SDO (Solar Dynamics Observatory)
SDO is a NASA satellite, considered to be a second-generation solar mission (also referred to as SOHO successor). SDO represents the first mission within NASA's LWS (Living With a Star) program, a space weather-focused and applications-driven research program. The goal of LWS is to understand the sun as a magnetic variable star and to measure its impact on life and society on Earth.
The overall SDO objective is to observe the dynamics of the solar interior, provide data on the sun's magnetic field structure, characterize the release of mass and energy from the sun into the heliosphere, and monitor variations in solar irradiance. The goal is to understand the dynamic state of the sun (its variability) on multiple temporal and spatial scales which influence life and technology on Earth - to enable the development of an operational capability for space weather prediction (the purpose of the LWS Program). 1) 2) 3) 4) 5)
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: 6)
1) What mechanisms drive the quasi-periodic 11-year cycle of solar activity?
2) How is active region magnetic flux synthesized, concentrated, and dispersed across the solar surface?
3) How does magnetic reconnection on small scales reorganize the large-scale field topology and current systems and how significant is it in heating the corona and accelerating the solar wind?
4) Where do the observed variations in the Sun‘s EUV spectral irradiance arise, and how do they relate to the magnetic activity cycles?
5) What magnetic field configurations lead to the coronal mass ejections (CMEs), filament eruptions, and flares that produce energetic particles and radiation?
6) Can the structure and dynamics of the solar wind near Earth be determined from the magnetic field configuration and atmospheric structure near the solar surface?
7) When will activity occur, and is it possible to make accurate and reliable forecasts of space weather and climate?
• To provide nearly continuous coverage of solar activity
• To provide coverage of the regimes (interior, photosphere, corona) in which the activity occurs
• To provide sufficient data on the types of phenomena which impact Earth, near-Earth space and humanity
• To observe the solar variability over the relevant timescales (seconds to years).
Figure 1: Artist's rendition of the deployed SDO spacecraft (image credit: NASA)
Figure 2: Top view of the SDO spacecraft (image credit: NASA)
The spacecraft is being designed and built at NASA/GSFC. The SDO design consists of a bus module and an instrument module (Figure 5); the instrument module employs a graphite composite structure to minimize thermal distortions The spacecraft bus module contains the S/C and instrument electronics. Redundant HGAs (High Gain Antennas) are mounted at the end of rigid booms (must be rigid due to required waveguides).
The spacecraft is 3-axis stabilized. The ACS (Attitude Control System) is a single-fault tolerant design. Its fully redundant attitude sensor complement includes 16 coarse sun sensors, a digital sun sensor (DSS), 3 two-axis inertial reference units (IRU), 2 star trackers (ST), and 4 guide telescopes. Attitude actuation is performed using 4 reaction wheel assemblies (RWA) and 8 thrusters, and a single main engine nominally provides velocity-change thrust. - The attitude control software has five nominal control modes: 3 wheel-based modes and 2 thruster-based modes. A wheel-based safehold running in the attitude control electronics box improves the robustness of the system as a whole. All six modes are designed on the same basic proportional-integral-derivative attitude error structure, with more robust modes setting their integral gains to zero. 7)
The ST and DSS combine to provide two-out-of-three single-fault tolerance fine attitude determination. Any one of the 4 AIA guide telescopes may be selected as the ACS CGT (Controlling Guide Telescope). Control is actuated using reaction wheel assemblies (RWA) and attitude control thrusters. Orbit change maneuvers can be accomplished using either the thrusters or a main engine, i.e. the RCS (Reaction Control Subsystem); the main engine will be used nominally for all long maneuvers performed in achieving geosynchronous orbit from the launch orbit.
The ACS supports five operational modes. These are: sun acquisition, inertial, science, ΔH and ΔV. One mode, namely safehold, operates solely in the ACE (Attitude Control Electronics) software. The SDO remains sun-pointing throughout most of its mission for the instruments to take measurements of the sun.
Figure 3: Overview of ACS components in the spacecraft (image credit: NASA)
Figure 4: Block diagram of the SDO attitude control electronics (image credit: NASA)
Onboard Ephemeris: The SDO onboard ephemeris predicts the locations of the Sun, Moon, spacecraft, and ground station in geocentric inertial coordinates, referred to the mean-equator-and-equinox of J2000 (GCI, mean-of-J2000). Each object's velocity is derived from differencing successive position vectors and dividing the result by the ephemeris task sample time (nominally 1 second). The solar ephemeris accuracy is better than 2 arcseconds during the 10 year SDO mission lifetime and has been validated by the JPL DE405 ephemeris.
The following key spacecraft technologies are being introduced:
• Ethernet chipset
• Ka-band transmitter
• APS (Active Pixel Sensor) star tracker.
SDO uses a bi-propellant propulsion system, an AKM (Apogee Kick Motor), to boost the spacecraft from a GTO (Geosynchronous Transfer Orbit) into a GSO (Geosynchronous Orbit). The Spacecraft design life is 5 years (10 years for expendables). The launch mass of SDO is about 3,200 kg.
Figure 5: Illustration of the SDO spacecraft (image credit: NASA)
Table 1: Overview of spacecraft parameters
Figure 6: Photo of the integrated SDO spacecraft (image credit: NASA)
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 7: 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.
• On January 30, 2014, the SDO got its own private solar eclipse showing from its geosynchronous orbital perch. Twice a year during new phase, the moon glides in front of the sun from the observatory’s perspective. The events are called lunar transits rather than eclipses since they’re seen from outer space. Transits typically last about a half hour, but at 2.5 hours, today’s was one of the longest ever recorded. The next one occurs on July 26, 2014. 10)
Figure 8: A lunar transit across the sun as seen by the SDO in six different color-coded wavelengths on January 30, 2014 (image credit: NASA, Universe Today)
Legend to Figure 8: The times of each photo are given in CST (Central Standard Time). In the last frame, the moon is silhouetted against the solar corona. At maximum about 90% of the sun was covered.
• January 07, 2014: An enormous sunspot, labeled AR1944 (Active Region 1944), slipped into view over the sun's left horizon late on Jan. 1, 2014. The sunspot steadily moved toward the right, along with the rotation of the sun, and now sits almost dead center, as seen in the image (Figure 9) from NASA's Solar Dynamics Observatory.
Sunspots are dark areas on the sun's surface that contain complex arrangements of strong magnetic fields that are constantly shifting. The largest dark spot in this configuration is approximately two Earths wide, and the entire sunspot group is some seven Earths across. 11)
Sunspots are part of what's known as active regions, which also include regions of the sun's atmosphere, the corona, hovering above the sunspots. Active regions can be the source of some of the sun's great explosions: solar flares that send out giant bursts of light and radiation due to the release of magnetic energy, or coronal mass ejections that send huge clouds of solar material out into space. As the sunspot group continues its journey across the face of the sun, scientists will watch how it changes and evolves to learn more about how these convoluted magnetic fields can cause space weather events that can affect spaceborne systems and technological infrastructure on Earth.
- On January 7, 2014 (18:18:34 UTC), as the giant AR1944 sunspot turned toward Earth, it erupted with a powerful X1.2-class flare.
Figure 9: One of the largest sunspots in the last nine years, labeled AR1944, was seen in early January 2014, as captured by NASA's SDO. An image of Earth has been added for scale (image credit: NASA)
• Nov. 9, 2013: The Sun is currently acting like it’s in solar maximum. Our Sun has emitted dozens of solar flares in since Oct. 23, 2013, with at least six big X-class flares.
Figure 10: The AIA instrument of SDO captured this image of the sun showing an X1.1 class flare on Nov. 8, 2013 (image credit: NASA)
• On October 21, 2013, NASA successfully launched a Black Brant IX sounding rocket at 10:00 hours UTC from the White Sands Missile Range, N.M., carrying instrumentation to support the calibration of the EVE (EUV Variability Experiment) aboard SDO. EVE measures the total extreme ultraviolet output of the sun, called its irradiance. 12)
As part of the planned SDO/EVE program, the rocket calibration flight occurs about once a year to accurately determine the long-term variations of the solar extreme ultraviolet irradiance. This kind of calibration is known as an under-flight. It uses a near-replica of the SDO/EVE instrument to gather a calibrated sounding rocket observation in coordination with the orbital satellite's observations.
Comparison of the two data sets then validates the accuracy of the SDO/EVE data, providing crucial calibration of any long-term changes in the orbital instrumentation. This was the fourth under-flight calibration for the EVE instrument. The previous flight was successfully conducted on June 23, 2012.
• May 15, 2013: According to “SpaceWeather.com,” AR1748 (sunspot active region 1748) has produced ”the strongest flares of the year so far, and they signal a significant increase in solar activity.” In only two days, sunspot AR1748 has produced four X-flares. The latest X-flare from this active sunspot occured on May 15th at 0152 UT. NASA's SDO (Solar Dynamics Observatory) captured the extreme ultraviolet flash. 13)
The AR1748 has produced an X1.7-class flare (0217 UT on May 13), an X2.8-class flare (1609 UT on May 13), an X3.2-class flare (0117 UT on May 14), and an X1-class flare (0152 on May 15). These are the strongest flares of the year, and they signal a significant increase in solar activity. 14)
Figure 11: An X3.2-class flare observed by SDO’s AIA instrument at 0114 UT, 14 May 2013 (NASA/SDO/AIA)
Legend to Figure 12: SID events make themselves known by the effect they have on low-frequency radio signals. When a SID passes by, the atmosphere overhead becomes a good reflector for radio waves, allowing signals to be received from distant transmitters. Battaiola monitored a faraway 21.75 kHz radio station to receive the SIDs over his location.
• On April 3, 2013 the SDO project performed an HMI roll maneuver. The entire SDO spacecraft is spun around so that HMI can verify its operation and measure some calibration data. 15)
- Twice a year, SDO performs a 360º roll maneuver about the axis on which it points toward the Sun. This produces some unique views; the rolls are necessary to help calibrate the instruments, particularly the HMI instrument, which is making precise measurements of the solar limb to study the shape of the Sun. The rolls also help the science teams to know how accurately the images are aligned with solar north. 16)
• The SDO mission and its payload are operating nominally in 2013. On Feb. 11, 2013, SDO was 3 years on orbit providing an enornous amount of information. 17)
Legend to Figure 13: These six images from SDO, chosen to show a representative image about every six months, track the rising level of solar activity since the mission first began to produce consistent images in May, 2010. The period of solar maximum is expected in 2013. The images were taken in the 171 Angstrom wavelength of extreme ultraviolet light.
• On April 4, 2012, the SDO spacecraft performed a 360º spin. It rolled completely about its axis– something it does twice a year . This maneuver helps the HMI (Helioseismic and Magnetic Imager ) instrument, one of three instruments onboard SDO, take measurements of the solar limb to study the shape of the sun. The roll helps scientists remove optical distortions from the images and to precisely determine the boundaries of the sun's horizon, or "limb". Accumulated over time, such data shows whether the sun's sphere changes in concert with the 11-year solar cycle, during which the sun moves through periods of greater and lesser activity as evidenced by the changing frequency of giant solar eruptions. 19)
• In 2012, the spacecraft and its payload are operating nominally. On Feb. 11, 2012, the SDO spacecraft was 2 years on orbit.
The SDO mission serves as a clear example of the importance of the systems engineering role across all phases of the mission development lifecycle and its contribution to mission success. The best metric to evaluate the success of this approach is in the successful launch and the on-orbit performance of the SDO mission itself, which, despite challenging technical drivers and development obstacles along way, is currently enabling ground-breaking science after only two years into its five-year mission life (Ref. 6).
Figure 14: AIA instrument image observed at 171 Ä showing the current conditions of the quiet corona and upper transition region of the Sun (image credit: NASA) 20)
• Sept. 2011 (the late phase of solar flares): Over the course of a year, the science team used the EVE (Extreme ultraviolet Variability Experiment) instrument on SDO to record data from many flares. EVE doesn't snap conventional images. T. Woods is the principal investigator for the EVE instrument and he explains that it collects all the light from the sun at once and then precisely separates each wavelength of light and measures its intensity. This doesn't produce pretty pictures the way other instruments on SDO do, but it provides graphs that map out how each wavelength of light gets stronger, peaks, and diminishes over time. EVE collects this data every 10 seconds, a rate guaranteed to provide brand new information about how the sun changes, given that previous instruments only measured such information every hour and a half or didn't look at all the wavelengths simultaneously – not nearly enough information to get a complete picture of the heating and cooling of the flare.
Recording extreme ultraviolet light, the EVE spectra showed four phases in an average flare’s lifetime (Figure 15). The first three have been observed and are well established (though EVE was able to measure and quantify them over a wide range of light wavelengths better than has ever been done).
- The first phase is the hard X-ray impulsive phase, in which highly energetic particles in the sun’s atmosphere rain down toward the sun’s surface after an explosive event in the atmosphere known as magnetic reconnection. They fall freely for some seconds to minutes until they hit the denser lower atmosphere, and then the second phase, the gradual phase, begins.
- Second phase: Over the course of minutes to hours, the solar material, called plasma, is heated and explodes back up, tracing its way along giant magnetic loops, filling the loops with plasma. This process sends off so much light and radiation that it can be compared to millions of hydrogen bombs.
- The third phase is characterized by the sun's atmosphere — the corona — losing brightness, and so is known as the coronal dimming phase. This is often associated with what's known as a CME (Coronal Mass Ejection), in which a great cloud of plasma erupts off the surface of the sun.
But the fourth phase, the late phase flare, spotted by EVE was new. Anywhere from one to five hours later for several of the flares, they saw a second peak of warm coronal material that didn't correspond to another X-ray burst. The late phase turns out to be different, the emissions happen substantially later, and it happens after the main flare exhibits that initial peak. 21) 22)
To try to understand what was happening, the team looked at the images collected from SDO's AIA (Advanced Imaging Assembly) as well. They could see the main phase flare eruption in the images and also noticed a second set of coronal loops far above the original flare site. These extra loops were longer and become brighter later than the original set (or the post-flare loops that appeared just minutes after that). These loops were also physically set apart from those earlier ones.
Figure 15: Graph of the EVE spectra showing the total intensity of any given EUV wavelength of light coming off of the sun (image credit: NASA)
Legend of Figure 15: This image shows a single moment from May 5, 2010. Instead of a conventional picture, the EVE produces graphs like this, called spectra, that show the total intensity of any given EUV wavelength of light coming off of the sun.
The intensity, the project recorded in those late phase flares, is usually dimmer than the X-ray intensity. But the late phase goes on much longer, sometimes for multiple hours, so it's putting out just as much total energy as the main flare that typically only lasts for a few minutes. Because this previously unrealized extra source of energy from the flare is equally important to impacting Earth’s atmosphere, Woods and his colleagues are now studying how the late phase flares can influence space weather (Ref. 21).
• In 2011, the spacecraft and its payload are operating nominally.
Figure 16: SDO image of the sun taken on Jan. 10, 2011 with the AIA instrument in the EUV range (image credit: NASA)
Legend to Figure 16: The image captures a dark coronal hole just about at sun center. Coronal holes are areas of the sun's surface that are the source of open magnetic field lines that head way out into space. They are also the source regions of the fast solar wind, which is characterized by a relatively steady speed of approximately 800 km/s. As the sun continues to rotate, the high speed solar wind particles blowing from this hole will likely reach Earth in a few days and may spark some auroral activity. 23)
• On Aug.1, 2010, a most unusual solar event occurred. Nearly the entire Earth-facing side of the Sun erupted in a tumult of activity, comprising a large solar flare, a solar tsunami, multiple filaments of magnetism lifting off the solar surface, radio bursts and half a dozen coronal mass ejections (CMEs). At the same time, NASA's three solar spacecraft, SDO and the two STEREO spacecraft, were ideally positioned to capture both the action on the Earth-facing side of the Sun, and most activity around the backside, leaving a wedge of only 30 degrees of the solar surface unobserved. 24) 25)
Explosions on the sun are not localized or isolated events, according to Karel Schrijver and Alan Title of LMSAL (Lockheed Martin, Solar & Astrophysics Laboratory). Instead, solar activity is interconnected by magnetism over breathtaking distances. Solar flares, tsunamis, coronal mass ejections--they can go off all at once, hundreds of thousands of miles apart, in a dizzyingly-complex concert of mayhem.
For several decades, scientists studying the sun have observed solar flares that appear to occur almost simultaneously but originated in completely different areas on the Sun. Solar physicists called them “sympathetic” flares, but it was thought these near-synchronous explosions in the solar atmosphere were too far apart – sometimes millions of kilometers distant – to be related. But now, with the continuous high-resolution and multi-wavelength observations with the SDO, combined with views from the twin STEREO spacecraft, the scientists are seeing how these sympathetic eruptions — sometimes on opposite sides of the sun — can connect through looping lines of the Sun’s magnetic field. 26)
Figure 17: Locations of key events are labeled in this extreme ultraviolet image of the sun, obtained by the Solar Dynamics Observatory during the Great Eruption of Aug. 1, 2010. White lines trace the sun's magnetic field (image credit: Karel Schrijver and Alan Title of LMSAL)
• On May 14, 2010, SDO passed a major milestone when it completed its post-launch check out (end of commissioning phase) and officially began its five-year science mission to study the sun (phase E). The project at NASA/GSFC declared SDO an operational mission. All of the instruments and the spacecraft are performing extremely well. SDO is now sending 1.5 TB of data/day to Earth, and will continue to do so at least until the end of the prime phase of the mission in 2015. 27) 28)
The SDO has allowed scientists for the first time to comprehensively view the dynamic nature of storms on the sun. Solar storms have been recognized as a cause of technological problems on Earth since the invention of the telegraph in the 19th century. 29)
• On April 19, 2010, SDO observed a massive eruption on the sun — one of the biggest in years. Astronomers have seen eruptions like this before, but rarely so large and never in such fluid detail. Coronal rain has long been a mystery. It's not surprising that plasma should fall back to the sun. After all, the sun's gravity is powerful. The puzzle of coronal rain is how slowly it seems to fall. The rain appears to be buoyed by a 'cushion' of hot gas.
Using the AIA (Atmospheric Imaging Assembly) instrument with an array of ultraviolet telescopes, SDO can remotely measure the temperature of gas in the sun's atmosphere. Coronal rain turns out to be relatively cool—"only" 60,000 K. When the rains falls, it is supported, in part, by an underlying cushion of much hotter material, between 1,000,000 and 2,200,000 K. 30)
Figure 18: Coronal rain. Encircled are two plasma streamers, one hitting the sun's surface and another incoming behind it (image credit: NASA)
Figure 19: A full-disk multiwavelength extreme ultraviolet image of the sun taken by the AIA instrument on March 30, 2010 (image credit: NASA)
Legend to Figure 19: False colors trace different gas temperatures. Reds are relatively cool (~60,000 K); blues and greens are hotter (> 1,000,000 K). SDO is able to monitor not just one small patch of sun, but rather the whole thing--full disk, atmosphere, surface, and even interior. 31)
Figure 20: An erupting prominence observed by the AIA instrument on March 30, 2010 (image credit: NASA)
• Following several precise propulsion burns to circularize its orbit, SDO arrived “on station” on March 16, 2010. All systems are operating nominally.
Sensor complement: (HMI, AIA, EVE)
The SDO sensor complement consists of three instruments which are pointed toward the sun to provide continuous, high cadence (cyclic) observations of the full solar disk and coronal imaging in multiple wavelengths to improve the understanding and forecasting of the sun's impact on our terrestrial environment. 32) 33)
• 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). 34) 35) 36) 37) 38)
Note: Since the two instruments, HMI and AIA, are both being developed at LMSAL, there is a lot of organizational synergism and cooperation between the two instruments on all levels.
HMI makes interference measurements of the motion of the solar photosphere to study solar oscillations and measurements of the polarization in a spectral line to study all three components of the photospheric magnetic field. HMI produces data to determine the interior sources and mechanisms of solar variability and how the physical processes inside the sun are related to surface magnetic field and activity. It also produces data to enable estimates of the coronal magnetic field for studies of variability in the extended solar atmosphere. HMI observations will enable establishing the relationships between the internal dynamics and magnetic activity in order to understand solar variability and its effects, leading to reliable predictive capability, one of the key elements of the LWS (Living With a Star) program.
The HMI observation goals are being addressed in a coordinated investigation in a number of parallel studies:
• Convection-zone dynamics and the solar dynamo
• Origin and evolution of sunspots, active regions and complexes of activity
• Sources and drivers of solar activity and disturbances
• Links between the internal processes and dynamics of the corona and heliosphere
• Precursors of solar disturbances for space-weather forecasts.
HMI will observe the full solar disk in the Fe I absorption line at 6173 Å (goal of 1 arcsecond resolution). The HMI instrument will produce measurements in the form of filtergrams in a set of polarizations and spectral line positions at a regular cadence for the duration of the mission that meet these basic requirements:
8) Full-disk Doppler velocity and line-of-sight magnetic flux images with 1.5 arcsec resolution at least every 50 seconds
9) Full-disk vector magnetic images of the solar magnetic field with 1.5 arc-sec resolution at least every 10 minutes.
The primary observables (Dopplergrams, longitudinal and vector magnetograms, and continuum intensity images) will be constructed from the raw filtergrams and will be made available at full resolution and cadence. Other derived products such as subsurface flow maps, far-side activity maps, and coronal and solar wind models that require longer sequences of observations shall be produced and made available.
In effect the solar turbulence is analogous to earthquakes. In manner similar to how seismologists can learn about the interior of the Earth by studying the waves generated in an earthquake. HMI's helioseismologists learn about the structure, temperature and flows in the solar interior.
The HMI instrument consists of a refracting telescope, a polarization selector, an image stabilization system (ISS), a narrow-band tunable filter. In addition, there are two 4096 x 4096 pixel CCD cameras with mechanical shutters and control electronics. The twin cameras of HMI operate independently. One is referred to as the “Doppler camera“; the objective is to measure the line-of-sight component of the magnetic field and velocity vectors. The second camera is referred to as “Magnetic camera”; the objective is to measure the vector magnetic field and line of sight velocities. 39)
The optics package consists of the following elements:
- Telescope section
- Polarization selectors - 3 rotating waveplates for redundancy
- Focus blocks
- ISS (Image Stabilization System)
- 5 element Lyot filter. One element tuned by rotating waveplate
- 2 tunable Michelson interferometers. 2 waveplates and 1 polarizer for redundancy
- Reimaging optics and beam distribution system
- 2 functionally identical CCD cameras - “Doppler” and “Magnetic”
The combined Lyot-Michelson filter system in HMI produces a transmission profile with a FWHM of 76 mÅ. The tuning positions are 69 mÅ apart from each other.
Figure 21: Principal optics package components of the HMI instrument (image credit: Stanford University)
Figure 22: Photo of the HMI instrument (image credit: NASA)
Figure 23: Optical layout of the HMI instrument (image credit: Stanford University)
Table 2: Overview of HMI observation requirements
PCU (Polarization Calibration Unit):
HMI polarization calibration requires the input of fixed polarization states into the instrument and the measurement of the observed parameters with the HMI. The PCU creates the polarization states by using a linear polarizer and retarder (wave plate) that can be inserted into the optical path and rotated independently. The PCU consists of a TCP/IP control interface (Newport XPS-C4) and two mechanical units (size: 787 mm x 508 mm x 203 mm), with 175 mm clear apertures that house the polarization optics. Each mechanical unit contains a linear and a rotational stage. The linear stages (Newport IMS300CC) move the polarization optics into and out of the optical path with a linear position resolution of 1.25 microns. The rotational stages (Newport RV240CC) move the calibration optics to any given angle with a resolution of 0.001º. 40)
Figure 24: HMI accommodation on SDO (image credit: Stanford University)
Figure 25: Functional block diagram of the HMI (image credit: Stanford University)
AIA (Atmospheric Imaging Assembly):
The AIA instrument is being designed and developed at LMSAL (Lockheed Martin Solar and Astrophysics Laboratory), Palo Alto, CA; (PI: Alan Title, LMSAL). The AIA science team includes scientists and engineers from many national and international institutions. The SAO (Smithsonian Astrophysical Observatory) has a major role in the AIA program.
The objective is to provide an unprecedented view of the solar corona, taking images that span at least 1.3 solar diameters in multiple wavelengths nearly simultaneously, at a resolution of about 1 arcsec and at a cadence of 10 seconds or better. The primary goal of the AIA science investigation is to use these data, together with data from other SDO instruments, as well as from other observatories, to significantly improve our understanding of the physics behind the activity displayed by the sun's atmosphere, which drives space weather in the heliosphere and in planetary environments. 41) 42)
Themes of the AIA Investigation
1) Energy input, storage, and release: the 3-D dynamic coronal structure.
2) Coronal heating and irradiance:thermal structure and emission.
3) Transients: sources of radiation and energetic particles
4) Connections to geospace: material and magnetic field output of the sun
5) Coronal seismology: a new diagnostic to access coronal physics
Table 3: Overview of AIA observation requirements for various science themes
AIA instrument design overview:
• Four ST (Science Telescopes), each with 8 science channels
- 7 EUV channels in a sequence of Fe line and He 304 Å
- 1 UV channel with CTN, 1600 Å, 1700 Å filters
• Active secondaries for image stabilization. Each ST is equipped with an ISS (Image Stabilization System)
• Four GT (Guide Telescopes)
• Four 4096 x 4096 pixel thinned back-illuminated CCDs (the sampling of 0.6 arcsec requires a 4096 x 4096 pixel detector). Note, the AIA and HMI CCDs: a 4096 x 4096 pixel science-grade CCD with 12 µm pixel pitch developed by ev2 and RAL, are currently the largest CCD to have ever flown in a space mission (Ref. 39).
• 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 26: Photo of the CCD device (image credit: ev2, LMSAL)
The AIA design provides the following instrument capabilities:
• Seven EUV (Extreme Ultraviolet) and three UV/visible channels. Four of the EUV wavelength bands open new perspectives on the solar corona, having never been imaged or imaged only during brief rocket flights. The set of six EUV channels that observe ionized iron allow the construction of relatively narrow-band temperature maps of the solar corona from below 1 MK to above 20 MK.
• A field of view (FOV) exceeding 41 arcmin (or 1.28 solar radii in the EW and NS directions), with 0.6 arcsec pixels
• A detector full well > 150,000 electrons and ~ 15 e/photon, with a camera readout noise of ≤ 25 electrons
• A sustained 10 second cadence during most of the mission
• A capability to adjust the observing program to changing solar conditions in order to implement observing programs that are optimized to meet the requirements of specific scientific objectives. This allows, for example, a 2 second cadence in a reduced field of view for flare studies.
• Provision of images in multiple EUV and UV pass bands. The basic observables are full-sun intensities at a range of wavelengths. Together, these will comprise the data archive, which is freely accessible to the research community and, with limitations dictated by resources, to other interested parties.
Derived data products, such as coronal thermal charts, maps of variability, and comparisons to HMI magnetograms and to (non-)potential field extrapolations will be made available regularly through the data-processing pipeline for a subset of the data for use in evaluation of the data and to aid the discovery of phenomena and cataloging of events. Software will be made available to researchers to create these data products for other datasets; a core library of easy-to-use, publicly-available software will be developed as part of the SolarSoft IDL environment to enable and support the investigations that are required to meet the primary AIA science goals
Table 4: Definition of AIA instrument spectral bands
Table 5: AIA instrument design characteristics
Figure 27: Illustration of a single AIA science telescope with quad selector (image credit: LMSAL)
Figure 28: AIA science telescope assembly (image: credit: LMSAL)
Figure 29: Optical layout of the AIA science telescope (image credit: LMSAL)
Figure 30: AIA telescope array mounted on IM (Instrument Module), image credit: LMSAL
AIA camera systems:
• The camera systems with CCD detectors are key elements of HMI & AIA. The HMI and AIA instrument use identical cameras and CCDs except that the AIA CCDs are back-side thinned.
• Each CCD detector array has a size of 4096 x 4096 pixels with 12 µm pixels (they were provided by e2v technologies ltd., Chelmsford, Essex, UK)
• CEB (Camera Electronics Box): 8 Mpixel/s via 2 Mpixel/s from 4 ports simultaneously
The AIA instrument has a data rate allocation of 67 Mbit/s (max, using data compression). The data is communicated over the IEEE 1355 high-rate science data bus (SpaceWire).
Camera readout electronics: Each AIA and HMI CCD (Figure 26) is driven and read out through its own dedicated CEB (Camera Electronics Box). It has dimensions of 152 mm x 131 mm x 95 mm and a mass of 2.9 kg. The enclosure walls are 5 mm thick aluminum to ensure sufficient attenuation of space radiation over mission life. During exposures the CCD and CEB consumes 12 W rising to 17 W during readout. The CEB contains four electronics cards mounted above a separately screened input filter and DC-DC power converter. A photo of the assembled unit, minus front panel and lid, is reproduced in Figure 31 (Ref. 39).
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 31: 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. 39).
Figure 32: Photo of the AIA telescope array (image credit: NASA)
EVE (EUV Variability Experiment)
The Extreme ultraviolet Variability Experiment (EVE) has been designed and developed at LASP (Laboratory for Atmospheric and Space Physics) of the University of Colorado (CU) at Boulder, CO (PI T. Woods). The science team consists of members from: CU/LASP, USC (university of Southern California), NRL (Naval Research Laboratory), MIT/LL (Massachusetts Institute of Technology/ Lincoln Laboratory), NOAA, and the University of Alaska, Utah State University. The objective is to measure the solar extreme ultraviolet (EUV) irradiance with unprecedented spectral resolution, temporal cadence, accuracy, and precision. Use of physics-based models of the solar EUV irradiance to advance the understanding of the solar EUV irradiance variations based on the activity of the solar magnetic features. 43) 44) 45) 46)
Specific EVE science objectives are:
1) Specify the solar EUV spectral irradiance and its variability on multiple time scales.
- EUV: 0-105 nm (0.1 nm resolution at >10 nm) and H I Lyman-á(121.6 nm)
- Time Scales: < 20 s cadence, continuous sequence
2) Advance current understanding of how and why the solar EUV spectral irradiance varies.
- Use AIA & HMI solar images to understand the interactions of the solar magnetic fields and the evolution of the solar features (e.g., plage, active network) and how these affect the solar EUV variations
3) Improve the capability to predict the EUV spectral irradiance variability
- Develop new forecast and nowcast models of the solar EUV irradiance for use in the NOAA space weather operations
4) Understand the response of the geospace environment to variations in the solar EUV spectral irradiance and the impact on human endeavors
- Use solar EUV irradiances with thermosphere and ionosphere models to better define the solar influences on Earth’s atmosphere
- Input EVE solar data near real-time into NOAA operational atmospheric models to improve accuracy of solar storm warnings and satellite drag calculations and to predict better communication disruptions
The EVE measurement approach is to observe simultaneously the solar EUV irradiance with different instrument types (multiple subsystems and technology) to meet the wavelength, resolution, and accuracy requirements.
Table 6: Overview of EVE instrument modules and measurements
Table 7: EVE instrument parameters
Figure 33: Overview of the EVE instrument (image credit: CU/LASP)
The EVE instrument consists of the following elements/modules: MEGS, ESP, and EEB.
• MEGS (Multiple EUV Grating Spectrograph). A set of 2 Rowland-circle grating spectrographs that measure the 5-105 nm spectral irradiance with 0.1 nm spectral resolution and with 10 second cadence. The MEGS have laminar groove profile (50% duty cycle of grooves) to suppress even orders.
- MEGS-A uses single, holographic, spherical grating at 80º grazing incidence
- MEGS-B uses dual, holographic, spherical grating, used near normal incidence
- CCD array type of size: 1024 x 2048 pixels (CCID-28 devices of MIT/LL, heritage: flown on Chandra and XMM/Newton)
- Back-thinned, back-illuminated
- Passively cooled to -100º C
Figure 34: Cross-section of the MEGS optics system (image credit: CU/LASP)
- MEGS-A has two slits and two filters: Slit 1: Mo/C, 5-20 nm; Slit 2: Si, 17.0 -37.0 nm
- MEGS-B has one slit and no primary filter. Additional removable filters for higher order checks.
Table 8: Overview of MEGS-A parameters
Figure 35: Schematic view of the MEGS-A device (image credit: CU/LASP)
Table 9: Overview of MEGS-B parameters
Figure 36: Schematic view of the MEGS-B device (image credit: CU/LASP)
• MEGS-SAM (Multiple EUV Grating Spectrograph-Solar Aspect Monitor). The objective is to provide pulse height analysis of X-ray photons. The device provides also MEGS pointing information with precision of 9 arcseconds. MEGS-SAM has a wavelength coverage of 0.1 -7 nm with a spectral resolution of 0.01-1 nm, and a spatial resolution with 10 arcsec/pixel. Detector: pinhole illuminates the MEGS-A CCD.
Table 10: Overview of MEGS-SAM parameters
Figure 37: Schematic of the MEGS-SAM device (image credit: CU/LASP)
• MEGS-P: Photometer for Lyman-α H I 121.6 nm and He I 58.4 nm emissions.
- Technique: grating + filter photometer
- MEGS-P channels are located in MEGS-B entrance baffles, providing a resolution of 5 nm
- Detector: IRD Si photodiode
- Filter: Acton Lyman-α filter and Al/Sn foil filter
Table 11: Parameters of the MEGS-P device
Figure 38: Schematic of the MEGS-P device (image credit: CU/LASP)
• ESP (EUV Spectrophotometer): A transmission grating spectrograph with stable Si photodiodes to provide solar X-ray measurement short of 5 nm, calibrations for MEGS sensitivity changes and higher time cadence (0.25 s). The ESP is very similar to the SOHO SEM instrument. ESP is of SEM instrument heritage flown on SOHO and also of TIMED heritage.
Table 12: Parameters of the ESP device
Figure 39: Optical layout of the ESP instrument (image credit: CU/LASP)
• EEB (EVE Electronics Box): Electronics that control the MEGS and ESP instruments and provides an interface to/from the SDO spacecraft.
EVE data products:
• Near real-time space weather data product of the solar EUV irradiance for NOAA SEC operations
• High quality solar EUV irradiances on 10 s cadence and averaged over 1 day provided daily to EVE's archive and FTP distribution center.
SDO ground system:
Data reception and spacecraft commanding will be conducted via a dedicated and newly implemented ground station at White Sands, NM. The SDO ground system consists of five major elements: 47)
1) SDOGS (SDO Ground Station), located at White Sands, NM and co-located with the WSGT (White Sands Ground Terminal) for TDRS service support. Two dual-feed antennas of 18 m diameter (S-band and Ka-band) are being allocated for SDO science data acquisition and TT&C operations support. A major function of the DDS is to continuously receive the high-rate science telemetry from the SDOGS Ka-band system and to deliver the science data to the SOCs in near real-time.
2) DDS (Data Distribution System), located at White Sands, NM.
• Receives the science telemetry data, processes it into files and distributes them to the instrument teams in near-real-time
• Provides a short-term (30 day) storage capability and supports data retransmissions as needed
• Provides the remote monitor and control capabilities of the DDS and SDOGS, from the MOC
3) MOC (Missions Operations Center), located at GSFC
• Supports the conventional real-time TT&C functions, which allows the Flight Operations Team (FOT) to monitor the health and status of the observatory and to control its operations
• Provides mission planning, trending and analysis, remote control and monitoring of DDS and ground station functions, and flight dynamics functions, including attitude determination and control and orbit maneuver computations and execution.
4) SOC (Science Operations Center). The 3 SOCs are located at the PI home institutions:
• They provide real-time health and safety monitoring as well as the command function for the science instrument
• Provision of science mission planning
• Science data processing, analysis, archiving, and distribution to the user community
5) GRN (Ground Communications Network)
• Provides connectivity between each of the ground system elements supporting all levels of data exchange and voice communications for SDO mission operations.
- One Optical Carrier Level 3 (OC3) network to AIA (67 Mbit/s) from DDS
- One Optical Carrier Level 3 (OC3) network to HMI (55 Mbit/s) from DDS
- One T3 circuit to EVE (7 Mbit/s) from DDS
- TT&C data: Four T1 circuits from MOC to/from SDOGS for S-band housekeeping telemetry and commands, two per SDOGS antenna site (restore time is < 1minute).
Figure 40: Ka-band end-to-end data flow configuration (image credit: NASA)
Figure 41: S-band end-to-end data flow configuration (image credit: NASA)
Figure 42: Overview of the SDO ground system (image credit: NASA)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.