NASA’s newest sun-watcher, the Interface Region Imaging Spectrograph, launched in 2013 with a specific goal: track how energy and heat coursed through a little understood region of the sun called the interface region.
Sandwiched between the solar surface and its outer atmosphere, the corona, the interface region is where the cooler temperatures of the sun’s surface transition to the hotter temperatures above. Moreover, all the energy to power the sun’s output — including eruptions such as solar flares and the sun’s constant outflow of particles called the solar wind — must make its way through this region.
Five papers based on IRIS data will 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. By looking at various regions of the interface region in unprecedented resolution, the papers offer clues to what heats the corona to unexplained temperatures of millions of degrees, far hotter than the surface of the sun itself, as well as what causes great writhing movement and accelerated particles throughout the solar atmosphere.
“This set of research really delivers on the promise of IRIS, which has been looking at a region of the sun with a level of detail that has never been done before,” said Bart De Pontieu the IRIS science lead at Lockheed Martin in Palo Alto, California. “The results focus on a lot of things that have been puzzling for a long time and they also offer some complete surprises.”
Solar Heat Bombs
One of the biggest surprises comes in the form of heat pockets of 200,000 F, low in the solar atmosphere – far lower down than where such high temperatures were expected. In a paper led by Hardi Peter of the Max Planck Institute for Solar System Research in Gottingen, Germany, the pockets were named bombs because of how much energy they release in such a short time.
Identifying different temperature material in the solar atmosphere is fairly straightforward, but it is much more complex to determine how high above the surface such material lies. Spotting such features relied heavily on IRIS’ high-resolution spectrograph, an instrument that divides incoming light into its separate wavelengths. Such spectra can then be analyzed to see what temperature material is present in a given area as well as how dense it is and how fast it is moving. IRIS showed this very hot material sandwiched between two cold layers at temperatures usually found only near the sun’s surface, thus giving information about its low-lying location that would have been otherwise hard to find.
“These unexpected results will likely lead to a reassessment of other phenomena in the low solar atmosphere,” said Alan Title, the IRIS principal investigator at Lockheed.
Resolving Unresolved Structure
A second paper highlights IRIS’ ability to zoom into part of the interface region, called the transition region, with unprecedented resolution. Early observations of the atmosphere hovering over the sun’s limb from Skylab in the 1970s pointed to the fact that this layer must be more complicated and structured than what could be seen in those images – the energy emissions coming from the region just didn’t seem to be physically possible based on the structure seen. But, like someone with nearsighted vision, Skylab didn’t have the necessary resolution to determine exactly what that structure was.
Able to see hundreds of times more detail than Skylab, IRIS has resolved numerous, small, low lying loops of material in the transition region, as described in a paper led by Viggo Hansteen, a solar scientist at the University of Oslo in Norway. These move quickly and last for only minutes at a time. At just a couple thousand miles high, the loops could not be seen with any previous instrument. Identifying such loops offers new insight into how the transition region emits the light and energy that we see: These loops show us a dynamic region where much heating occurs and is quickly released.
A third paper uses IRIS to determine speeds of structures in the other part of the interface region just above the sun’s surface, called the chromosphere. The solar features seem to be twisting – one side of the loop appears to be moving away from the viewer, the other side is moving toward it.
“We conclude that the gas in the chromosphere is often twisting like a tornado twirling around a central magnetic tube,” said De Pontieu, who is also the first author on this paper. “These twisting motions are often associated with heating to hundreds of thousands of degrees.”
While fairly small by solar standards, these mini-tornadoes twist at up to 12 miles per second, and are scattered throughout the chromosphere. The tornadoes are signatures of a magnetic feature called Alfven waves, which are known to be able to carry energy and heat throughout the solar material.
High Speed Jets
A fourth paper led by Hui Tian at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, reveals insight about the very creation of the solar wind. It has generally been thought that the solar wind originates in a gentle way, evaporating from funnels of solar material that are rooted in strong magnetic field regions called networks on the sun’s surface. However, these ideas might need to be updated based on IRIS observations of high-speed jets, traveling at speeds of 90 miles per second — faster than any previously reported jet structures in the interface region.
The IRIS observations show such intermittent jets coming out of areas of weaker magnetic fields and less dense material in the solar atmosphere, called coronal holes, which are typically thought to be a source of the solar wind. Scientists’ next steps are to determine whether the jets are indeed the origins of the solar wind – in contrast to previous predictions that the solar wind traveled through the interface region at just a couple miles per second – or at least how it relates to the solar wind.
Additionally, the jets show temperatures of 200,000 F and can be many times taller than the relatively thin transition region itself, which is about 300 miles high. Hui states that the jets appear to be one of the most important basic structures in the transition region – knowing more about them could help explain why the amount of energy emitted from the transition region is brighter than models would predict.
Accelerated Electrons in Nanoflares
The final paper spotted the effects of ubiquitous nanoflares throughout the corona. Large solar flares are initiated by a mechanism called magnetic reconnection, during which magnetic field lines cross and explosively realign, often sending particles zooming off at near-light speeds. Nanoflares are smaller versions of these that have long been hypothesized to drive coronal heating as they might release enough energy sufficient to heat the entire corona. In this paper, Paola Testa, also at the Harvard-Smithsonian Center for Astrophysics, used IRIS’ ability to measure velocity to determine how the material at the footpoints of magnetic loops react to the accelerated particles that slam down into the interface region.
“Nanoflares have long been associated with coronal heating,” said Title. “With this research we can show the properties of these high energy electrons and how they affect the interface region – the area where the bulk of solar atmospheric heating is known to occur.”
This research has applications beyond just understanding the sun: These high-speed electrons also occur in other stars. Understanding what accelerates them on the sun can translate to better understanding of a host of astrophysical events.
“These five papers show clearly what IRIS offers to our studies of the sun,” said Adrian Daw, the mission scientist for IRIS at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We’ve never before been able to look at this low level of the atmosphere with such detail. Now, IRIS is uncovering a trove of information on how material moves and heats up there. ”
This trove of new IRIS results has revealed a region of the sun more complicated than thought. Instead of homogenous layers of material of one temperature, the interface region is rife with heated zones threading and twisting through it. Together, this information will help scientists map the dynamic interface region for the first time to improve our understanding of how the vast deposits of magnetic energy and twisting solar material of the solar surface are transferred into the million degree temperatures above.