking about ways it could get out from closed field – especially in the context of the solar wind,” Brooks said. “But I think the fact that the material was found in the core of the region, where the magnetic fields are very strong, makes it harder for those processes to work.”
- The surprising result raises new questions about how SEPs escape the Sun, questions ripe for future work. Still, pinpointing one event’s source is a big step forward.
- “Normally, you have to infer this kind of thing – you’d say, ‘look we saw an SEP and a solar flare, and the SEP probably came from the solar flare,’” said Wilson, who wasn’t involved in the study. “But this is direct evidence tying these two phenomena together.”
- Brooks and Yardley also demonstrate one way to use NASA’s growing Heliophysics System Observatory, combining multi-spacecraft observations to do science that previously wasn’t possible.
- “It's a way of thinking about all the spacecraft that are in flight that you can use to do a single study,” Wilson said. “It's like having a bunch of weather stations — you start to get a much better picture of what the weather is doing on a larger scale, and you can actively start to try to predict it.”
- “These authors have done a remarkable job combining the right data sets and applying them to the right questions,” Savage said. “The search for the origins of potentially harmful energetic particles has been critically narrowed thanks to this effort.”
Figure 11: Closed magnetic field lines loop back to the Sun, surrounded by open field lines that reach out into space, as depicted in this illustration (image credits: NASA’s Goddard Space Flight Center/Lisa Poje/Genna Duberstein)
• January 19, 2021: Scientists have combined NASA data and cutting-edge image processing to gain new insight into the solar structures that create the Sun’s flow of high-speed solar wind, detailed in new research published today in The Astrophysical Journal. This first look at relatively small features, dubbed “plumelets,” could help scientists understand how and why disturbances form in the solar wind. 13)
- The Sun’s magnetic influence stretches billions of miles, far past the orbit of Pluto and the planets, defined by a driving force: the solar wind. This constant outflow of solar material carries the Sun’s magnetic field out into space, where it shapes the environments around Earth, other worlds, and in the reaches of deep space. Changes in the solar wind can create space weather effects that influence not only the planets, but also human and robotic explorers throughout the solar system — and this work suggests that relatively small, previously-unexplored features close to the Sun’s surface could play a crucial role in the solar wind’s characteristics.
- Like all solar material, which is made up of a type of ionized gas called plasma, the solar wind is controlled by magnetic forces. And the magnetic forces in the Sun’s atmosphere are particularly complex: The solar surface is threaded through with a constantly-changing combination of closed loops of magnetic field and open magnetic field lines that stretch out into the solar system.
- It’s along these open magnetic field lines that the solar wind escapes from the Sun into space. Areas of open magnetic field on the Sun can create coronal holes, patches of relatively low density that appear as dark splotches in certain ultraviolet views of the Sun. Often, embedded within these coronal holes are geysers of solar material that stream outward from the Sun for days at a time, called plumes. These solar plumes appear bright in extreme ultraviolet views of the Sun, making them easily visible to observatories like NASA’s Solar Dynamics Observatory satellite and other spacecraft and instruments. As regions of particularly dense solar material in open magnetic field, plumes play a large role in creating the high-speed solar wind — meaning that their attributes can shape the characteristics of the solar wind itself.
- Using high-resolution observations from NASA’s SDO (Solar Dynamics Observatory) satellite, along with an image processing technique developed for this work, Uritsky and collaborators found that these plumes are actually made up of much smaller strands of material, which they call plumelets. While the entirety of the plume stretches out across about 70,000 miles in SDO’s images, the width of each plumelet strand is only a few thousand miles across, ranging from around 2,300 miles at the smallest to around 4,500 miles in width for the widest plumelets observed.
- Though earlier work has hinted at structure within solar plumes, this is the first time scientists have observed plumelets in sharp focus. The techniques used to process the images reduced the “noise” in the solar images, creating a sharper view that revealed the plumelets and their subtle changes in clear detail.
- Their work, focused on a solar plume observed on July 2-3, 2016, shows that the plume’s brightness comes almost entirely from the individual plumelets, without much additional fuzz between structures. This suggests that plumelets are more than just a feature within the larger system of a plume, but rather the building blocks of which plumes are made.
Figure 12: Scientists used image processing on high-resolution images of the Sun to reveal distinct “plumelets” within structures on the Sun called solar plumes. The full-disk Sun and the left side of the inset image were captured by NASA’s SDO in a wavelength of extreme ultraviolet light and processed to reduce noise. The right side of the inset has been further processed to enhance small features in the images, revealing the edges of the plumelets in clear detail. These plumelets could help scientists understand how and why disturbances in the solar wind form (image credits: NASA/SDO/Uritsky, et al.)
- “People have seen structure in and at the base of plumes for a while,” said Judy Karpen, one of the authors of the study and chief of the Space Weather Laboratory in the Heliophysics Science Division at NASA Goddard. “But we’ve found that the plume itself is a bundle of these denser, flowing plumelets, which is very different from the picture of plumes we had before.”
- They also found that the plumelets move individually, each oscillating on its own — suggesting that the small-scale behavior of these structures could be a major driver behind disruptions in the solar wind, in addition to their collective, large-scale behavior.
Searching for plumelet signatures
- The processes that create the solar wind often leave signatures in the solar wind itself — changes in the wind’s speed, composition, temperature, and magnetic field that can provide clues about the underlying physics on the Sun. Solar plumelets may also leave such fingerprints, revealing more about their exact role in the solar wind’s creation, even though finding and interpreting them can be its own complex challenge.
- One key source of data will be NASA’s Parker Solar Probe, which has flown closer to the Sun than any other spacecraft — reaching distances as close as 4 million miles from the solar surface by the end of its mission — captures high-resolution measurements of the solar wind as it swings by the Sun every few months. Its observations, closer to the Sun and more detailed than those from prior missions, could reveal plumelet signatures.
- In fact, one of Parker Solar Probe’s early and unexpected findings might be connected to plumelets. During its first solar flyby in November 2018, Parker Solar Probe observed sudden reversals in the magnetic field direction of the solar wind, nicknamed “switchbacks.” The cause and the exact nature of the switchbacks is still a mystery to scientists, but small-scale structures like plumelets could produce similar signatures.
Figure 13: During its first solar flyby in November 2018, NASA’s Parker Solar Probe observed switchbacks — sudden reversals in the magnetic field of the solar wind, illustrated here. Newly-observed solar plumelets might produce similar signatures to switchbacks (image credits: NASA's Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez)
- Finding signatures of the plumelets within the solar wind itself also depends on how well these fingerprints survive their journey away from the Sun — or whether they would be smudged out somewhere along the millions of miles they travel from the Sun to our observatories in space.
- Evaluating that question will rely on remote observatories, like ESA and NASA’s Solar Orbiter, which has already taken the closest-ever images of the Sun, including a detailed view of the solar surface — images that will only improve as the spacecraft gets closer to the Sun. NASA’s upcoming PUNCH mission — led by Craig DeForest, one of the authors on the plumelets study — will study how the Sun’s atmosphere transitions to the solar wind and could also provide answers to this question.
- “PUNCH will directly observe how the Sun’s atmosphere transitions to the solar wind,” said Uritsky. “This will help us understand if the plumelets can survive as they propagate away from the Sun — if can they actually be injected into the solar wind.”
• January 15, 2021: NASA’s images of the Sun’s dynamic and — dazzling beauty have captivated the attention of millions. In 2021, the US Postal Service is showcasing the Sun’s many faces with a series of Sun Science forever stamps that show images of solar activity captured by NASA’s Solar Dynamics Observatory, or SDO. 14)
Figure 14: The United States Post office announced on Jan. 15, 2021, that they would be releasing a series of stamps highlighting images of the Sun captured by NASA's Solar Dynamics Observatory (image credits: NASA/SDO/USPS)
- “I have been a stamp collector all my life and I can’t wait to see NASA science highlighted in this way,” said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate (SMD) in Washington. “I feel that the natural world around us is as beautiful as art, and it’s inspiring to be able to share the import and excitement of studying the Sun with people around the country.”
- The 20-stamp set features ten images that celebrate the science behind NASA’s ongoing exploration of our nearest star. The images display common events on the Sun, such as solar flares, sunspots and coronal loops. SDO has kept a constant eye on the Sun for over a decade. Outfitted with equipment to capture images of the Sun in multiple wavelengths of visible, ultraviolet, and extreme ultraviolet light, SDO has gathered hundreds of millions of images during its tenure to help scientists learn about how our star works and how its constantly churning magnetic fields create the solar activity we see.
- That solar activity can drive space weather closer to Earth that can interfere with technology and radio communications in space. In addition to this immediate relevancy to our high-tech daily lives, the study of the Sun and its influence on the planets and space surrounding it – a field of research known as heliophysics – holds profound implications for the understanding of our solar system and the thousands of solar systems that have been discovered beyond our own. As our closest star, the Sun is the only nearby star that humans are able to study in great detail, making it a vital source of data.
• January 4, 2021: A secret behind the workings of sunquakes – seismic activity on the Sun during solar flares – might be hidden beneath the solar surface. 15)
- These earthquake-like events release acoustic energy in the form of waves that ripple along the Sun’s surface, like waves on a lake, in the minutes following a solar flare – an outburst of light, energy, and material seen in the Sun’s outer atmosphere.
Figure 15: Movie of a sunquake – the earthquake-like waves that ripple through our star. Left frame shows the active region in visible light (amber) and extreme ultraviolet (red) on July 30, 2011. Right frame shows the ripples on Sun’s outlying surface up to 42 minutes after the onset of the flare, which is marked by the label “IP” for impulsive flare (image credit: NASA/SDO)
- Scientists have long suspected that sunquakes are driven by magnetic forces or heating of the outer atmosphere, where the flare occurs. These waves were thought to dive down through the Sun’s surface and deep into its interior. But new results, using data from NASA’s SDO (Solar Dynamics Observatory), have found something different.
- In July 2011, SDO observed a sunquake with unusually sharp ripples emanating from a moderately strong solar flare. Scientists were able to track the waves that caused these ripples back to their source, using a technique called helioseismic holography. This technique, which used SDO’s Helioseismic and Magnetic Imager to measure how the solar surface was moving, has previously been used to track acoustic waves from a variety of other sources in the Sun.
- Instead of the waves traveling into the Sun from above, the scientists saw the surface ripples of a sunquake emerging from deep beneath the solar surface right after a flare occurred. The results, published in the journal Astrophysical Journal Letters, found the acoustic source was around 700 miles below the surface of the Sun – not above the surface as previously was thought. 16)
- The scientists believe that these waves were driven by a submerged source, which was in turn somehow triggered by the solar flare in the atmosphere above. The new findings might help explain a long-standing mystery about sunquakes: why some of their characteristics look remarkably different from the flares that trigger them.
- The scientists still haven’t identified exactly what mechanism actually causes sunquakes, though the results do provide the clue that their origins likely lurk beneath the surface. The scientists plan to continue searching for a mechanism by looking at other sunquakes to see if they have similarly submerged sources.
• September 21, 2020. Solar flares are violent explosions on the sun that fling out high-energy charged particles, sometimes toward Earth, where they disrupt communications and endanger satellites and astronauts. 17)
- But as scientists discovered in 1996, flares can also create seismic activity — sunquakes — releasing impulsive acoustic waves that penetrate deep into the sun’s interior.
Figure 16: An X-class solar flare (X9.3) emitted on September 6, 2017, and captured by NASA’s Solar Dynamics Observatory in extreme ultraviolet light (image credit: NASA/GSFC/SDO)
- While the relationship between solar flares and sunquakes is still a mystery, new findings suggest that these “acoustic transients” — and the surface ripples they generate — can tell us a lot about flares and may someday help us forecast their size and severity.
- A team of physicists from the United States, Colombia and Australia has found that part of the acoustic energy released from a flare in 2011 emanated from about 1,000 kilometers beneath the solar surface — the photosphere — and, thus, far beneath the solar flare that triggered the quake.
- The results, reported today in The Astrophysical Journal Letters, come from a diagnostic technique called helioseismic holography, introduced in the late 1900s by French scientist Francoise Roddier and extensively developed by U.S. scientists Charles Lindsey and Douglas Braun, now at NorthWest Research Associates in Boulder, Colorado, and co-authors of the paper. 18) 19)
- Helioseismic holography allows scientists to analyze acoustic waves triggered by flares to probe their sources, much as seismic waves from megaquakes on Earth allow seismologists to locate their epicenters. The technique was first applied to acoustic transients released from flares by a graduate student in Romania, Alina-Catalina Donea, under the supervision of Lindsey and Braun. Donea is now at Monash University in Melbourne, Australia.
- “It‘s the first helioseismic diagnostic specifically designed to directly discriminate the depths of the sources it reconstructs, as well as their horizontal locations,” Braun said.
Figure 17: NASA’s Solar Dynamics Observatory captured this image of a medium-class (M8.1) solar flare (bright area at right) on September 8, 2017. The image blends two different wavelengths of extreme ultraviolet light (image credit: NASA/GSFC/SDO)
- “We can’t see the sun’s inside directly. It is opaque to the photons that show us the sun’s outer atmosphere, from where they can escape to reach our telescopes,” said co-author Juan Camilo Buitrago-Casas, a University of California, Berkeley, doctoral student in physics from Colombia. “The way we can know what happens inside of the sun is via seismic waves that make ripples on the solar surface similar to those caused by earthquakes on our planet. A big explosion, such as a flare, can inject a powerful acoustic pulse into the sun, whose subsequent signature we can use to map its source in some detail. The big message of this paper is that the source of at least some of this noise is deeply submerged. We are reporting the deepest source of acoustic waves so far known in the sun.”
How sunquakes produce ripples on the sun’s surface
- The acoustic explosions that cause sunquakes in some flares radiate acoustic waves in all directions, primarily downward. As the downward-traveling waves move through regions of ever-increasing temperature, their paths are bent by refraction, ultimately heading back up to the surface, where they create ripples like those seen after throwing a pebble in a pond. The time between the explosion and the arrival of the ripples is about 20 minutes.
Figure 18: Solar flares trigger acoustic waves (sunquakes) that travel downward but, because of increasing temperatures, are bent or refracted back to the surface, where they produce ripples that can be seen from Earth-orbiting observatories. Solar physicists have discovered a sunquake generated by an impulsive explosion 1,000 kilometers below the flare (top), suggesting that the link between sunquakes and flares is not simple. (image credit: UC Berkeley, cartoon by Juan Camilo Buitrago-Casas)
- “The ripples, then, are not just a surface phenomenon, but the surface signature of waves that have gone deep beneath the active region and then back up to the outlying surface in the succeeding hour,” Lindsey said. Analyzing the surface ripples can pinpoint the source of the explosion.
- “It has been widely supposed that the waves released by acoustically active flares are injected into the solar interior from above. What we are finding is the strong indication that some of the source is far beneath the photosphere,” said Juan Carlos Martínez Oliveros, a solar physics researcher at UC Berkeley’s Space Sciences Laboratory and a native of Colombia. “It seems like the flares are the precursor, or trigger, to the acoustic transient released. There is something else happening inside the sun that is generating at least some part of the seismic waves.”
- “Using an analogy from medicine, what we (solar physicists) were doing before is like using X-rays to look at one snapshot of the interior of the sun. Now, we are trying to do a CAT scan, to view the solar interior in three dimensions,” added Martínez Oliveros.
- The Colombians, including students Ángel Martínez and Valeria Quintero Ortega at Universidad Nacional de Colombia, in Bogotá, are co-authors of the ApJ“We have known about acoustic waves from flares for a little over 20 years now, and we have been imaging their sources horizontally since that time. But we have only recently discovered that some of those sources are submerged below the solar surface,” said Lindsey. “This may help explain a great mystery: Some of these acoustic waves have emanated from locations that are devoid of local surface disturbances that we can directly see in electromagnetic radiation. We have wondered for a long time how this can happen.” Letters paper with their supervisor, Benjamín Calvo-Mozo, associate professor of astronomy.
- “We have known about acoustic waves from flares for a little over 20 years now, and we have been imaging their sources horizontally since that time. But we have only recently discovered that some of those sources are submerged below the solar surface,” said Lindsey. “This may help explain a great mystery: Some of these acoustic waves have emanated from locations that are devoid of local surface disturbances that we can directly see in electromagnetic radiation. We have wondered for a long time how this can happen.”
A seismically active sun
- For more than 50 years, astronomers have known that the sun reverberates with seismic waves, much like the Earth and its steady hum of seismic activity. This activity, which can be detected by the Doppler shift of light emanating from the surface, is understood to be driven by convective storms that form a patchwork of granules about the size of Texas, covering the sun’s surface and continually rumbling.
Figure 19: Time-lapse sequence of the July 30, 2011, solar flare observed by NASA’s SDO. The left frame shows visible light emissions in amber and excess extreme ultraviolet emissions in red. The right frame shows the line-of-sight Doppler velocity of the solar surface emissions. Between 20 to 40 minutes following the impulsive phase of the flare (IP on timeline), a strong acoustic disturbance released downward into the underlying solar interior has refracted back to the outlying surface, tens of thousands of kilometers from the site of the flare, to elicit outwardly propagating surface ripples (right frame). The movie is 200 times faster than real time; the ripples are amplified by a factor of three in the right frame compared to the left (video credit: Charles Lindsey)
- Amid this background noise, magnetic regions can set off violent explosions releasing waves that make the spectacular ripples that then appear on the sun’s surface in the succeeding hour, as discovered 24 years ago by astronomers Valentina Zharkova and Alexander Kosovichev.
- As more sunquakes have been discovered, flare seismology has blossomed, as have the techniques to explore their mechanics and their possible relationship to the architecture of magnetic flux underlying active regions.
- Among the open questions: Which flares do and don’t produce sunquakes? Can sunquakes occur without a flare? Why do sunquakes emanate primarily from the edges of sunspots, or penumbrae? Do the weakest flares produce quakes? What is the lower limit?
- Until now, most solar flares have been studied as one-offs, since strong flares, even during times of maximum solar activity, may occur only a few times a year. The initial focus was on the largest, or X-class, flares, classified by the intensity of the soft X-rays they emit. Buitrago-Casas, who obtained his bachelor’s and master’s degrees from Universidad Nacional de Colombia, teamed up with Lindsey and Martínez Oliveros to conduct a systematic survey of relatively weak solar flares to increase their database, for a better understanding of the mechanics of sunquakes.
- Of the 75 flares captured between 2010 and 2015 by the RHESSI satellite — a NASA X-ray satellite designed, built and operated by the Space Sciences Laboratory and retired in 2018 — 18 produced sunquakes. One of Buitrago-Casas’s acoustic transients, the one released by the flare of July 30, 2011, caught the eyes of undergraduate students Martínez, now a graduate student, and Quintero Ortega.
- “We gave our student collaborators at the National University the list of flares from our survey. They were the first ones who said, ‘Look at this one. It’s different! What happened here?’” Buitrago-Casas said. “And so, we found out. It was super exciting!”
- Martínez and Quintero Ortega are the first authors on a paper describing the extreme impulsivity of the waves released by that flare of July 30, 2011, that appeared in the May 20, 2020, issue of The Astrophysical Journal Letters. These waves had spectral components that gave the researchers unprecedented spatial resolution of their source distributions.
- Thanks to superb data from NASA’s Solar Dynamics Observatory satellite, the team was able to pinpoint the source of the explosion that generated the seismic waves 1,000 kilometers below the photosphere. This is shallow, relative to the sun’s radius of nearly 700,000 kilometers, but deeper than any previously known acoustic source in the sun.
- A source submerged below the sun’s photosphere with its own morphology and no conspicuous directly overlying disturbance in the outer atmosphere suggests that the mechanism that drives the acoustic transient is itself submerged.
- “It may work by triggering a compact explosion with its own energy source, like a remotely triggered earthquake,” Lindsey said. “The flare above shakes something beneath the surface, and then a very compact unit of submerged energy gets released as acoustic sound,” he said. “There is no doubt that the flare is involved, it’s just that the existence of this deep compact source suggests the possibility of a separate, distinctive, compact, submerged energy source driving the emission.”
- About half of the medium-sized solar flares that Buitrago-Casas and Martínez Oliveros have catalogued have been associated with sunquakes, showing that they commonly occur together. The team has since found other submerged sources associated with even weaker flares.
- The discovery of submerged acoustic sources opens the question of whether there are instances of acoustic transients being released spontaneously, with no surface disturbance, or no flare, at all.
- “If sunquakes can be generated spontaneously in the sun, this might lead us to a forecasting tool, if the transient can come from magnetic flux that has yet to break the sun’s surface,” Martínez Oliveros said. “We could then anticipate the inevitable subsequent emergence of that magnetic flux. We may even forecast some details about how large an active region is about to appear and what type — even, possibly, what kinds of flares — it might produce. This is a long shot, but well worth looking into.”
• September 15, 2020: Solar Cycle 25 has begun. During a media event on Tuesday, experts from NASA and the National Oceanic and Atmospheric Administration (NOAA) discussed their analysis and predictions about the new solar cycle – and how the coming upswing in space weather will impact our lives and technology on Earth, as well as astronauts in space. 20)
- The Solar Cycle 25 Prediction Panel, an international group of experts co-sponsored by NASA and NOAA, announced that solar minimum occurred in December 2019, marking the start of a new solar cycle. Because our Sun is so variable, it can take months after the fact to declare this event. Scientists use sunspots to track solar cycle progress; the dark blotches on the Sun are associated with solar activity, often as the origins for giant explosions – such as solar flares or coronal mass ejections – which can spew light, energy, and solar material into space.
- “As we emerge from solar minimum and approach Cycle 25’s maximum, it is important to remember solar activity never stops; it changes form as the pendulum swings,” said Lika Guhathakurta, solar scientist at the Heliophysics Division at NASA Headquarters in Washington.
- NASA and NOAA, along with the Federal Emergency Management Agency and other federal agencies and departments, work together on the National Space Weather Strategy and Action Plan to enhance space weather preparedness and protect the nation from space weather hazards. NOAA provides space weather predictions and satellites to monitor space weather in real time; NASA is the nation’s research arm, helping improve our understanding of near-Earth space, and ultimately, forecasting models.
- Space weather predictions are also critical for supporting Artemis program spacecraft and astronauts. Surveying this space environment is the first step to understanding and mitigating astronaut exposure to space radiation. The first two science investigations to be conducted from the Gateway will study space weather and monitor the radiation environment in lunar orbit. Scientists are working on predictive models so they can one day forecast space weather much like meteorologists forecast weather on Earth.
- “There is no bad weather, just bad preparation,” said Jake Bleacher, chief scientist for NASA’s Human Exploration and Operations Mission Directorate at the agency’s Headquarters. “Space weather is what it is – our job is to prepare.”
- Understanding the cycles of the Sun is one part of that preparation. To determine the start of a new solar cycle, the prediction panel consulted monthly data on sunspots from the World Data Center for the Sunspot Index and Long-term Solar Observations, located at the Royal Observatory of Belgium in Brussels, which tracks sunspots and pinpoints the solar cycle’s highs and lows.
- “We keep a detailed record of the few tiny sunspots that mark the onset and rise of the new cycle,” said Frédéric Clette, the center’s director and one of the prediction panelists. “These are the diminutive heralds of future giant solar fireworks. It is only by tracking the general trend over many months that we can determine the tipping point between two cycles.”
Figure 20: This split image shows the difference between an active Sun during solar maximum (on the left, captured in April 2014) and a quiet Sun during solar minimum (on the right, captured in December 2019). December 2019 marks the beginning of Solar Cycle 25, and the Sun’s activity will once again ramp up until solar maximum, predicted for 2025 (image credit: NASA/SDO)
- With solar minimum behind us, scientists expect the Sun’s activity to ramp up toward the next predicted maximum in July 2025. Doug Biesecker, panel co-chair and solar physicist at NOAA’s Space Weather Prediction Center (SWPC) in Boulder, Colorado, said Solar Cycle 25 is anticipated to be as strong as the last solar cycle, which was a below-average cycle, but not without risk.
- “Just because it’s a below-average solar cycle, doesn’t mean there is no risk of extreme space weather,” Biesecker said. “The Sun’s impact on our daily lives is real and is there. SWPC is staffed 24/7, 365 days a year because the Sun is always capable of giving us something to forecast.”
- Elsayed Talaat, director of Office of Projects, Planning, and Analysis for NOAA’s Satellite and Information Service in Silver Spring, Maryland, described the nation’s recent progress on the Space Weather Action Plan as well as on upcoming developments, including NOAA’s Space Weather Follow-On L-1 observatory, which launches in 2024, before Solar Cycle 25’s predicted peak.
- “Just as NOAA’s National Weather Service makes us a weather-ready nation, what we’re driving to be is a space weather-ready nation,” Talaat said. “This is an effort encompassing 24 agencies across the government, and it has transformed space weather from a research perspective to operational knowledge.”
• August 18, 2020: Solar flares emit sudden, strong bursts of electromagnetic radiation from the Sun's surface and its atmosphere, and eject plasma and energetic particles into inter-planetary space. Since large solar flares can cause severe space weather disturbances affecting Earth, to mitigate their impact their occurrence needs to be predicted. However, as the onset mechanism of solar flares is unclear, most flare prediction methods so far have relied on empirical methods. 21)
- The research team led by Professor Kanya Kusano (Director of the Institute for Space-Earth Environmental Research, Nagoya University) recently succeeded in developing the first physics-based model that can accurately predict imminent large solar flares. The work was published in the journal Science on July 31, 2020. 22)
- The new method of flare prediction, called the kappa scheme, is based on the theory of "double-arc instability," that is a magnetohydrodynamic (MHD) instability triggered by magnetic reconnection. The researchers assumed that a small-scale reconnection of magnetic field lines can form a double-arc (m-shape) magnetic field and trigger the onset of a solar flare (Figure 21). The kappa scheme can predict how a small magnetic reconnection triggers a large flare and how a large solar flare can occur.
Figure 21: The process of solar flare production in the physics-based prediction method. A: Electric currents flow along magnetic field lines across the magnetic polarity inversion line on the solar surface, where the magnetic field changes its polarity. B: Magnetic field lines are reconnected and form a double-arc loop that moves away from the surface due magnetohydrodynamic instability. C: The upward motion of the double-arc loop induces further magnetic reconnection. A solar flare begins to burst out from the base points of the reconnected field lines. D: More magnetic reconnections amplify the instability and the solar flare expands (image credit: Institute for Space-Earth Environmental Research, Nagoya University)
The predictive model was tested on about 200 active regions during solar cycle 24 from 2008 to 2019 using data obtained by NASA's Solar Dynamics Observatory (SDO) satellite. It was demonstrated that with few exceptions, the kappa-scheme predicts most imminent solar flares, as well as the precise location they will emerge from (Figure 22). The researchers also discovered that a new parameter — the "magnetic twist flux density" close to a magnetic polarity inversion line on the solar surface — determines when and where solar flares probably occur and how large they are likely to be.
Figure 22: The magnetic field on the solar surface and the initial brightening of the largest solar flare (GOES class X9.3) during solar cycle 24 in NOAA Active Region (AR) 12673 on Sep. 6, 2017. This was observed by the Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) onboard the NASA's Solar Dynamics Observatory (SDO) satellite. A: The magnetic field on the solar surface before the onset of the large flare at 11:45 UT. White and black indicates the intensity of the magnetic field along the line of sight out of and toward the plane. B: An expanded view of the vertical magnetic field in AR 12673. A white circle indicates the location where a large flare was predicted by this study. The black contour shows the magnetic polarity inversion (PIL). C: Bright flare ribbon observed by SDO/AIA1600Â at 11:52 UT. Figures B and C are based on Figure 3 of the research paper by Kusano et al. (2020), published in Science (image credit: NASA/SDO the AIA and HMI science teams)
- Previous flare prediction methods have relied on empirical relations in which the predictions of the previous day tend to continue into the next day even if flare activity changes. In contrast, the kappa-scheme predicts large solar flares through a physics-based approach regardless of previous flare activity. While it takes a lot more work to implement the scheme in real-time operational forecasting, this study shows that the physics-based approach may open a new direction for flare prediction research.
• July 28, 2020: In a study published in Nature Astronomy, an international team of researchers has presented a new, detailed look inside the “central engine” of a large solar flare accompanied by a powerful eruption first captured on Sept. 10, 2017 by the Owens Valley Solar Array (EOVSA) — a solar radio telescope facility operated by New Jersey Institute of Technology’s (NJIT) Center for Solar-Terrestrial Research (CSTR). 23)
- The new findings, based on EOVSA’s observations of the event at microwave wavelengths, offer the first measurements characterizing the magnetic fields and particles at the heart of the explosion. The results have revealed an enormous electric current “sheet” stretching more than 40,000 kilometers through the core flaring region where opposing magnetic field lines approach each other, break and reconnect, generating the intense energy powering the flare. 24)
- Notably, the team’s measurements also indicate a magnetic bottle-like structure located at the top of the flare’s loop-shaped base (known as the flare arcade) at a height of nearly 20,000 kilometers above the Sun’s surface. The structure, the team suggests, is likely the primary site where the flare’s highly energetic electrons are trapped and accelerated to nearly the speed of light.
- ”...the measured magnetic field profile of the current sheet beautifully matched the theoretical prediction made decades ago.
- Researchers say the study’s new insight into the central engine that drives such powerful eruptions may aid future space weather predictions for potentially catastrophic energy releases from solar flares — the solar system’s most powerful explosions, capable of severely disrupting technologies on Earth such as satellite operations, GPS navigation and communication systems, among many others.
- "One of the major goals of this research is to better understand the fundamental physics of solar eruptions,” said Bin Chen, the paper’s lead author and professor of physics at NJIT. “It has been long suggested that the sudden release of magnetic energy through the reconnection current sheet is responsible for these major eruptions, yet there has been no measurement of its magnetic properties. With this study we’ve finally measured the details of the magnetic field of a current sheet for the first time, giving us a new understanding of the central engine of the Sun’s major flares.”
- “The place where all the energy is stored and released in solar flares has been invisible until now. To play on a term from cosmology, it is the Sun’s ‘dark energy problem,’ and previously we’ve had to infer indirectly that the flare’s magnetic reconnection sheet existed,” said Dale Gary, EOVSA director at NJIT and co-author of the paper. “EOVSA’s images made at many microwave frequencies showed we can capture radio emissions to illuminate this important region. Once we had that data, and the analysis tools created by co-authors Gregory Fleishman and Gelu Nita, we were able to start analyzing the radiation to enable these measurements.”
Figure 23: Researchers provide an unprecedented look inside the “central engine” of a large solar flare, a site were dramatic bursts of energy are released, and particles are accelerated to relativistic energies (photo credit: NJIT-CSTR, B. Chen, S. Yu; NASA SDO)
- Earlier this year in the journal Science, the team reported it could finally provide quantitative measurements of the evolving magnetic field strength directly following the flare's ignition.
- Continuing their investigation, the team’s latest analysis combined numerical simulations performed at Center for Astrophysics | Harvard & Smithsonian (CfA) with EOVSA’s spectral imaging observations and multiwavelength data — spanning radio waves to X-rays — collected from the X8.2-sized solar flare. The flare is the second largest to have occurred from the past 11-year solar cycle, occurring with a fast coronal mass ejection (CME) that drove a large-scale shock in the upper solar corona.
- Among the study’s surprises, the researchers found that the measured profile of the magnetic field along the flare’s current sheet feature closely matched predictions from the team’s numerical simulations, which were based on a well-known theoretical model for explaining solar flare physics, first proposed in the 1990s with an analytical form.
- “It surprised us that the measured magnetic field profile of the current sheet beautifully matched the theoretical prediction made decades ago,” said Chen.
- “The force of the Sun’s magnetic field plays a key role in accelerating plasma during an eruption. Our model was used for computing the physics of the magnetic forces during this eruption, which manifests as a highly twisted ‘rope’ of magnetic field lines, or magnetic flux rope,” explained Kathy Reeves, astrophysicist at CfA and co-author of the study. “It is remarkable that this complicated process can be captured by a straightforward analytical model, and that the predicted and measured magnetic fields match so well.”
Figure 24: Observations of the Sept. 10, 2017 solar flare and the standard solar flare model. Left: Observations in extreme ultraviolet (grayscale background) and microwave (red to blue indicate increasing frequencies). Light orange curves are selected magnetic field lines from the matching theoretical model. Right: Numerical simulation of the flare. The reconnection current sheet is shown as the thin orange-purple feature located between the erupting magnetic flux rope and the flare arcade. Microwave sources from relativistic electrons are observed to fill the entire region surrounding the current sheet. (image credit: NJIT-CSTR, B. Chen, S. Yu; CfA, C. Shen; Solar Dynamics Observatory)
- The simulations, performed by Chengcai Shen at CfA, were developed to numerically solve governing equations for quantifying the behavior of electrically conducting plasma throughout the flare’s magnetic field. By applying an advanced computational technique known as “adaptive mesh refinement,” the team was able to resolve the thin reconnection current sheet and capture its detailed physics at superfine spatial scales to below 100 km.
- “Our simulation results match both the theoretical prediction on magnetic field configuration during a solar eruption and reproduce a set of observable features from this particular flare, including magnetic strength and plasma inflow/outflows around the reconnecting current sheet,” Shen noted.
- The team’s measurements and matching simulation results revealed that the flare’s current sheet features an electric field that produces a shocking 4,000 volts per meter. Such a strong electric field is present over a 40,000 km region, greater than the length of three Earths placed together side by side.
- The analysis also showed a huge amount of magnetic energy being pumped into the current sheet at an estimated rate of 10-100 billion trillion (1022-1023) joules per second — that is, the amount of energy being processed at the flare’s engine, within each second, is equivalent to the total energy released by the explosion of about a hundred thousand of the most powerful hydrogen bombs (50-megaton-class) at the same time.
- “Such an enormous energy release at the current sheet is mind-blowing. The strong electric field generated there can easily accelerate the electrons to relativistic energies, but the unexpected fact we found was that the electric field profile in the current sheet region did not coincide with the spatial distribution of relativistic electrons that we measured,” said Chen. “In other words, something else had to be at play to accelerate or redirect these electrons. What our data showed was a special location at the bottom of the current sheet — the magnetic bottle — appears to be crucial in producing or confining the relativistic electrons.”
- “While the current sheet seems to be the place where the energy is released to get the ball rolling, most of the electron acceleration appears to be occurring in this other location, the magnetic bottle. ... Similar magnetic bottles are under development for confining and accelerating particles in some laboratory fusion reactors.” added Gary. “Others have proposed such a structure in solar flares before, but we can truly see it now in the numbers.”
- Approximately 99% of the flare’s relativistic electrons were observed congregating at the magnetic bottle throughout the duration of the five-minute-long emission.
- For now, Chen says the group will be able to apply these new measurements as a comparative baseline to study other solar flare events, as well as explore the exact mechanism that accelerates particles by combining the new observations, numerical simulations and advanced theories. Because of the breakthrough capabilities of EOVSA, NJIT was recently selected to participate in a joint NASA/NSF DRIVE Science Center Collaboration on Solar Flare Energy Release (SolFER).
- “Our goal is to develop a full understanding of solar flares, from their initiation until they finally spray out highly energized particles into the solar wind, and eventually, into the space environment of Earth,” said Jim Drake, professor of physics at the University of Maryland and principal investigator of SolFER who was not involved in this study. “These first observations are already suggesting that relativistic electrons might be trapped in a large magnetic bottle produced as the magnetic fields of the corona ‘reconnect’ to release their energy. ... The EOVSA observations will continue helping us unravel how the magnetic field drives these energetic electrons.”
- “Further investigating the role of the magnetic bottle in particle acceleration and transport will require more advanced modeling to compare with EOVSA’s observations,” said Chen. “There are certainly huge prospects out there for us to study that address these fundamental questions.”
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. 25) 26)
• 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). 27) 28) 29) 30) 31)
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. 32)
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 25: Principal optics package components of the HMI instrument (image credit: Stanford University)
Figure 26: Photo of the HMI instrument (image credit: NASA)
Figure 27: 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. 33)
Figure 28: HMI accommodation on SDO (image credit: Stanford University)
Figure 29: 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. 34) 35)
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. 32).
• 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 31: Illustration of a single AIA science telescope with quad selector (image credit: LMSAL)
Figure 32: AIA science telescope assembly (image: credit: LMSAL)
Figure 33: Optical layout of the AIA science telescope (image credit: LMSAL)
Figure 34: 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 30) 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 35 (Ref. 32).
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. 32).
Figure 36: 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. 36) 37) 38) 39)
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 37: 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 38: 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 39: Schematic view of the MEGS-A device (image credit: CU/LASP)
Table 9: Overview of MEGS-B parameters
Figure 40: 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 41: 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 42: 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 43: 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: 40)
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 44: Ka-band end-to-end data flow configuration (image credit: NASA)
Figure 45: S-band end-to-end data flow configuration (image credit: NASA)
Figure 46: 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).