In this spring's issue of Nature Astronomy, researchers from the Rosseland Centre for Solar Physics (RoCS) have identified a connection between the heating around sunspots in the chromosphere and corona, and the high temperatures in these layers.
Solar physicists Souvik Bose, Bart De Pontieu, and Viggo Hansteen have combined observations from the chromosphere and corona with machine learning, bringing them closer to solving the big question: Why is it hotter around the Sun than on its surface?
A new explanatory model
Lead author Souvik Bose explains that the researchers used high-resolution observations from the chromosphere and corona, combined with advanced numerical simulations. They utilized data from IRIS, a small space telescope that studies the chromosphere and the Sun's interface region, as well as from a sounding rocket. These observations were combined with numerical simulations and a data code developed at RoCS.
– "The Bifrost code is designed to model the outer layers of the solar atmosphere and includes all the relevant physics needed to interpret the observations," explains Viggo Hansteen.
With this, they may have solved part of the puzzle that has puzzled researchers for the past 25 years:
- The Sun's core reaches around 15 million degrees Celsius.
- The coldest layer, just above the photosphere, measures around 4,100 degrees.
- The temperature rises to 10,000 degrees in the chromosphere.
In the outermost layer, the corona, the temperature exceeds 1 million degrees and can rise to over 10 million degrees in certain areas.
"Moss" on the Sun
Souvik Bose explains that the physical mechanism heating up the "moss-like" areas around active, strong magnetic fields on the Sun to extremely high temperatures might be the key to understanding this heating. He emphasizes that it is still uncertain how widespread this mechanism is in other areas on the Sun, but the researchers are pleased that their work contributes to solving the puzzle of the Sun's corona.
– "Future high-resolution observations from the upcoming NASA Multi-Slit Solar Explorer (MUSE) will be necessary to fully understand how the corona and moss are heated," says Bose.
RoCS researchers are contributing to this new spacecraft.
Discovery of "invisible" magnetic fields
The researchers discovered small, "moss-like" patches around sunspots on the Sun—areas with extra strong, almost invisible magnetic fields. These field lines generate electric currents that provide additional heating to the gas around them, on top of the heat already streaming from the overlying corona. This could explain why the "moss" around sunspots becomes much hotter than the Sun's surface, a phenomenon that has been challenging to understand due to the extreme heat.
– "The results align well with simulations and provide deep insights into the nature of the heating mechanism in active solar regions. This discovery helps explain why the Sun's outer atmosphere, the corona, is so much hotter than the Sun's surface," say Souvik Bose and Professor Viggo Hansteen.
The researchers affiliated with RoCS have taken a significant step forward.
Scientific Article:
Souvik Bose, Bart De Pontieu, Viggo Hansteen et al., "Chromospheric and coronal heating in an active region plage by dissipation of currents from braiding," Nature Astronomy, April 2024.