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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).


Figure 2: Artist's rendition of the IRIS spacecraft (image credit: NASA)


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)


Figure 4: Photo of the fully integrated IRIS spacecraft in a cleanroom of Lockheed Martin Space Systems (image credit: NASA, Lockheed Martin) 10)


Figure 5: Photo of the IRIS spacecraft in launch configuration (image credit: NASA, Lockheed Martin)

The SMEX (Small Explorer) IRIS spacecraft has a launch mass of ~236 kg. It is ~ 2.18 m in length,and ~ 3.7 m across with its solar panels deployed. The mission design life is 2 years.


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. 11) 12)

Orbit: Sun-synchronous orbit, altitude of 596 km x 666 km, inclination = 97.9º. The orbit allows eclipse-free continuous viewing for 8 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.



Mission status:

• 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 6: 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. 13)

- 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 7: 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. 14) 15) 16)

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 8: 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 9: 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. 14)

• 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 10). 17) 18) 19)

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.


Figure 10: These two images show a section of the sun as seen by IRIS (right) and by the SDO mission (left), acquired on July 17, 2013 (image credit: NASA)

• In its first step towards science operations since launch, . 20)

• 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. 21)


Figure 11: Artist's rendition of the IRIS spacecraft on orbit (image credit: NASA/GSFC)



IRIS instrument:

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. 22) 23) 24) 25)

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 Ä) and near UV (NUV, from 2783 to 2835 Ä) 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 12: Conceptual design of the IRIS instrument (image credit: NASA/LMSAL, Ref. 7)

The IRIS telescope is feeding a stigmatic UV spectrograph and a 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.

In December 2010, the IRIS instrument passed the CDR (Critical Design Review) conducted at LMSAL (PI: Alan Title). 26) 27)


Figure 13: Diagram of spectrograph and slit-jaw imager with part of the internal structure and baffling (image credit: NASA/LMSAL)


Figure 14: 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.

Primary diameter

19 cm

Effective focal length

6.895 m

FOV (Field of View)

175 arsec x 175 arcsec (SJI), ? arcsec x 175 arcsec (SG – slit), 130 arcsec x 175 arcsec (SG – raster)

Spatial scale (pixel)

0.167 arsec

Spatial resolution

1/3 arcsec FUV (Far-UV), 0.4 arcsec NUV (Near-UV)

Spectral scale (pixel)

12.8 mÄ (FUV), 25.6 mÄ (NUV)

Spectral resolution

26 mÄ (FUV SG), 53 mÄ (NUV SG)


55 Ä (FUV SJI), 4 Ä (NUV SJI)

CCD detectors

Four e2v 2061 x 1056 pixels, thinned, back-illuminated

CCD cameras

Two x 4-port readout cameras (SDO flight spares)

Detector full well

150,000 electrons

Typical exposure times

0.5 to 30 seconds

Flight Computer

BAe RAD 6000


Instrument = 87 kg, spacecraft = 96 kg, Total = 183 kg


Instrument = 55 W, spacecraft = 247 W, Total = 302 W

Science telemetry

Average downlink rate = 0.7 Mbit/s, X-band downlink rate = 13 Mbit/s, Total data volume ~ 20 GB(day (uncompressed)

Table 1: 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.


Wavelength (Ä)

Dispersion (mÄ)





EA (cm2)

Temperature (log T)




























Table 2: IRIS spectrograph channels. Dispersion, Camera Electronics Box (CEB) and Effective Area (EA) vary for the three band passes


Figure 15: 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 16) 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 16). 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 16: 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 17: Photo of the IRIS telescope (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.