Dynamic, low-lying magnetic loops dominate the ultraviolet emission from the solar transition region. The enhanced spatial and temporal resolution of the solar observation satellite Interface Region Imaging Spectrograph (IRIS) has made it possible to study these structures adequately.
In collaboration with the University of Colorado Boulder and NASA’s Marshall Space Flight Center, scientists from Rice University detected a phenomenon in the solar wind that could solve a long-standing mystery about the sun: why the solar atmosphere is millions of degrees hotter than the surface.
Scientists 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.
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.
After studying IRIS images, scientists just solved the details of these transition region loops and detected pockets of super-hot plasma. Scientists also studied the movements and temperatures of ions within the loops via the light they emit, read as spectral lines that serve as chemical “fingerprints.”
Rice solar physicist Stephen Bradshaw said, “It’s in the emission lines where all the physics is imprinted. 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 pictures uncovered hot-spot spectra where the lines were widened by thermal and Doppler impacts, showing the components engaged with nanoflares as well as their temperatures and velocities.
At the problem spots, they discovered reconnecting jets containing silicon ions moved toward (blue-shifted) and away from (red-shifted) the spectator (IRIS) at speeds up to 100 kilometers for every second. No Doppler shift was distinguished for the lighter oxygen particles.
Scientists studied two components of the mechanism: how the energy gets out of the magnetic field and how it heats the plasma.
Bradshaw said, “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. 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.”
In this process, the magnetic fields of the plasma strands break. They then reconnect at braiding sites into lower energy states and emit stored magnetic energy. Where this takes place, the plasma becomes superheated.
However, it remains obscure how the plasma gets heated by the released magnetic energy. To find out the answer, scientists took a gander at the regions in these little loop structures where reconnection takes place. They then measured the emission lines from the ions, chiefly silicon, and oxygen.
What they found was surprising. They found that the spectral lines of the silicon ions were much broader than the oxygen. It means preferential heating of the silicon ions. There’s a kinetic process called ion cyclotron heating that favors heavy heating ions over lighter ones.
Bradshaw 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.”
Shah Mohammad Bahauddin, now a research faculty member at the Laboratory for Atmospheric and Space Physics at Colorado, said, “In the solar wind, heavier ions are significantly hotter than lighter ions. That’s been definitively measured. Our study shows for the first time that this is also a property of the transition region. It 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 is whether such phenomena are happening at the same rate all over the sun. Most probably, the answer is no. 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. I believe IRIS will be able to tell us about the chromospheric pieces soon. That will help us build a unified and global theory of the sun’s atmosphere.”
- Bahauddin, S.M., Bradshaw, S.J. & Winebarger, A.R. The origin of reconnection-mediated transient brightenings in the solar transition region. Nat Astron (2020). DOI: 10.1038/s41550-020-01263-2