IRIS Tackles Coronal Mystery

Researchers hope NASA's newest solar mission IRIS, launched last Thursday, June 27th, will unravel the mystery surrounding the narrow transition region between the Sun’s surface and its blazing hot atmosphere.

Scientists still wrestle with basic questions about our star, and none is more challenging than why its temperature soars from a hot 6,000 kelvins at the photosphere (visible surface) to a blazing 1,000,000 K atmosphere in the lower corona, across just 1,200 miles (2,000 km). NASA scientists hope that this newest solar sentinel — the Interface Region Imaging Spectrograph (IRIS), launched last Thursday, June 27th — will help unravel this and other solar mysteries.

You’d expect that the farther away you are from a heat source, the cooler it is. But something strange happens in the narrow band, dubbed the “interface region” by the mission’s scientists,between the chromosphere (at the bottom of the Sun’s atmosphere) and the transition region (sandwiched between the chromosphere and corona). This area emits most of the Sun’s ultraviolet radiation while the corona drives the solar wind, both of which impact Earth, so scientists are eager to find out where its energy comes from.

Equipped with an 8-inch-aperture (0.2-m) ultraviolet telescope, the 403-pound (183-kg) IRIS spacecraft will stare at just 1% of the Sun’s surface, snapping one shot about every second to catch the fast-changing activity in the chromosphere. The physics in this region is so complex and dynamic (because the chromosphere hosts both hot and cold plasma) that, until recently, researchers couldn’t model or capture what was going on. But IRIS can tease out features as small as 150 miles (240 km) across and with record speed.

The most recent spacecraft tasked to study the interface was the Transition Region and Coronal Explorer (TRACE), which was retired in 2010. TRACE had a spatial resolution of 1 arcsecond, and its spectra sampled gas from 6,000 to 1,000,000 K. IRIS surpasses TRACE’s spatial resolution and covers a wider spectral range, from 4,500 to 10,000,000 K. The telescope aboard IRIS closely imitates the Atmospheric Imaging Assembly (AIA) aboard NASA’s Solar Dynamics Observatory (SDO) , which studies the Sun’s corona, but offers more detailed imaging. IRIS’s optics have been described as a “microscope to SDO’s telescope”.

“Improved resolution is like zooming in on Google maps. The value of the spatial resolution is that we think the energy transfer isn't happening on large scales,” explains Kelly Korreck (Harvard-Smithsonian Center for Astrophysics), but instead in small, finer structures.

The launch was scheduled for June 26th but got delayed due to a power outage in and around California's Vandenberg Air Force Base, where the control center is located. The following evening, at 7:27:44 p.m. Pacific Daylight Time on June 27th , IRIS rode to space aboard a Pegasus XL rocket that was fired off the California coast. Here’s a video showing the launch.

IRIS now circles Earth north-to-south (in a Sun-synchronous polar orbit) every 97 minutes at an average altitude of 400 miles (645 km). These next couple of months will be spent performing quality tests and calibrations to ensure all systems are “go,” and then the initial data from IRIS’s two-year-long study will become available to the scientific community.

Solar scientists now have a wide range of other spacecraft to keep tabs on the Sun and its effects on Earth. These include:

• Solar and Heliospheric Observatory (SOHO), launched in 1995, orbits the Sun-facing side of Earth and studies a range of solar features from the visible surface to the outer atmosphere.
• Hinode (previously called Solar B) took over for the late Yohkoh (Solar A) to investigate links between magnetic activity in the photosphere and eruptions in the corona).
• Two identical spacecraft comprise the Solar Terrestrial Relations Observatory (STEREO), which uses 3D imaging to investigate the cause of solar ejections.
• Solar Dynamics Observatory (SDO), launched just 3 years ago, studies changes in the Sun’s magnetic field. SDO’s main instrument, a cluster of four telescopes called the Atmospheric Imaging Assembly (AIA), monitors activity in the solar corona.