IRIS Satellite Images Shake up Solar Science

New high-resolution, high-speed spectrographs reveal surprising atmospheric structure

IRIS images reveal the fine structure of solar prominences
New high-speed, high-resolution spectrographic images from IRIS reveal the structure and motion of the Sun's little-understood transitional region with never-before-seen detail.

The first images from NASA’s latest solar observing satellite are in, and they show unprecedented detail—and unexpected complexity—in the roiling lower layers of the Sun’s atmosphere. Already, the images have revealed a previously-unseen fibrous inner structure of many solar features, including the familiar earth-size prominences that can erupt into solar flares and the less-well-known, 500-kilometer-wide spicules that jet up into the corona at speeds of 20 km/s.

Although the data has just started to come in, the early results are enough to challenge the current numerical models of solar behavior.

The pictures from the IRIS (Interface Region Imaging Spectrograph) Observatory, launched 27 June this year, capture images that are sharply defined in space, time, and wavelength. The instrument combines an ultraviolet telescope with a high-precision spectrograph. The imager can resolve solar features 250 km in diameter (see the comparison photos below). The spectral data is used to calculate the atmosphere’s temperature and, thanks to Doppler shifts, its detailed motion (to within one kilometer per second).

Photo: NASA
This image compares the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) at 1600 Angstroms (on left) to the IRIS' Si IV (on right).

The spectrograph can record targeted transition emissions over the range of temperatures from 4500 K to 10 000 000 K, focusing on specific emission lines of magnesium, silicon, carbon, oxygen, and iron, which vary with temperature. The imager also takes pictures—covering an area about 170 arc-seconds (equal to 124 000 km at the sun)—at wavelengths corresponding to a temperature range between 4500 K and 65 000 K. This allows the researchers to separately image the photosphere, chromosphere, interface region, and corona.

Because the spectrograph is fast, IRIS can combine big images with very fine spectra of every 240-km-wide slice of the scene. This reveals the temperature, density, composition, and motion of the solar atmosphere with detail previously unachievable.  (IRIS's design is described in a 2012 IEEE Aerospace Conference paper.)

IRIS astronomers are particularly interested in the interface region between the 6000-K surface and the 2 000 000-K corona. The mechanisms of this transfer are poorly understood, and shedding, er, sunlight on its workings is critical to modeling and forecasting solar weather among many other applications.

The first look at IRIS results came 9 December in a presentation and press conference at the American Geophysical Union fall meeting in San Francisco, by principal investigator Alan Title of the Lockheed-Martin Solar and Astrophysics Laboratory (LMSAL, Palo Alto, Calif.), Bart De Pontieu (also of LMSAL), Mats Carlsson of the University of Oslo (in Norway, of course), and Scott McIntosh of the National Center for Atmospheric Research’s High Altitude Observatory (in Boulder, Colo.).  

“We were not aware of how complicated, and how prevalent, the connections were within sunspots,” said Title. He later added that, “the quality of images and spectra we are receiving from IRIS is amazing.”

“We were flabbergasted by the spectra,” said De Pontieu of some slit-jaw analyses of prominences (video below). “We could see four, five, six different velocities,” implying multiple jets traveling in different directions at various levels along the same line of sight.

Title went on to stress that “we’re getting this kind of quality from a smaller, less expensive mission, which took only 44 months to build.” He was referring to the fact that IRIS is a product of NASA’s Small Explorers (SMEX) program, conceived to produce less expensive probes on shorter development cycles.

Other IRIS Observatory collaborators include the Harvard-Smithsonian Center for Astrophysics (which built the telescope), Montana State University (which designed the spectrograph), NASA Ames Research Center (which handled mission operations and ground data systems). 


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