IRIS (Interface Region Imaging Spectrograph) Observatory
IRIS is a science mission in NASA's SMEX (Small Explorer) program to study the interface between the photosphere and corona of the sun. The IRIS investigation is centered on three themes of broad significance to solar and plasma physics, space weather, and astrophysics, aiming to understand how internal convective flows power atmospheric activity:
1) Which types of non-thermal energy dominate in the chromosphere and beyond?
2) How does the chromosphere regulate mass and energy supply to corona and heliosphere?
3) How do magnetic flux and matter rise through the lower atmosphere, and what role does flux emergence play in flares and mass ejections?
The complex processes and enormous contrasts of density, temperature and magnetic field within this interface region require instrument and modeling capabilities that are only now within reach. The IRIS team will use advances in instrumental and computational technology, its extensive experience, and its broad technological heritage to build a state-of-the-art instrument to provide unprecedented access to the plasma-physical processes in the interface region. 1) 2) 3)
IRIS will provide key insights into all these processes, and thereby advance our understanding of the solar drivers of space weather from the corona to the far heliosphere, by combining high-resolution imaging and spectroscopy for the entire chromosphere and adjacent regions. IRIS will resolve in space, time, and wavelength the dynamic geometry from the chromosphere to the low-temperature corona to shed much-needed light on the physics of this magnetic interface region.
In June 2010, NASA selected the IRIS mission proposal of LMATC (Lockheed Martin Advanced Technology Center, Palo Alto, CA (PI: Alan M. Title). The IRIS mission uses a solar telescope and spectrograph to explore the solar chromospheres. This is a crucial region for understanding energy transport into the solar wind and an archetype for stellar atmospheres. Recent discoveries have shown the chromosphere is significantly more dynamic and structured than previously thought. The unique instrument capabilities, coupled with state of the art 3-D modeling, will explore this dynamic region in detail. The mission will greatly extend the scientific output of existing heliophysics spacecraft that follow the effects of energy release processes from the sun to Earth. 4) 5) 6) 7)
The IRIS science investigation includes the following team members:
Lockheed Martin Solar and Astrophysics Lab (LMSAL); Lockheed Martin Sensing and Exploration Systems (LMS&ES), SAO (Smithsonian Astrophysical Observatory), Cambridge, MA, USA; MSU (Montana State University), Bozeman, MT, USA; Institute for Theoretical Astrophysics, UiO (University of Oslo), Norway; HAO (High Altitude Observatory) of NCAR, Boulder, CO, USA; Stanford University, Stanford, CA, USA, NASA/ARC (Ames Research Center), Mountain View, CA, USA; NASA/GSFC (Goddard Space Flight Center), Greenbelt, MD, USA; NSO (National Solar Observatory), Space Sciences Lab, AURA (Association of Universities for Research in Astronomy), USA; UCB (University of California, Berkeley); PPPL (Princeton Plasma Physics Laboratory), Princeton, NJ, USA; Sydney Institute for Astronomy, University of Sydney, Australia; Center for Plasma Astrophysics, University of Leuven, The Netherlands; MSSL (Mullard Space Science Laboratory), London, UK; RAL (Rutherford-Appleton Laboratory), UK; ESA (European Space Agency); MPS (Max Planck Institute for Solar Research), Katlenburg-Lindau, Germany; NAO (National Astronomical Observatory), Tokyo, Japan; Niels Bohr Institute, University of Copenhagen, Sweden.
Background: The chromosphere and transition region (TR) form a complex interface between the solar surface and corona. Almost all of the mechanical energy that drives solar activity and solar atmospheric heating is converted into heat and radiation within this interface region, with only a small amount leaking through to power coronal heating and drive the solar wind. The chromosphere requires a heating rate that is between one and two orders of magnitude larger than that of the corona. Yet despite the importance of the interface region for solar activity, the heating of the corona, and the genesis of the solar wind, the chromosphere and TR have received much less attention than the photosphere or corona. This is in part because the interface region is highly complex. 8)
The transition between high and low plasma β (the ratio of plasma pressure to magnetic pressure) occurs somewhere between photosphere and corona, so that in the interface region, the magnetic field and plasma compete for dominance (with a variety of impacts on, e.g., waves such as mode coupling, refraction and reflection). Within this region, the density drops by 6 orders of magnitude; the temperature rapidly increases from 5,000 to 1 million K, with strong gradients across the magnetic field evident from high-resolution images of the chromosphere (Figure 1).
Figure 1: Hα line center image taken at the Swedish Solar Telescope on 16-June-2003 showing the fine scale structuring of the upper chromosphere, with the thinnest fibrils having diameters less than 200 km (image credit: LMSAL)
The plasma transitions from partially ionized in the chromosphere (which leads to a variety of interesting plasma physics effects) to fully ionized in the corona, and shows evidence of supersonic and super-Alfvenic motions. To top it off, the chromosphere is partially opaque, with non- LTE (Local Thermodynamic Equilibrium) effects dominating the radiative transfer, so that interpreting the radiation, and determining the local energy balance and ionization state, is non-intuitive and requires advanced computer models. The highly dynamic nature of the chromosphere, as observed with Hinode and ground-based telescopes, further complicates attempts to better understand the interface region. This is both because high cadence observations are required (better than 15 seconds), and because the ionization state of some elements (e.g., hydrogen) reacts only slowly to changes in the energy balance, and thus depends on the history of the plasma. IRIS will exploit recent advances in novel, high throughput and high-resolution instrumentation, efficient numerical simulation codes, and powerful, massively parallel supercomputers, to open a new window into the physics of the interface region Ref. 8).
Construction, integration and testing of the IRIS spacecraft will be done by the Lockheed Martin Space Systems ATC (Advanced Technology Center) in Palo Alto, CA. Mission operations and some system engineering will be the responsibility of NASA/ARC.
IRIS is a 3-axis stabilized, sun-pointed mission that studies the chromospheres in the FUV (Far Ultraviolet) and NUV (Near Ultraviolet) spectral region with 0.33 arcsec spatial resolution, 0.4 km/s velocity resolution and a FOV (Field of View) of 171 arcsec. This two-year mission fills a critical observational data gap by providing simultaneous, co-spatial and comprehensive coverage from photosphere (~4,500 K) up to corona (≤ 10 MK). IRIS consists of a 20 cm aperture telescope assembly that feeds an imaging spectrograph and a separate imaging camera system with wavelengths in the FUV and NUV. A spacecraft bus based upon heritage designs (TRACE) supports the science mission and provides pointing, power, and data communications for the mission.
• A guide telescope is used for fine pointing
• The pointing range is anywhere within the 1.2 Rsun from disk center
• RAD 750 CPU
• Power generation of 340 W, 28 V. The two solar arrays measure 0.6 m x 1.3 m each, with a total surface area of 1.7 m2.
• RF communications: X-band for payload downlink at 15 Mbit/s including the overhead of LDPC (Low Density Parity Checking ) 7/8s encoding. The effective downlink rate is 13 Mbit/s (excluding overhead) during up to 15 passes per day with the antennas of KSAT (Kongsberg Satellite Services) in Svalbard, Norway, as well as some passes from NASA’s NEN (Near Earth Network) in Alaska and Wallops. Onboard data storage capacity of 48 Gbit in solid state memory. IRIS is equipped with two omnidirectional S-band antennas for uplinking of commands and downlinking of engineering data.The S-band provides uplink at 2 kbit/s and downlink at 256 kbit/s.
There is no propulsion system and there are no consumables on board. The ACS can point the telescope IRIS boresight to any location on the solar disk or above above the limb within 21 arcminutes of disk center, and roll the spacecraft (and thus, the spectrograph slit), up to ±90º (at 0º the slit is oriented parallel to N-S on the Sun).
Figure 3: Overview of the IRIS observatory showing the 20 cm UV telescope, with and without solar panels (image credit: (NASA, LMSAL) 9)
Mission development: 10)
The cost, schedule, and size constraints were part of all design trades. The result was to use existing hardware and software designs as much as possible and to use commercially supplied components where feasible. Single string systems and subsystems were used throughout the observatory. The observatory was designed for autonomous on-orbit operations with minimal human support from the ground. Flight spares, brassboards, and engineering units were kept to a minimum. Deferral of subsystem testing was carried out to streamline the integration and test effort, and qualification by similarity was used for components with flight heritage. A small core team was responsible for the design, development, integration, test, launch, and on-orbit checkout of the observatory.
For the spacecraft, prior missions and design studies were used for reference in the design of the IRIS spacecraft bus. The LM team designed the spacecraft architecture including the bus structure, the harness and interconnects, the MLI (Multi-Layer Insulation) blanketing, the ACS (Attitude Control Subsystem) flight software, the fault management scheme, and the EGSE (Electrical Ground Support Equipment). The solar arrays, deployment mechanisms, and EGSE were developed in house. The majority of the spacecraft components were procured from vendors with a majority being catalog items such as the reaction wheel assemblies, torque rods, IAU (Integrated Avionics Unit) which consisted of the C&DH and EPS subsystems, the coarse and digital sun sensors, the magnetometer, and the star trackers. The flight and test batteries were spares from the GRAIL program (Ref. 10).
The S-band and X-band RF communications units were vendor developed to meet the system requirements and size constraints of the IRIS spacecraft structure. Cost and schedule saving features included the design of a gyro-less ACS system enabled by the use of the fine pointing signal from the instrument guide telescope and the use of auto generated ACS code from the Simulink system model. The only engineering units procured for the spacecraft development were two IAU engineering models used for development and test of the spacecraft and ACS software. The communications vendor developed engineering units for the X- and S- band units to aid in their development, but they were not contract deliverables.
A RWA (Reaction Wheel Assembly) engineering unit was loaned from the GRAIL program to aid in an anomaly investigation, but was not transferred to the IRIS program and the magnetometer vendor loaned an engineering unit for system and model validation. No spacecraft flight spares were procured. The architecture was single string with some limited redundancy from the use of 4 RWAs (Reaction Wheel Assemblies, can operate in degraded mode with 3) and the star tracker and magnetometer electronics units.
Figure 4: Spacecraft schematic (image credit: LMSAL)
The assembly, integration, and test operations were also planned with cost, schedule, and risk in mind. The traditional flight development program would have a progressive level of flight qualification and acceptance testing as components are developed and integrated into higher levels of assembly. On an enhanced Class D mission such as IRIS, all flight acceptance tests are performed, but the program has the ability to trade at what level the tests are carried out, as long as all subsystems are exposed to the required test environments at some time in the test flow. The majority of the vendor supplied hardware was acceptance tested prior to delivery to LM. One exception was the telescope assembly for which the thermal vacuum, vibration, and EMI/EMC (Electromagnetic Interference/ Electromagnetic Compatibility) tests were deferred to higher levels of assembly.
For the instrument, vibration testing was deferred to the observatory level due to the complexity of replicating the mounting to the spacecraft and to reduce the risk of over test. However, extensive thermal vacuum and optical performance testing was performed at the instrument level due to the new spectrograph design and the complexity of optical stimulation at the observatory level. Instrument level EMI/EMC testing was deferred to the observatory level to ensure all electrical interfaces were tested in the most flight like configuration. For the spacecraft, vibration, thermal vacuum, and EMI/EMC tests were all deferred to the observatory level. The decision as to which level of integration to carry out the environmental and acceptance testing was based on balancing schedule, test complexity, and risk with many of the decisions reducing the overall risk to the mission.
Figure 5: Photo of the fully integrated IRIS spacecraft in a cleanroom of Lockheed Martin Space Systems (image credit: NASA, Lockheed Martin) 11)
Figure 6: Photo of the IRIS spacecraft in launch configuration (image credit: NASA, Lockheed Martin)
Table 1: Key performance parameters of IRIS
Launch: The IRIS spacecraft was launched on June 28, 2013 (2.27 UTC) aboard a Pegasus XL vehicle of OSC (Orbital Sciences Corporation) from VAFB (Vandenberg Air Force Base), CA, USA. The L-1011 aircraft took off from VAFB and flew to the drop point over the Pacific Ocean, where the aircraft released the Pegasus XL from beneath its belly. 12) 13)
Orbit: Sun-synchronous orbit, altitude of 620 km x 670 km, inclination = 97.9º. The orbit allows eclipse-free continuous viewing for 7 months per year. The instrument will not be operated in the 4 months in which eclipses occur.
NASA/ARC (Ames Research Center), Moffett Field, CA, is responsible for mission operations and the ground data system. The NSC (Norwegian Space Center) captures the IRIS data with their antennas in Svalbard, inside the Arctic Circle, in northern Norway (Spitsbergen). The science data will be managed by the Joint Science Operations Center of the Solar Dynamics Observatory, run by Stanford and Lockheed Martin. NASA's Goddard Space Flight Center in Greenbelt, Md., oversees the SMEX project.
• February 19, 2021: For decades after its discovery, observers could only see the solar chromosphere for a few fleeting moments: during a total solar eclipse, when a bright red glow ringed the Moon’s silhouette. 14)
Figure 7: The chromosphere, photographed during the 1999 total solar eclipse. The red and pink hues – light emitted by hydrogen – earned it the name chromosphere, from the Greek “chrôma” meaning color (image credit: Luc Viatour)
- More than a hundred years later, the chromosphere remains the most mysterious of the Sun’s atmospheric layers. Sandwiched between the bright surface and the ethereal solar corona, the Sun’s outer atmosphere, the chromosphere is a place of rapid change, where temperature rises and magnetic fields begin to dominate the Sun’s behavior.
- Now, for the first time, a triad of NASA missions have peered into the chromosphere to return multi-height measurements of its magnetic field. The observations – captured by two satellites and the CLASP2 (Chromospheric Layer Spectropolarimeter 2) mission, aboard a small suborbital rocket – help reveal how magnetic fields on the Sun’s surface give rise to the brilliant eruptions in its outer atmosphere. The paper was published today in Science Advances. 15)
Figure 8: The chromosphere lies between the photosphere, or bright surface of the Sun that emits visible light, and the super-heated corona, or outer atmosphere of the Sun at the source of solar eruptions. The chromosphere is a key link between these two regions and a missing variable determining the Sun’s magnetic structure (image credit: NASA’s Goddard Space Flight Center)
- A major goal of heliophysics – the science of the Sun’s influence on space, including planetary atmospheres – is to predict space weather, which often begins on the Sun but can rapidly spread through space to cause disruptions near Earth.
- Driving these solar eruptions is the Sun’s magnetic field, the invisible lines of force stretching from the solar surface to space well past Earth. This magnetic field is difficult to see – it can only be observed indirectly, by light from the plasma, or super-heated gas, that traces out its lines like car headlights traveling a distant highway. Yet how those magnetic lines arrange themselves – whether slack and straight or tight and tangled – makes all the difference between a quiet Sun and a solar eruption.
- “The Sun is both beautiful and mysterious, with constant activity triggered by its magnetic fields,” said Ryohko Ishikawa, solar physicist at the National Astronomical Observatory of Japan in Tokyo and lead author of the paper.
- Ideally, researchers could read out the magnetic field lines in the corona, where solar eruptions take place, but the plasma is way too sparse for accurate readings. (The corona is far less than a billionth as dense as air at sea level.)
- Instead, scientists measure the more densely packed photosphere – the Sun’s visible surface – two layers below. They then use mathematical models to propagate that field upwards into the corona. This approach skips measuring the chromosphere, which lies between the two, instead, hoping to simulate its behavior.
- Unfortunately the chromosphere has turned out to be a wildcard, where magnetic field lines rearrange in ways that are hard to anticipate. The models struggle to capture this complexity.
- “The chromosphere is a hot, hot mess,” said Laurel Rachmeler, former NASA project scientist for CLASP2, now at the NOAA ( National Oceanic and Atmospheric Administration). “We make simplifying assumptions of the physics in the photosphere, and separate assumptions in the corona. But in the chromosphere, most of those assumptions break down.”
- Institutions in the U.S., Japan, Spain and France worked together to develop a novel approach to measure the chromosphere’s magnetic field despite its messiness. Modifying an instrument that flew in 2015, they mounted their solar observatory on a sounding rocket, so named for the nautical term “to sound” meaning to measure. Sounding rockets launch into space for brief, few-minute observations before falling back to Earth. More affordable and quicker to build and fly than larger satellite missions, they’re also an ideal stage to test out new ideas and innovative techniques.
- Launching from the White Sands Missile Range in New Mexico, the rocket shot to an altitude of 170 miles (274 km) for a view of the Sun from above Earth’s atmosphere, which otherwise blocks certain wavelengths of light. They set their sights on a plage, the edge of an “active region” on the Sun where the magnetic field strength was strong, ideal for their sensors.
- As CLASP2 peered at the Sun, NASA’s IRIS (Interface Region Imaging Spectrograph) and the JAXA/NASA Hinode satellite, both watching the Sun from Earth orbit, adjusted their telescopes to look at the same location. In coordination, the three missions focused on the same part of the Sun, but peered to different depths.
- Hinode focused on the photosphere, looking for spectral lines from neutral iron formed there. CLASP2 targeted three different heights within the chromosphere, locking onto spectral lines from ionized magnesium and manganese. Meanwhile, IRIS measured the magnesium lines in higher resolution, to calibrate the CLASP2 data. Together, the missions monitored four different layers within and surrounding the chromosphere.
- Eventually the results were in: The first multi-height map of the chromosphere’s magnetic field.
- “When Ryohko first showed me these results, I just couldn't stay in my seat,” said David McKenzie, CLASP2 principal investigator at NASA’s Marshall Space Flight Center in Huntsville, Alabama. “I know it sounds esoteric – but you've just showed the magnetic field at four heights at the same time. Nobody does that!”
- The most striking aspect of the data was just how varied the chromosphere turned out to be. Both along the portion of the Sun they studied and at different heights within it, the magnetic field varied significantly.
- “At the Sun’s surface we see magnetic fields changing over short distances; higher up those variations are much more smeared out. In some places, the magnetic field didn't reach all the way up to the highest point we measured whereas in other places, it was still at full strength.”
- The team hopes to use this technique for multi-height magnetic measurements to map the entire chromosphere’s magnetic field. Not only would this help with our ability to predict space weather, it will tells us key information about the atmosphere around our star.
- “I'm a coronal physicist – I'm really interested in the magnetic fields up there,” Rachmeler said. “Being able to raise our measurement boundary to the top of the chromosphere would help us understand so much more, help us predict so much more – it would be a huge step forward in solar physics.”
- They’ll have a chance to take that step forward soon: A re-flight of the mission was just greenlit by NASA. Though the launch date isn’t yet set, the team plans to use the same instrument but with a new technique to measure a much broader swath of the Sun.
- “Instead of just measuring the magnetic fields along the very narrow strip, we want to scan it across the target and make a two-dimensional map,” McKenzie said.
Measuring Magnetic Fields
- To measure magnetic field strength, the team took advantage of the Zeeman effect, a century-old technique. (The first application of the Zeeman effect to the Sun, by astronomer George Ellery Hale in 1908, is how we learned that the Sun was magnetic.) The Zeeman effect refers to the fact that spectral lines, in the presence of strong magnetic fields, splinter into multiples. The farther apart they split, the stronger the magnetic field.
Figure 9: The Zeeman effect. This animated image shows a spectrum with several absorption lines – spectral lines produced when atoms at specific temperatures absorb a specific wavelength of light. When a magnetic field is introduced (shown here as blue magnetic field lines emanating from a bar magnetic), absorption lines split into two or more. The number of splits and the distance between them reveals the strength of the magnetic field. Note that not all spectral lines split in this way, and that the CLASP2 experiment measured spectral lines in the ultraviolet range, whereas this demo shows lines in the visible range (image credits: NASA’s Goddard Space Flight Center/Scott Weissinger)
- The chaotic chromosphere, however, tends to “smear” spectral lines, making it difficult to tell just how far apart they split – that’s why previous missions had trouble measuring it. CLASP2’s novelty was in working around this limitation by measuring “circular polarization,” a subtle shift in the light’s orientation that happens as part of the Zeeman effect. By carefully measuring the degree of circular polarization, the CLASP2 team could discern how far apart those smeared lines must have split, and thereby how strong the magnetic field was.
• December 7, 2020: A phenomenon first detected in the solar wind may help solve a long-standing mystery about the sun: why the solar atmosphere is millions of degrees hotter than the surface. 16)
- Images from the IRIS (Interface Region Imaging Spectrograph) satellite and the AIA (Atmospheric Imaging Assembly) show evidence that low-lying magnetic loops are heated to millions of degrees Kelvin.
- Researchers at Rice University, the University of Colorado Boulder and NASA’s Marshall Space Flight Center make the case that heavier ions, such as silicon, are preferentially heated in both the solar wind and in the transition region between the sun’s chromosphere and corona.
Figure 10: Images of the sun captured by the IRIS mission show new details of how low-lying loops of plasma are energized and may also reveal how the hot corona is created (video credit: Rice University/NASA)
- There, loops of magnetized plasma arc continuously, not unlike their cousins in the corona above. They’re much smaller and hard to analyze, but have long been thought to harbor the magnetically driven mechanism that releases bursts of energy in the form of nanoflares.
- Rice solar physicist Stephen Bradshaw and his colleagues were among those who suspected as much, but none had sufficient evidence before IRIS.
- The high-flying spectrometer was built specifically to observe the transition region. In the NASA-funded study, which appears in Nature Astronomy, the researchers describe “brightenings” in the reconnecting loops that contain strong spectral signatures of oxygen and, especially, heavier silicon ions. 17)
- The team of Bradshaw, his former student and lead author Shah Mohammad Bahauddin, now a research faculty member at the Laboratory for Atmospheric and Space Physics at Colorado, and NASA astrophysicist Amy Winebarger studied IRIS images able to resolve details of these transition region loops and detect pockets of super-hot plasma. The images allow them to analyze the movements and temperatures of ions within the loops via the light they emit, read as spectral lines that serve as chemical “fingerprints.”
- “It’s in the emission lines where all the physics is imprinted,” said Bradshaw, an associate professor of physics and astronomy. “The idea was to learn how these tiny structures are heated and hope to say something about how the corona itself is heated. This might be a ubiquitous mechanism that operates throughout the solar atmosphere.”
- The images revealed hot-spot spectra where the lines were broadened by thermal and Doppler effects, indicating not only the elements involved in nanoflares but also their temperatures and velocities.
- At the hot spots, they found reconnecting jets containing silicon ions moved toward (blue-shifted) and away from (red-shifted) the observer (IRIS) at speeds up to 100 kilometers per second. No Doppler shift was detected for the lighter oxygen ions.
- The researchers studied two components of the mechanism: how the energy gets out of the magnetic field, and then how it actually heats the plasma.
- The transition region is only about 10,000 degrees Fahrenheit, but convection on the sun’s surface affects the loops, twisting and braiding the thin magnetic strands that comprise them, and adds energy to the magnetic fields that ultimately heat the plasma, Bradshaw said. “The IRIS observations showed that process taking place and we’re reasonably sure at least one answer to the first part is through magnetic reconnection, of which the jets are a key signature,” he said.
- In that process, the magnetic fields of the plasma strands break and reconnect at braiding sites into lower energy states, releasing stored magnetic energy. Where this takes place, the plasma becomes superheated.
- But how plasma is heated by the released magnetic energy has remained a puzzle until now. “We looked at the regions in these little loop structures where reconnection was taking place and measured the emission lines from the ions, chiefly silicon and oxygen,” he said. “We found the spectral lines of the silicon ions were much broader than the oxygen.”
- That indicated preferential heating of the silicon ions. “We needed to explain it,” Bradshaw said. “We had a look and a think and it turns out there’s a kinetic process called ion cyclotron heating that favors heating heavy ions over lighter ones.”
- He said ion cyclotron waves are generated at the reconnection sites. The waves carried by the heavier ions are more susceptible to an instability that causes the waves to “break” and generate turbulence, which scatters and energizes the ions. This broadens their spectral lines beyond what would be expected from the local temperature of the plasma alone. In the case of the lighter ions, there might be insufficient energy left over to heat them. “Otherwise, they don’t exceed the critical velocity needed to trigger the instability, which is faster for lighter ions,” he said.
- “In the solar wind, heavier ions are significantly hotter than lighter ions,” Bradshaw said. “That’s been definitively measured. Our study shows for the first time that this is also a property of the transition region, and might therefore persist throughout the entire atmosphere due to the mechanism we have identified, including heating the solar corona, particularly since the solar wind is a manifestation of the corona expanding into interplanetary space.”
- The next question, Bahauddin said, is whether such phenomena are happening at the same rate all over the sun. “Most probably the answer is no,” he said. “Then the question is, how much do they contribute to the coronal heating problem? Can they supply sufficient energy to the upper atmosphere so that it can maintain a multimillion-degree corona?
- “What we’ve shown for the transition region was a solution to an important piece of the puzzle, but the big picture requires more pieces to fall in the right place,” Bahauddin said. “I believe IRIS will be able to tell us about the chromospheric pieces in the near future. That will help us build a unified and global theory of the sun’s atmosphere.”
• September 21, 2020: In a paper published today in Nature Astronomy, researchers report the first ever clear images of nanojets — bright thin lights that travel perpendicular to the magnetic structures in the solar atmosphere, called the corona — in a process that reveals the existence of one of the potential coronal heating candidates: nanoflares. 18) 19)
- In pursuit of understanding why the Sun’s atmosphere is so much hotter than the surface, and to help differentiate between a host of theories about what causes this heating, researchers turn to NASA’s Interface Region Imaging Spectrograph (IRIS) mission. IRIS was finely tuned with a high-resolution imager to zoom in on specific hard-to-see events on the Sun.
Figure 11: In pursuit of understanding why the Sun's atmosphere is so much hotter than the surface, and to help differentiate between a host of theories about what causes this heating, researchers turn to NASA's Interface Region Imaging Spectrograph (IRIS) mission (video credit: NASA's Goddard Space Flight Center/Scientific Visualization Studio)
- Nanoflares are small explosions on the Sun – but they are difficult to spot. They are very fast and tiny, meaning they are hard to pick out against the bright surface of the Sun. On April 3, 2014, during what’s known as a coronal rain event when streams of cooled plasma fall from the corona to the Sun’s surface looking almost like an enormous waterfall, researchers noticed bright jets appearing near the end of the event. These telltale flashes are nanojets — heated plasma traveling so fast that they appear on images as bright thin lines seen within the magnetic loops on the Sun. Nanojets are considered a “smoking gun,” key evidence of the presence of nanoflares. Each nanojet is believed to be initiated by a process known as magnetic reconnection where twisted magnetic fields explosively realign. One reconnection can set off another reconnection, creating an avalanche of nanojets in the corona of the Sun, a process that could create the energy that is heating the corona. In the visualization above, the Solar Dynamic Observatory gives us a full view of the Sun before zooming into IRIS’s up close view of the nanojets, which briefly light up in the magnetic loops.
- IRIS gathers its high resolution images by focusing in on a small portion of the Sun at a time. So observing specific events is a combination of educated guesswork and looking at the right place at the right time. Once the nanojets were identified against the backdrop of the coronal rain, researchers coordinated with NASA’s Solar Dynamics Observatory (SDO) and the Hinode observatory, a partnership among the Japan Aerospace Exploration Agency, ESA (European Space Agency), and NASA to get a complete view of the Sun, and confirm whether they were detecting nanojets, and assess their effects on the corona.
- The researchers combined the many observations with advanced simulations to recreate the events they saw on the Sun. The models showed that the nanojets were a telltale signature of magnetic reconnection and nanoflares, contributing to coronal heating in the simulations. More studies will need to be done to establish the frequency of nanojets and nanoflares all over the Sun, and how much energy they contribute to heating the solar corona. Going forward, missions like Solar Orbiter and Parker Solar Probe can give more detail into the processes that heat the solar corona.
• February 19, 2019: Scientists have discovered tadpole-shaped jets coming out of regions with intense magnetic fields on the Sun. Unlike those living on Earth, these “tadpoles” — formally called pseudo-shocks — are made entirely of plasma, the electrically conducting material made of charged particles that account for an estimated 99 percent of the observable universe. The discovery adds a new clue to one of the longest-standing mysteries in astrophysics. 20)
Figure 12: Images from IRIS show the tadpole-shaped jets containing pseudo-shocks streaking out from the Sun (image credit: Abhishek Srivastava IIT (BHU)/Joy Ng, NASA’s Goddard Space Flight Center)
- For 150 years scientists have been trying to figure out why the wispy upper atmosphere of the Sun — the corona — is over 200 times hotter than the solar surface. This region, which extends millions of miles, somehow becomes superheated and continually releases highly charged particles, which race across the solar system at supersonic speeds.
- When those particles encounter Earth, they have the potential to harm satellites and astronauts, disrupt telecommunications, and even interfere with power grids during particularly strong events. Understanding how the corona gets so hot can ultimately help us understand the fundamental physics behind what drives these disruptions.
- In recent years, scientists have largely debated two possible explanations for coronal heating: nanoflares and electromagnetic waves. The nanoflare theory proposes bomb-like explosions, which release energy into the solar atmosphere. Siblings to the larger solar flares, they are expected to occur when magnetic field lines explosively reconnect, releasing a surge of hot, charged particles. An alternative theory suggests a type of electromagnetic wave called Alfvén waves might push charged particles into the atmosphere like an ocean wave pushing a surfer. Scientists now think the corona may be heated by a combination of phenomenon like these, instead of a single one alone.
- The new discovery of pseudo-shocks adds another player to that debate. Particularly, it may contribute heat to the corona during specific times, namely when the Sun is active, such as during solar maximums — the most active part of the Sun’s 11-year cycle marked by an increase in sunspots, solar flares and coronal mass ejections.
- The discovery of the solar tadpoles was somewhat fortuitous. When recently analyzing data from NASA’s IRIS (Interface Region Imaging Spectrograph), scientists noticed unique elongated jets emerging from sunspots — cool, magnetically-active regions on the Sun’s surface — and rising 3,000 miles up into the inner corona. The jets, with bulky heads and rarefied tails, looked to the scientists like tadpoles swimming up through the Sun’s layers.
- “We were looking for waves and plasma ejecta, but instead, we noticed these dynamical pseudo-shocks, like disconnected plasma jets, that are not like real shocks but highly energetic to fulfill Sun's radiative losses,” said Abhishek Srivastava, scientist at the Indian Institute of Technology (BHU) in Varanasi, India, and lead author on the new paper in Nature Astronomy. 21)
- Using computer simulations matching the events, they determined these pseudo-shocks could carry enough energy and plasma to heat the inner corona.
Figure 13: A computer simulation shows how the pseudo-shock is ejected and becomes disconnected from the plasma below (green), image credit: Abhishek Srivastava IIT (BHU)/Joy Ng, NASA’s Goddard Space Flight Center)
- The scientists believe the pseudo-shocks are ejected by magnetic reconnection — an explosive tangling of magnetic field lines, which often occurs in and around sunspots. The pseudo-shocks have only been observed around the rims of sunspots so far, but scientists expect they’ll be found in other highly magnetized regions as well.
- Over the past five years, IRIS has kept an eye on the Sun in its 10,000-plus orbits around Earth. It’s one of several in NASA’s Sun-staring fleet that have continually observed the Sun over the past two decades. Together, they are working to resolve the debate over coronal heating and solve other mysteries the Sun keeps.
- “From the beginning, the IRIS science investigation has focused on combining high-resolution observations of the solar atmosphere with numerical simulations that capture essential physical processes,” said Bart De Pontieu research scientist at Lockheed Martin Solar & Astrophysics Laboratory in Palo Alto, California. “This paper is a nice illustration of how such a coordinated approach can lead to new physical insights into what drives the dynamics of the solar atmosphere.”
- The newest member in NASA’s heliophysics fleet, Parker Solar Probe, may be able to provide some additional clues to the coronal heating mystery. Launched in 2018, the spacecraft flies through the solar corona to trace how energy and heat move through the region and to explore what accelerates the solar wind as well as solar energetic particles. Looking at phenomena far above the region where pseudo-shocks are found, Parker Solar Probe’s investigation hopes to shed light on other heating mechanisms, like nanoflares and electromagnetic waves. This work will complement the research conducted with IRIS.
- “This new heating mechanism could be compared to the investigations that Parker Solar Probe will be doing,” said Aleida Higginson, deputy project scientist for Parker Solar Probe at Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “Together they could provide a comprehensive picture of coronal heating.”
Figure 14: The tadpole-shaped pseudo-shocks, shown in dashed white box, are ejected from highly magnetized regions on the solar surface (image credit: Abhishek Srivastava IIT (BHU)/Joy Ng, NASA’s Goddard Space Flight Center)
Table 2: Extended life for ESA's science missions 22)
• October 30, 2018: IRIS (Interface Region Imaging Spectrograph) is a NASA small explorer mission that provides high-resolution spectra and images of the Sun in the 133 – 141 nm and 278 – 283 nm wavelength bands. The IRIS data are archived in calibrated form and made available to the public within seven days of observing. The calibrations applied to the data include dark correction, scattered light and background correction, flat fielding, geometric distortion correction, and wavelength calibration. In addition, the IRIS team has calibrated the IRIS absolute throughput as a function of wavelength and has been tracking throughput changes over the course of the mission. 23)
• December 1, 2016: IRIS collects data on the temperature and movement of solar material throughout this region to determine how it helps drive the constant changes we see on our sun. This data is crucial for answering outstanding questions about our sun, such as why its million-degree upper atmosphere, the corona, is several hundred times hotter than the fiery surface below. The interface region feeds solar material into the corona and the solar wind, the constant stream of charged particles flowing from the sun. This particular region is also responsible for generating most of the ultraviolet emission that reaches Earth. Our space weather and environment are continuously influenced by both these emissions and the solar wind. 24)
• Sept. 19,2016: The IRIS mission, built and operated by Lockheed Martin for NASA, has received more time to deliver groundbreaking space science. A new NASA contract extends the Lockheed Martin’s support for the orbiting observatory through September 2018, with a further extension possible through September 2019. 25)
- Since its launch in June 2013, IRIS has taken more than 24 million images or spectral measurements of the sun. Scientists at NASA, Lockheed Martin and other institutions around the world have used IRIS to make exciting discoveries about what causes the heating of the solar atmosphere and how solar flares are triggered and release magnetic energy. The observatory views only a small part of the sun at any time, but through careful planning by the IRIS science planning team, IRIS was able to catch nine of the largest flares (X-class) and almost 100 of the second largest class of flares (M-class) and numerous weaker C-class flares.
• On July 24, 2016, NASA’s IRIS (Interface Region Imaging Spectrograph) satellite captured a mid-level solar flare: a sudden flash of bright light on the solar limb – the horizon of the sun – as seen at the beginning of this video. Solar flares are powerful explosions of radiation. During flares, a large amount of magnetic energy is released, heating the sun’s atmosphere and releasing energized particles out into space. Observing flares such as this helps the IRIS mission study how solar material and energy move throughout the sun’s lower atmosphere, so we can better understand what drives the constant changes we can see on our sun. 26)
- As the video continues, solar material cascades down to the solar surface in great loops, a flare-driven event called post-flare loops or coronal rain. This material is plasma, a gas in which positively and negatively charged particles have separated, forming a superhot mix that follows paths guided by complex magnetic forces in the sun's atmosphere. As the plasma falls down, it rapidly cools – from millions down to a few tens of thousands of kelvins. The corona is much hotter than the sun’s surface; the details of how this happens is a mystery that scientists continue to puzzle out. Bright pixels that appear at the end of the video aren’t caused by the solar flare, but occur when high-energy particles bombard IRIS’s CCD device camera – an instrument used to detect photons.
Figure 15: Photo of a mid-sized flare on the solar limb, acquired by IRIS on July 24, 2016 (image credit: NASA/GSFC)
• The IRIS spacecraft and its payload are operating nominally in 2016. 27)
• July 2015: The IRIS mission is approved by NASA to continue its first extended mission and to plan against the current budget guidelines. Any changes to the guidelines will be handled through the budget formulation process. The IRIS mission will be invited to the 2017 Heliophysics Senior Review. 28)
- The IRIS (Interface Region Imaging Spectrograph) Observatory provides spectra and images from the solar photosphere into the low corona. It is designed to make spectroscopic observations of the Sun at high spatial, spectral, and temporal resolution. The spacecraft carries a 20 cm UV Cassegrain telescope that feeds a dual range UV slit spectrograph and slit-jaw imager. IRIS has an effective spatial resolution between 0.33 and 0.44 arcseconds, and a maximum field of view of 120 arcseconds. The far-UV channel covers the 133.0–141.0 nm range, with a 4 x 10-3 nm spectral resolution. The near-UV channel covers the 278.0–284.0 nm range, with an 8 x 10-3 nm spectral resolution. The slit-jaw imaging covers similar ranges in the far-UV (4.0 nm bandpass) and near-UV (0.4 nm bandpass). IRIS has a very high data rate, that allows images to be taken every 5–10 s and spectra every 1–2 s. IRIS travels in a polar sun-synchronous orbit (twilight orbit) that allows for eight months of continuous observations per year and maximizes eclipse-free viewing of the Sun.
- On June 28, 2015, the IRIS observatory was 2 years on orbit. IRIS observations have advanced our understanding of what role the interface region, which lies between the sun’s photosphere and corona, plays in powering its dynamic million-degree atmosphere and driving the solar wind. Since its first birthday last summer, IRIS data has revealed unexpected complexity in the interface region and investigated the source of the extremely high temperatures of the solar atmosphere, or corona. 29)
• June 2015: The 2015 Heliophysics Senior Review panel undertook a review of 15 missions currently in operation in April 2015. The panel found that all the missions continue to produce science that is highly valuable to the scientific community and that they are an excellent investment by the public that funds them. 30)
- The IRIS mission deliberately combines unprecedented observations of the solar chromosphere and transition region with advanced numerical simulations, including the development of publicly available sophisticated radiative transfer codes. During its prime mission phase IRIS has already achieved significant milestones in reaching its primary objective of understanding the processes that energize the solar atmosphere. For example, the detection of non-thermal electrons in coronal nano-flares provides new insight into mechanisms behind non-thermal energy generation specifically which dominate in the chromosphere and corona. The IRIS high-resolution spectra have shown for the first time the existence of regions of very dense plasma in the photosphere and low chromosphere that are rapidly accelerated and heated to 80,000 K. This discovery has pushed numerical simulations to explore specific reconnection processes that will explain these newly observed features and their impact regarding the mass and energy supply for the corona. One of the most challenging problems in solar physics today is to better understand the nature of solar flares and CMEs (Coronal Mass Ejections). IRIS has provided critical data that demonstrate, for example, the important role played by electron beams in the chromosphere during flares (which challenges some flare models) and evidence for tether-cutting magnetic reconnection as a trigger for both flares and associated CMEs.
- The IRIS Science Plan for its extended phase is quite compelling. It is focused on five prioritized science goals: 1) Study the fundamental physical processes in the solar atmosphere; 2) Investigate the (in)stability of the magnetized atmosphere; 3) Analyze the energy and mass transfer between photosphere, chromosphere, and corona; 4) Quantify the variations of far and near ultraviolet solar radiation over the solar cycle; 5) Explore the solar-stellar connection. The accomplishment of these goals will provide a new insight into basic physical processes and improve the current numerical models.
- IRIS Science Strengths: IRIS provides the highest spectral resolution observations of the chromosphere and transition region in the far-UV and near-UV ranges of any mission. The subarcsecond spatial resolution combined with the very high temporal cadence of the observations makes IRIS a unique instrument for detailed studies of the magnetic field reconnection and its thermodynamic effects in flares. IRIS has a strong program of coordinated observations with Solar-B/Hinode and SDO (Solar Dynamics Observatory). IRIS chromospheric and transition region spectra and images form a perfect complement to Hinode’s high-resolution photospheric magnetograms and coronal spectra and images, and to SDO’s contextual full-disk magnetograms and coronal images. The synergy with Hinode and SDO allows studies of how the magnetic field drives and mediates dynamics and heating in the coronal and solar wind. The high quality of the data and the new research opportunities offered by this mission are confirmed by 31 refereed papers published in the 16 months since the data became public, a rate similar to the corresponding phase of other Heliophysics missions.
- The research topics outlined in the proposal are highly relevant to the heliophysics research objectives. During its extended phase the IRIS mission will focus in further developing advanced numerical simulations to improve the diagnostic value of its observations, and further strengthening collaborations with other spaceborne and ground-based observatories. IRIS science goals for the next five years will address a broad range of unsolved questions pertinent to the physics of the solar chromosphere and low corona, such as which physical processes dominate the heating of the chromosphere, which mechanisms drive white light flares and initiate coronal mass ejections, and how jets supply mass to the corona and to the fast/slow solar wind. The transition layers of the solar atmosphere are crucial to understand in terms of the basic physics of our star, but they are so highly structured and dynamic that only the unique capabilities of the IRIS mission are presently poised to provide answers.
- IRIS is a great asset to the HSO (Heliophysics Systems Observatory). It has an excellent track record of coordinating with HSO missions. The current synergy with Hinode, SDO and RHESSI is an example of a system’s approach to build up a better understanding of such a complex system as the solar atmosphere. Future collaboration/coordination plans include ground-based instrumentation that will soon be operational, such as the ALMA (Atacama Large Millimeter/submillimeter Array), the German GREGOR 1.6 m telescope, and the NSF-funded 4 m DKIST (Daniel K. Inouye Solar Telescope). IRIS observations will be used to investigate solar abundance details and provide physical insight into the cyclic- and feature-driven variability of the solar UV output—both of which have far-reaching impact in heliospheric and astrophysical science.
- IRIS spacecraft / instrument health and status: The IRIS spacecraft contains no consumables, and has performed almost flawlessly on orbit for two years. No subsystem shows signs of significant degradation.
- IRIS Data Operations (accessibility, quality control, archiving) The IRIS data are available to the science community in each level of reduction as soon as they are available at the SDO JSOC (Joint Science Operations Center), where they are archived and processed. Calibrated data are available within seven days of the observations, while preliminary calibrated quick-look images are provided within hours of the observations. The Heliophysics Coverage Registry provides a specific search engine that enables easy co-temporal/spatial IRIS and Hinode data comparison. In addition, the modeling output are considered part of the IRIS mission data and made available for community analysis through the same distribution channel.
• On May 6, 2015, the IRIS mission completed its 10,000th orbit of the Earth. On June 27, 2015, IRIS was two years on orbit. 31)
Figure 16: In this photo, the IRIS spacecraft captured several large solar prominences on the edge of the sun, acquired on April 28, 2015 (image credit: NASA)
• October 16, 2014: The IRIS mission has provided scientists with five new findings into how the sun’s atmosphere, or corona, is heated far hotter than its surface, what causes the sun’s constant outflow of particles called the solar wind, and what mechanisms accelerate particles that power solar flares. - The new information will help researchers better understand how our nearest star transfers energy through its atmosphere and track the dynamic solar activity that can impact technological infrastructure in space and on Earth. 32) 33)
Five papers, based on IRIS data, highlight different aspects of the energy’s journey from the sun’s surface through its atmosphere in the Oct. 17, 2014, issue of Science magazine. 34)
- Solar heat bombs: The first result identified heat pockets of 200,000 degrees Fahrenheit (~111100º C), lower in the solar atmosphere than ever observed by previous spacecraft. Scientists refer to the pockets as solar heat bombs because of the amount of energy they release in such a short time. Identifying such sources of unexpected heat can offer deeper understanding of the heating mechanisms throughout the solar atmosphere.
- Resolving unresolved structure: For its second finding, IRIS observed numerous, small, low lying loops of solar material in the interface region for the first time. The unprecedented resolution provided by IRIS will enable scientists to better understand how the solar atmosphere is energized.
- Mini-tornadoes: A surprise to researchers was the third finding of IRIS observations showing structures resembling mini-tornadoes occurring in solar active regions for the first time. These tornadoes move at speeds as fast as 20 km/s and are scattered throughout the chromosphere, or the layer of the sun in the interface region just above the surface. These tornados provide a mechanism for transferring energy to power the million-degree temperatures in the corona.
- High-speed jets: Another finding uncovers evidence of high-speed jets at the root of the solar wind. The jets are fountains of plasma that shoot out of coronal holes, areas of less dense material in the solar atmosphere and are typically thought to be a source of the solar wind.
- Accelerated electrons in nanoflares: The final result highlights the effects of nanoflares throughout the corona. Large solar flares are initiated by a mechanism called magnetic reconnection, whereby magnetic field lines cross and explosively realign. These often send particles out into space at nearly the speed of light. Nanoflares are smaller versions that have long been thought to drive coronal heating. IRIS observations show high energy particles generated by individual nanoflare events impacting the chromosphere for the first time.
Figure 17: Illustration of ubiquitous nanoflares throughout the corona (image credit: NASA)
• August 2014: The science and technical performance of the observatory has been excellent. All systems are currently operating within specification and the science continues to exceed level one requirements. The observatory operates and executes the science timelines autonomously with the MOC uploading commands via S-band once per day, five days a week. The science team develops the timelines based on recent active areas and regions of interest and based on collaborations with other missions and observatories; the timelines are loaded between one and three days before execution. The science data is transmitted to the ground via X-band using ground stations at Svalbard, Alaska, and Wallops. The data is transmitted via internet protocol to the MOC and then to Stanford for archiving, processing, and distribution to the science community (Ref. 10).
- The technical performance on-orbit has met or exceeded the program requirements. The ACS pointing performance of the pitch and yaw axis during nominal science operations has been better than 0.5 arcsec, which is an order of magnitude better than the requirements. The peak to peak roll error is 7.7 arcsec which translates to a line of sight error on the sun to less than 0.05 arcsec. While there are periodic disturbances from the large filter wheel in the instrument and from RWA momentum unloading, these motions are corrected by the instrument’s image stabilization system.
- Over the past year, IRIS has carried out greater than 5000 orbits of solar observations including long observations of active regions and studies of: coronal holes, solar plage, solar flares, emergence of filaments, spicules, coronal rain, and other solar features.
• June 28, 2014: One year ago today, NASA's newest solar observatory was launched into orbit around Earth. IRIS observes the low level of the sun's atmosphere — a constantly moving area called the interface region — in better detail than has ever been done before. 35)
- During its first year in space, IRIS provided detailed images of this area, finding even more turbulence and complexity than expected. The interface region lies at the core of many outstanding questions about the sun's atmosphere, such as how solar material in the corona reaches millions of degrees, several thousand times hotter than the surface of the sun itself or how the sun creates giant explosions like solar flares and coronal mass ejections. The interface region is also where most of the ultraviolet emission is generated that impacts the near-Earth space environment and Earth’s climate.
- In its first year, IRIS fortuitously witnessed dozens of solar flares, including one X-class flare, and the foot points of a coronal mass ejection, or CME. IRIS must commit to pointing at certain sections of the sun at least a day in advance, so catching these eruptions in the act involves educated guesses and a little bit of luck.
- There have been several CMEs during the past year, but, due to the small field of view, IRIS must be pointed at the location when the CME occurs as finally happened on May 9, 2014.
• A CME (Coronal Mass Ejection) surged off the side of the sun on May 9, 2014, and NASA's newest solar observatory caught it in extraordinary detail. This was the first CME observed by IRIS. 36)
Legend to Figure 18: The four panels show the evolution of the CME as captured in the SJI channel; the sequence starts in the top left panel and progresses from frame a-d. The images cover a 175 arcsec x 175 arcsec area of the sun. The central line is the 0.33 arcsec wide spectrograph slit.
• On March 29, 2014, an X-class flare (CME) erupted from the right side of the sun... and vaulted into history as the best-observed flare of all time. The flare was witnessed by four different NASA spacecraft and one ground-based observatory — three of which had been fortuitously focused in on the correct spot as programmed into their viewing schedule a full day in advance. 37)
Figure 19: This combined image shows the March 29, 2014, X-class flare as seen through the eyes of different observatories. SDO is on the bottom/left, which helps show the position of the flare on the sun. The darker orange square is IRIS data. The red rectangular inset is from Sacramento Peak. The violet spots show the flare's footpoints from RHESSI (image credit: NASA/GSFC)
The telescopes involved were:
- NASA's IRIS (Interface Region Imaging Spectrograph)
- NASA's SDO (Solar Dynamics Observatory)
- NASA's RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager)
- JAXA's Solar-B/Hinode Observatory
- National Solar Observatory's Dunn Solar Telescope located at Sacramento Peak in New Mexico.
Numerous other spacecraft provided additional data about what was happening on the sun during the event and what the effects were at Earth.
- NASA's STEREO mission and the SOHO mission of ESA and NASA both watched the great cloud of solar material that erupted off the sun with the flare.
- NOAA's GOES satellite tracked X-rays from the flare, and other spacecraft measured the effects of the flare as it came toward Earth.
This event was particularly exciting for the IRIS team, as this was the first X-class flare ever observed by IRIS (launched on June 28, 2013). IRIS provided scientists with the first detailed view of what happens in this region during a flare.
Figure 20: X1-class solar flare on March 29, 2014 as seen by NASA’s IRIS spacecraft (image credit: NASA/IRIS/SDO) 38)
Coordinated observations are crucial to understanding such eruptions on the sun and their effects on space weather near Earth. Where terrestrial weather watching involves thousands of sensors and innumerable thermometers, solar observations still rely on a mere handful of telescopes. The instruments on the observatories are planned so that each shows a different aspect of the flare at a different heights off the sun's surface and at different temperatures. Together the observatories can paint a three-dimensional picture of what happens during any given event on the sun.
• On Jan. 28, 2014, NASA's IRIS (Interface Region Imaging Spectrograph) mission witnessed its strongest solar flare since it launched in the summer of 2013. Solar flares are bursts of X-rays and light that stream out into space, but scientists don't yet know the fine details of what sets them off.
Figure 21: On Jan. 28, 2014, the IRIS observatory observed its strongest solar flare to date (image credit: NASA/GSFC, IRIS, SDO)
- IRIS peers into a layer of the sun's lower atmosphere just above the surface, called the chromosphere, with unprecedented resolution. However, IRIS can't look at the entire sun at the same time, so the team must always make decisions about what region might provide useful observations. On Jan. 28, scientists spotted a magnetically active region on the sun and focused IRIS on it to see how the solar material behaved under intense magnetic forces. At 19:40 UTC, a moderate flare, labeled an M-class flare — which is the second strongest class flare after X-class — erupted from the area, sending light and X-rays into space. 39)
- IRIS studies the layer of the sun’s atmosphere called the chromosphere that is key to regulating the flow of energy and material as they travel from the sun's surface out into space. Along the way, the energy heats up the upper atmosphere, the corona, and sometimes powers solar events such as this flare.
Figure 22: NASA's IRIS witnessed its strongest solar flare since it launched in the summer of 2013 (image credit: NASA, IRIS)
• December 2013: Over its first six months, IRIS has thrilled scientists with detailed images of the interface region, finding even more turbulence and complexity than expected. IRIS scientists presented the mission's early observations at a press conference at the Fall American Geophysical Union meeting in San Francisco, CA, on Dec. 9, 2013. 40) 41) 42)
For the first time, IRIS is making it possible to study the explosive phenomena in the interface region in sufficient detail to determine their role in heating the outer solar atmosphere. The mission’s observations also open a new window into the dynamics of the low solar atmosphere that play a pivotal role in accelerating the solar wind and driving solar eruptive events.
Tracking the complex processes in the interface region requires instrument and modeling capabilities that are only now within our technological reach. IRIS captures both images and what's known as spectra, which display how much of any given wavelength of light is present. This, in turn, corresponds to how much material in the solar atmosphere is present at specific velocities, temperatures and densities. IRIS's success is due not only to its high spatial and temporal resolution, but also because of parallel development of advanced computer models. The combined images and spectra have provided new imagery of a region that was always known to be dynamic, but shows it to be even more violent and turbulent than imagined.
The project is seeing rich and unprecedented images of violent events in which gases are accelerated to very high velocities while being rapidly heated to hundreds of thousands of degrees. Bart De Pontieu, the IRIS science lead at Lockheed Martin, has been culling images of two particular types of events on the sun that have long been interesting to scientists.
- One is known as a prominence, which are cool regions within the interface region that appear as giant loops of solar material rising up above the solar surface. When these prominences erupt they lead to solar storms that can reach Earth. IRIS shows highly dynamic and finely structured flows sweeping throughout the prominence.
- The second type of event is called a”spicule”, which are giant fountains of gas – as wide as a state and as long as Earth – that zoom up from the sun's surface at 150,000 miles per hour. Spicules may play a role in distributing heat and energy up into the sun's atmosphere, the corona. IRIS imaging and spectral data allows the project to see at high resolution, for the first time, how the spicules evolve. In both cases, observations are more complex than what existing theoretical models predicted.
Mats Carlsson of the University of Olso helps support the crucial computer model component of IRIS' observations. The computer models require an intense amount of power. Modeling just an hour of events on the sun can take several months of computer time. IRIS relies on supercomputers at NASA/ARC, the Norwegian supercomputer collaboration and the Partnership for Advanced Computing in Europe.
Such computer models had helped design the IRIS instruments by providing a basis for the instrument performance requirements. Currently, they are used for analysis of IRIS data, as they represent the state of knowledge about what scientists understand about the interface region. By comparing models with actual observations, researchers figure out where the models fail, and therefore where the current state of knowledge is not complete.
Figure 23: The fine detail in images of prominences in the sun's atmosphere from NASA's IRIS mission – such as the red swirls shown here – are challenging the way scientists understand such events (image credit: NASA, LMSAL, IRIS collaboration)
Figure 24: IRIS provides novel views of the mass cycle at the interface between the cool surface and hot atmosphere; the image of the active transition region was acquired on Oct. 2, 2013 (image credit: IRIS collaboration, Ref. 40)
• On July 17, 2013, the IRIS team opened the IRIS telescope door and captured its first observations of a region of the sun that is now possible to observe in detail: the lowest layers of the sun's atmosphere (Figure 25). 43) 44) 45)
The first images from IRIS show the solar interface region in unprecedented detail. They reveal dynamic magnetic structures and flows of material in the sun's atmosphere and hint at tremendous amounts of energy transfer through this little-understood region. These features may help power the sun's dynamic million-degree atmosphere and drive the solar wind that streams out to fill the entire solar system.
IRIS capabilities are tailored to let scientists observe the interface region in exquisite detail. The energy flowing through it powers the upper layer of the sun's atmosphere, the corona, to temperatures greater than 1 million ºC. That is almost a thousand times hotter than the sun's surface. Understanding the interface region is important because it drives the solar wind and forms the ultraviolet emission that impacts near-Earth space and Earth's climate.
• In its first step towards science operations since launch, . 46)
• A 60 day check out period began at launch. The first 30 days, which ended on July 27, consisted of tests and spacecraft system checks. The team will use the remaining 30 days for initial observing runs to fine tune instrument observations. If all is nominal, the team plans to begin normal science mode by August 26, 2013. 47)
Figure 26: Artist's rendition of the IRIS spacecraft on orbit (image credit: NASA/GSFC)
The IRIS instrument is a multi-channel imaging spectrograph with a 20 cm UV telescope. The objective is to obtain UV spectra and images with high resolution in space (1/3 arcsec) and time (1s) focused on the chromosphere and the transition region of the sun, a complex dynamic interface region between the photosphere and corona. In this region, all but a few percent of the non-radiative energy leaving the sun is converted into heat and radiation. Here, magnetic field and plasma exert comparable forces, resulting in a complex, dynamic region whose understanding remains a challenge. 48) 49) 50) 51)
The IRIS instrument uses a Cassegrain telescope with a 19 cm primary mirror and an active secondary mirror with a focus mechanism. The telescope has a FOV of about 3 arcmin x 3 arcmin and feeds far UV (FUV, from 1332 to 1407 Å) and near UV (NUV, from 2783 to 2835 Å) light into a spectrograph box. Dielectric coatings throughout the optical path ensure visible and IR radiation is suppressed. Most of the solar energy passes through the ULE substrate of the primary mirror and is radiated back into space.
Figure 27: Conceptual design of the IRIS instrument (image credit: NASA/LMSAL)
The IRIS telescope is feeding a stigmatic UV spectrograph and a SJI (Slit-Jaw Imager) that provide an unprecedented combination of 1/3 arcsec imaging with rapid high-resolution spectroscopy. Simultaneous intensity and velocity maps (spectroheliograms) in multiple UV emission lines covering a range of chromospheric, transition region, and coronal temperatures are acquired at a cadence that is comparable to pure imaging instruments. The UV slit-jaw imager provides high resolution, high cadence imaging in selected spectral bands. The instrument package builds extensively on heritage technology: TRACE, SXI (GOES), SECCHI-EUVI (STEREO), FPP (Hinode), and HMI/AIA of SDO.
Figure 28: Diagram of spectrograph and slit-jaw imager with part of the internal structure and baffling (image credit: NASA/LMSAL)
Figure 29: The IRIS instrument block diagram (image credit: LMSAL)
IRIS will obtain spectra along a slit (1/3 arcsec wide), and slit-jaw images. The CCD detectors will have 1/6 arcsec pixels. IRIS will have an effective spatial resolution between 0.33 and 0.4 arcsec and a maximum field of view of 120 arcsec.
Table 3: IRIS instrument characteristics (Ref. 7)
IRIS will have a high data rate (0.7 Mbit/s on average) so that the baseline cadence is: 5 s for slit-jaw images, 1 s for six spectral windows, including rapid rastering to map solar regions.
Table 4: IRIS spectrograph channels. Dispersion, Camera Electronics Box (CEB) and Effective Area (EA) vary for the three band passes
Figure 30: Spectrograph optical layout (image credit: NASA)
Many mechanical and electronic parts, drawings and designs are being re-used from the successful TRACE, Solar-B/Hinode, SECCHI and AIA/HMI programs. For example, the IRIS telescope, built at SAO (Smithsonian Astrophysical Observatory), Cambridge, MA, is based on the design of one of the AIA (Atmospheric Imaging Assembly) telescopes. The spectrograph is being built by MSU and LMSAL and NASA/ARC (Ames Research Center) will provide mission operations support.
The data handling and pipeline will be based on the existing AIA data pipeline, with Stanford University playing a major role in the operation of the data pipeline. Data will be downlinked through an X-band antenna at ground stations in Svalbard in Norway (funded by the Norwegian Space Center) and in Alaska and other NASA sites.
IRIS science investigation (Ref. 8):
The IRIS spectra will cover temperatures from 4,500 K to 10 MK, with the images covering temperatures from 4,500 K to 65,000 K. The CCD detectors will have 1/6 arc second pixels. IRIS will have an effective spatial resolution between 0.33 and 0.4 arcsec and a maximum field of view of 170 arcsec x 170 arcsec.
IRIS will have thermal coverage from the photosphere (neutral lines, wings of Mg II h/k) through the chromosphere (Mg II h/k) and transition region (C II, Si IV, O IV) into the corona (Fe XII and Fe XXI). This will allow the project to fully trace and identify the connections between all regions in the solar atmosphere. The high throughput of the instrument will allow short exposure times that enable measurements of the intensity, Doppler shift (down to 1 km/s), line width., and reconstruct images.
Deeper exposures will also reveal the full shape of the spectral line profiles (e.g., asymmetries). The short exposure times and flexible rastering schemes (Figure 31) will allow rapid scans of small regions on the Sun at very high spatial resolution of order 0.33-0.40 arcsec. IRIS will function as a microscope for instruments onboard Solar-B/Hinode and the SDO (Solar Dynamics Observatory), which have a spatial and temporal resolution that is significantly reduced compared to IRIS.
The sparse raster option will allow rapid scans of much larger areas, which can be used, for example, for flare or CME (Coronal Mass Ejection) watch programs (Figure 31). The simultaneous images will have broader spectral range, so they will contain a mixture of continuum and upper chromospheric (Mg II k) or transition region (C II, Si IV) emission. The upper chromospheric and transition region contributions are estimated to be in excess of 50% of the total emission of in these spectral regions.
Figure 31: High throughput allows for rapid rasters of high S/N spectra that enable line centroid velocity determination down to 1 km/s precision within 1 s exposures for the brightest lines (image credit: LMSAL)
IRIS will be operated in a manner that is similar to TRACE and Solar-B/Hinode, with observing programs uploaded 5 times per week, and the data made publicly available within a day of the observation. The project will operate IRIS in full coordination with Solar-B/Hinode and SDO. To augment the IRIS data, the project will have a special focus on coordination with ground-based observations that obtain chromospheric spectral line profiles over a large field of view (using Fabry-Perot interferometers). The Mg II h/k lines are optically thick lines, so require careful analysis for a proper interpretation. This can be done using 3D radiative MHD (Magneto-Hydrodynamic) models and non-LTE radiative transfer diagnostic software tools such as MULTI and RH. This approach is essential because of the highly dynamic and rapidly spatially varying nature the chromosphere.
In summary, the IRIS science investigation will focus on combining IRIS data with Solar-B/Hinode, SDO and ground-based observations, together with numerical MHD and multi-fluid plasma simulations to develop a comprehensive picture of the flow of energy and mass in the solar atmosphere. Given the complexity of the interface region, the interplay between observations and simulations will be very important. The IRIS science investigation has a strong theory/numerical modeling component. State-of-the-art radiative 3D MHD numerical simulations and synthetic (non-LTE) diagnostics in, e.g., optically thick lines like Mg II h/k, will allow creation of simulated data for detailed comparisons with IRIS observations (Ref. 8).
Figure 32: Photo of the IRIS telescope (image credit: NASA)
The MOS (Mission Operations System) and GDS (Ground Data System) were developed by NASA/ARC (Ames Research Center). The MMOC (Multi-Mission Operations Center) is housed and staffed by NASA Ames personnel. The GDS is supported by NASA’s NEN (Near-Earth Network) and the NSC (Norwegian Space Center) with ground stations in Svalbard, Alaska, and Wallops Island. The observatory operates autonomously with one command pass during nominal working days (5 days per week). Science team members prepare observation timelines that include pointing targets, scans and slews, and coordinated observation sequences with other missions and observatories. The timelines are prepared the day prior to the command load and cover weekends and holidays (Ref. 10).
The science data is downlinked via 13 ground station passes per day (7 days per week). The MOC is staffed for the command passes with lights out operation the remainder of the time. The data from each ground station pass is transferred autonomously using standard internet protocol. The observatory has a fault management system that monitors subsystem telemetry points and determines if immediate action is required or if alerts are to be sent to the ground. The team has a health and safety web page viewable by all team members and an Alert Notification System on the ground that monitors telemetry and sends out text messages if any program parameters go out of range. IRIS generates more than 45 Gb of science data/day. The data is transferred from the NASA MMOC to the SDO (Solar Dynamics Observatory) Science Operations Center at Stanford, where preliminary processing is performed. The quicklook data is typically available on an open access basis to the science community within 6 hours of receipt on the ground. Calibrated data is available 7 days after receipt on the ground.
Figure 33: IRIS mission concept (image credit: LMSAL)
Throughout design, development, integration, and test, the instrument, spacecraft, MOS/GDS, and science teams were integrated and co-located as much as possible. Many personnel who worked on the instrument and spacecraft subsystems and components were part of the program from design phase to launch and on-orbit checkout. This was an important part of the program as it ensured: continuity of knowledge from design to test, that subsystem test scripts could be modified and carried into observatory test, and that anomaly and root cause investigations could be carried out in a timely manner. This also enabled team members to cross-train on other subsystems allowing the relatively small team to carry out the environmental, comprehensive and functional tests, launch operations, and on-orbit checkout and calibration.
A common ground system EGSE operating system, ITOS (Integrated Test and Operations System), was selected for the instrument, spacecraft, and mission operations systems. This allowed test scripts and display screens developed during subassembly integration and test to be migrated to the system level testing and to the MOC. EGSE imaging and analysis routines from AIA and HMI were ported for use on IRIS. The spacecraft EGSE was developed for the IRIS program with a few components such as the Solar Array Simulators and RF test equipment made available from prior programs. The optical stimulus GSE – used for performance testing of the instrument throughout integration and test - was based on the AIA stimulus system including use of a spare telescope tube. In addition, a set of company processes and procedures were tailored to meet the scope and pace of the mission.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).