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SDO (Solar Dynamics Observatory)

Spacecraft     Launch    Mission Status     Sensor Complement    Ground Segment    References

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?

The observation requirements are:

• To provide nearly continuous coverage of solar activity

• To provide coverage of the regimes (interior, photosphere, corona) in which the activity occurs

• To provide sufficient data on the types of phenomena which impact Earth, near-Earth space and humanity

• To observe the solar variability over the relevant timescales (seconds to years).


Figure 1: Artist's rendition of the deployed SDO spacecraft (image credit: NASA)


Figure 2: Top view of the SDO spacecraft (image credit: NASA)


The spacecraft is being designed and built at NASA/GSFC. The SDO design consists of a bus module and an instrument module (Figure 5); the instrument module employs a graphite composite structure to minimize thermal distortions The spacecraft bus module contains the S/C and instrument electronics. Redundant HGAs (High Gain Antennas) are mounted at the end of rigid booms (must be rigid due to required waveguides).

The spacecraft is 3-axis stabilized. The ACS (Attitude Control System) is a single-fault tolerant design. Its fully redundant attitude sensor complement includes 16 coarse sun sensors, a digital sun sensor (DSS), 3 two-axis inertial reference units (IRU), 2 star trackers (ST), and 4 guide telescopes. Attitude actuation is performed using 4 reaction wheel assemblies (RWA) and 8 thrusters, and a single main engine nominally provides velocity-change thrust. - The attitude control software has five nominal control modes: 3 wheel-based modes and 2 thruster-based modes. A wheel-based safehold running in the attitude control electronics box improves the robustness of the system as a whole. All six modes are designed on the same basic proportional-integral-derivative attitude error structure, with more robust modes setting their integral gains to zero. 8)

The ST and DSS combine to provide two-out-of-three single-fault tolerance fine attitude determination. Any one of the 4 AIA guide telescopes may be selected as the ACS CGT (Controlling Guide Telescope). Control is actuated using reaction wheel assemblies (RWA) and attitude control thrusters. Orbit change maneuvers can be accomplished using either the thrusters or a main engine, i.e. the RCS (Reaction Control Subsystem); the main engine will be used nominally for all long maneuvers performed in achieving geosynchronous orbit from the launch orbit.

The ACS supports five operational modes. These are: sun acquisition, inertial, science, ΔH and ΔV. One mode, namely safehold, operates solely in the ACE (Attitude Control Electronics) software. The SDO remains sun-pointing throughout most of its mission for the instruments to take measurements of the sun.


Figure 3: Overview of ACS components in the spacecraft (image credit: NASA)


Figure 4: Block diagram of the SDO attitude control electronics (image credit: NASA)

Onboard Ephemeris: The SDO onboard ephemeris predicts the locations of the Sun, Moon, spacecraft, and ground station in geocentric inertial coordinates, referred to the mean-equator-and-equinox of J2000 (GCI, mean-of-J2000). Each object's velocity is derived from differencing successive position vectors and dividing the result by the ephemeris task sample time (nominally 1 second). The solar ephemeris accuracy is better than 2 arcseconds during the 10 year SDO mission lifetime and has been validated by the JPL DE405 ephemeris.

The following key spacecraft technologies are being introduced:

• Ethernet chipset

• Ka-band transmitter

• APS (Active Pixel Sensor) star tracker.

SDO uses a bi-propellant propulsion system, an AKM (Apogee Kick Motor), to boost the spacecraft from a GTO (Geosynchronous Transfer Orbit) into a GSO (Geosynchronous Orbit). The Spacecraft design life is 5 years (10 years for expendables). The launch mass of SDO is about 3,200 kg.


Figure 5: Illustration of the SDO spacecraft (image credit: NASA)

Spacecraft mass

Total mass of the spacecraft at launch is 3200 kg (payload 270 kg, fuel 1400 kg)

Spacecraft dimensions

- The overall length along the sun-pointing axis is 4.5 m, and each side is 2.22 m
- The span of the extended solar panels is 6.25 m

Spacecraft power

Total available power is 1540 W from 6.5 m2 of solar arrays (efficiency of 16%)
Battery (ABSL): Li-ion with a capacity of 156 Ah, mass = 40 kg

Spacecraft orientation

The high-gain antennas rotate once each orbit to follow the Earth

Spacecraft design life

5 years (10 years for expendables)

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)

Launch: The SDO spacecraft was launched on February 11, 2010 on an Atlas-V vehicle from KSC at Cape Canaveral, FLA. The launch provider was ILS (International Launch Services). 9) 10)

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.

SDO mission status and imagery in the period 2018-2010

Mission status for the period of 2020-2019

• 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. 11)

- 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 9: 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. 12)

- 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. 13)

- 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 10: 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. 14) 15)

- 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 11: 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 12: 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 13: 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. 16)

- 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 14: 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. 17)

- 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. 18)

- 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 15). The kappa scheme can predict how a small magnetic reconnection triggers a large flare and how a large solar flare can occur.


Figure 15: 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 16). 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 16: 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). 19)

- 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. 20)

- 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 17: 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 18: 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.

Shocking Measurements

- 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.”

• 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. 21)

Figure 19: 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. 22)

- 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. 23)

- 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 20: 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 21: 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. 24)

- 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 22: 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 23: 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. 25)

- “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.”

Tsunami connections

- 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. 26)

- 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. 27)

- 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. 28)


Figure 24: 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 25: 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 26: 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. 29)

Figure 27: 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 28: 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. 30)

- 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. 31)

- "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 29: 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) 32)


Figure 30: 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. 33) 34)

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). 35) 36) 37) 38) 39)

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.

HMI Instrument:

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. 40)

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

- Shutters

- 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 31: Principal optics package components of the HMI instrument (image credit: Stanford University)


Figure 32: Photo of the HMI instrument (image credit: NASA)


Figure 33: Optical layout of the HMI instrument (image credit: Stanford University)

Center wavelength

6173.3 Å ± 0.1 Å(Fe I line)

Filter bandwidth, Filter tuning range

76 mÅ ± 10 mÅ FWHM, 680 mÅ ± 68 mÅ

Center wavelength drift

< 10 mÅ during any 1 hour period

FOV (Field of View), Angular resolution

> 2000 arcsec, < 1.5 arcsec

Focus adjustment range

±4 depths of focus

Pointing jitter reduction factor

> 40dB with servo bandwidth > 30 Hz

Image stabilization offset range

> ±14 arcsec in pitch and yaw

Pointing adjustment range

> ±200 arcsec in pitch and yaw

Pointing adjustment step size

< 2 arc-seconds in pitch and yaw

Dopplergram cadence, Image cadence for each camera

< 50 seconds, < 4 seconds

Full image readout rate

< 3.2 seconds

Exposure knowledge, Timing accuracy

< 5 µs, < 0.1 seconds of ground reference time

Detector format, Detector resolution

≥ 4000 x 4000 pixels, 0.50 ±0.01 arc-second / pixel

Science telemetry compression

To fit without loss in allocated telemetry

Eclipse recovery

< 60 minutes after eclipse end

Instrument design life

5 years

Allocated data rate for instrument

55 Mbit/s

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. 41)


Figure 34: HMI accommodation on SDO (image credit: Stanford University)


Figure 35: 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. 42) 43)

Themes of the AIA Investigation

1) Energy input, storage, and release: the 3-D dynamic coronal structure.
3-D configuration of the solar corona; mapping magnetic free energy; evolution of the corona towards unstable configurations; the life-cycle of atmospheric field

2) Coronal heating and irradiance:thermal structure and emission.
Contributions to solar (E)UV irradiance by types of features; physical properties of irradiance-modulating features; physical models of the irradiance-modulating features; physics-based predictive capability for the spectral irradiance

3) Transients: sources of radiation and energetic particles
Unstable field configurations and initiation of transients; evolution of transients; early evolution of CME's; particle acceleration

4) Connections to geospace: material and magnetic field output of the sun
Dynamic coupling of the corona and heliosphere; solar wind energetics; propagation of CMEs and related phenomena; vector field and velocity

5) Coronal seismology: a new diagnostic to access coronal physics
Evolution, propagation, and decay of transverse and longitudinal waves; probing coronal physics with waves; the role of magnetic topology in wave phenomena.

Requirement ->

Spatial coverage FOV
Δx=1 Mm

Temporal coverage

Thermal coverage

Intensity coverage

Science theme



Δ log T



Dynamic range

1) Energy input storage & release,
dynamic coronal structure

Full corona 40'-46'

~10 s

Full disk passage


0.7-8 MK (full corona)


Large for simultaneous obs. of faint & bright structures

2) Coronal heating & irradiance

Active regions

< 1 min, a few s in flares


0.3 for DEM investigation

0.7-20 MK(full corona)


> 1000

3) Transients,
Sources of radiation & energetic particles

Majority of disk

A few s in flares

At least days for buildup

~0.3 for T< 5MK, ~0.6 for T> 5MK

5000 K - 20 MK


> 1000 in quiescent channels

4) Connections to geospace

Full disk + off-limb

~ 10 s

Continuous observing

~ 0.3

5000 K - 20 MK

10% for thermal structure

Large to study high coronal field

5) Coronal seismology

Active regions

As short as possible

Continuous for discovery

~ 0.5 to limit LOS confusion

multi-T observations for thermal evolution

10% for density

> 10

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. 40).

• Full CCD readout in 2.5 seconds

• Reconfiguration of all mechanisms in 1 second (filter wheels, sector shutter, focal plane shutters)

• Onboard data compression via several lookup tables


Figure 36: Photo of the CCD device (image credit: ev2, LMSAL)

The AIA design provides the following instrument capabilities:

• Seven EUV (Extreme Ultraviolet) and three UV/visible channels. Four of the EUV wavelength bands open new perspectives on the solar corona, having never been imaged or imaged only during brief rocket flights. The set of six EUV channels that observe ionized iron allow the construction of relatively narrow-band temperature maps of the solar corona from below 1 MK to above 20 MK.

• A field of view (FOV) exceeding 41 arcmin (or 1.28 solar radii in the EW and NS directions), with 0.6 arcsec pixels

• A detector full well > 150,000 electrons and ~ 15 e/photon, with a camera readout noise of ≤ 25 electrons

• A sustained 10 second cadence during most of the mission

• A capability to adjust the observing program to changing solar conditions in order to implement observing programs that are optimized to meet the requirements of specific scientific objectives. This allows, for example, a 2 second cadence in a reduced field of view for flare studies.

• Provision of images in multiple EUV and UV pass bands. The basic observables are full-sun intensities at a range of wavelengths. Together, these will comprise the data archive, which is freely accessible to the research community and, with limitations dictated by resources, to other interested parties.

Derived data products, such as coronal thermal charts, maps of variability, and comparisons to HMI magnetograms and to (non-)potential field extrapolations will be made available regularly through the data-processing pipeline for a subset of the data for use in evaluation of the data and to aid the discovery of phenomena and cataloging of events. Software will be made available to researchers to create these data products for other datasets; a core library of easy-to-use, publicly-available software will be developed as part of the SolarSoft IDL environment to enable and support the investigations that are required to meet the primary AIA science goals

Band name

Δλ (Å), FWHM

Primary ion(s)

Region of the sun's atmosphere

Charac.log temperature (K)






1700 Å



Temperature minimum, photosphere


304 Å



Chromosphere, transition region


1600 Å


C IV+ continuum

Transition region+ upper photosphere


171 Å



Quiet corona, upper transition region


193 Å



Corona and hot flare plasma

6.1, 7.3

211 Å



Active region corona


335 Å



Active region corona


94 Å



Flaring regions


151 Å



Flaring regions

7.0, 7.2

Table 4: Definition of AIA instrument spectral bands

Implementation requirement


High Angular Resolution

~0.6 arcsec pixels

Large FOV (field and irradiance)

full sun + 2 pressure scale heights

Large dynamic range

> 1000

Complete coronal temperature coverage

~105 -107 K in 6 EUV Fe-line channels


UV/WL (White Light) and He II 304 Å imaging

Adequate photo-/chromospheric coverage

10 s baseline cadence, 2 s fastest

Time resolution (dynamics and irradiance)

Brightness histogram feedback

Dynamic exposure control

Continuous observations up to many weeks, spanning half a cycle

Long-term coverage

Adequate aperture, filters, detector system

Table 5: AIA instrument design characteristics


Figure 37: Illustration of a single AIA science telescope with quad selector (image credit: LMSAL)


Figure 38: AIA science telescope assembly (image: credit: LMSAL)


Figure 39: Optical layout of the AIA science telescope (image credit: LMSAL)


Figure 40: 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 36) 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 41 (Ref. 40).

The upper-most card carries four video processing and digitization ASICs operating in parallel at 2 Mpixel/s and each connected to one of the CCD's quadrant readout amplifiers. The second card in the stack provides all of the CCD's low-noise DC bias voltages. Supplies to each of the CCD's output amplifiers are buffered separately to minimize crosstalk between channels. An 8-channel 10-bit DAC ASIC enables software programming and fine adjustment of the bias supplies. Telemetry circuitry internal to the CEB allows monitoring of the CEB's secondary power rails, CCD bias voltages and the CCD and CEB operating temperatures. The third card carries a waveform generator and sequencer ASIC and sufficient clock driver buffers to enable CCD readout through any or all of its quadrant readout amplifiers. The final card provides a SpaceWire communications interface with the main AIA or HMI control electronics. A single link is used for programming the CEB's ASICs and registers, commanding a CCD readout and the return of the CCD's digitized video data at 200 Mbit/s.


Figure 41: Photo of the CEB (Camera Electronics Box), image credit: RAL

A key component of the camera electronics is a custom-designed and space-qualified CCD video signal processing and digitization ASIC. It provides 2 Mpixel/s video amplification, CDS processing and 16 bit digitization of a 1 V input signal. The design is fully-differential to aid rejection of common-mode noise. A 10-bit DAC enables ± 500 mV of programmable DC offset to be introduced into the video signal and a 7-bit programmable x1-x3 gain amplifier enables the ADC to be matched to the required CCD signal swing. The ADC is a 16 bit fully-differential pipelined converter using feedback capacitor switching in the amplifier stages, and over-ranging at intervals in order to minimize differential non-linearity due to capacitor mismatching and amplifier gain errors. Triple-voting logic is used to enhance the single-event upset tolerance of the logic and registers. The ASIC was manufactured on a 0.35 µm 3.3 V CMOS process known for its excellent tolerance to ionizing radiation. With its inputs grounded, the ASIC's noise is 3.5 ADU rms in 16 bits or 53 µV rms. The CCD provides ~ 4.5 µV/ e- and so the equivalent noise is ~ 12 e- rms. The combined noise floor of the CCD and electronics is ~ 4 ADU rms or ~ 16 e- rms. The power consumption from a 3.3 V supply is 400 mW (Ref. 40).


Figure 42: 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. 44) 45) 46) 47)

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.




Spectral range (nm)


Reflective grating spectrograph

0.1 nm

5 - 36


Reflective grating spectrograph

0.1 nm

35 - 105


Solar aspect monitor

0.002-1 nm

0 - 7


Set of filter photometers
H I Lyman-α proxy for other H I emissions
H e I Lyman-α proxy for other He I emissions

5 nm
5 nm

H I 121.6, He 58.4


EUV Spectrophotometer

4 nm
7 nm

17.5, 25.6, 30.4, 36, 58.4
0-7 (zeroth order)

Table 6: Overview of EVE instrument modules and measurements

Instrument mass, power

54.2 kg, 47.2 W (average)

Instrument size

99 cm x 61 cm x 36 cm

Data rate

2 kbit/s (engineering), 7 Mbit/s (science)

Table 7: EVE instrument parameters


Figure 43: 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 detectors:

- 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 44: 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.

Wavelength coverage (λ)

5 - 37 nm

Δλ resolution

0.1 nm

Time cadence

10 s


± 2º

Aperture door


Filter wheel

5 positions

CCD detector

1024 x 2048 pixels

Power, data rate

11 W, 3.4 Mbit/s

Table 8: Overview of MEGS-A parameters


Figure 45: Schematic view of the MEGS-A device (image credit: CU/LASP)

Wavelength coverage (λ)

34 - 105 nm

Δλ resolution

0.1 nm

Time cadence

10 s


± 2º

Aperture door


Filter wheel

5 positions

CCD detector

1024 x 2048 pixels

Power, data rate

11 W, 3.4 Mbit/s

Table 9: Overview of MEGS-B parameters


Figure 46: 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.

Wavelength coverage (λ)

0.1 - 7 nm

Δλ resolution

0.01 - 1 nm

Time cadence

10 s


± 2º

Aperture door


Filter wheel

5 positions

Table 10: Overview of MEGS-SAM parameters


Figure 47: 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

Wavelength coverage (λ)

121.6 nm

Δλ resolution

1 nm

Time cadence

0.25 s

FOV (Field of View)

± 2º

Aperture door

Behind MEGS-B mechanism

Filter wheel

Behind MEGS-B mechanism

Si photodiode

1 cm x 1 cm

Power, data

0.2 W, 1 kbit/s

Table 11: Parameters of the MEGS-P device


Figure 48: 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.

Wavelength coverage (λ)

- 1st order at 18.4, 25.5, 30.4, 35.5 nm
- Zero (0 th) order: 0.1-7 nm

Δλ resolution

1st: 2 nm; 0th: 7 nm

Time cadence

0.25 s

FOV (Field of View)

± 2º

Aperture door


Filter wheel

5 positions

Si photodiodes

0.6 cm x 1.6 cm


1.9 W, 7 kbit/s

Table 12: Parameters of the ESP device


Figure 49: 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: 48)

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
through the DDS/SDOGS Interface Manager (DSIM) which is part of the DDS design

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 50: Ka-band end-to-end data flow configuration (image credit: NASA)


Figure 51: S-band end-to-end data flow configuration (image credit: NASA)


Figure 52: 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 (

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