This image shows a filament, taken on 19 October 2013. Filaments are ribbons of cooler solar material that thread through the Sun's atmosphere and are made up of many threads. Credit: JAXA/NASA/Hinode
The Sun’s corona is several hundred times hotter than its surface and researchers might have finally found out why.
It’s long been supposed that magnetic waves are the cause but it’s only now that the mechanism behind the plasma heating has become clear.
The wispy aura of plasma surrounding the Sun, the corona, is usually only visible from Earth during an eclipse or with a specialist coronagraph.
It’s been known for some time that the corona can reach temperatures as high as 1,000,000K, several hundred times hotter than the 6,000K of the Sun’s observable surface, the photosphere.
While most of the Sun’s energy is from nuclear fusion, it’s been unclear where the energy for the huge coronal temperatures comes from.
Magnetic waves that oscillate through the Sun, called Alvénic waves, are thought to be the main cause but exactly how they transfer their energy to the corona is uncertain.
“For over 30 years scientists hypothesized a mechanism for how these waves heat the plasma,” says Patrick Antolin of the National Astronomical Observatory of Japan.
“An essential part of this process is called resonant absorption -- and we have now directly observed resonant absorption for the first time.”
Resonant absorption is a process where repeated waves add energy to the solar material, similar to repeatedly pushing a swing to make it go higher.
As the waves become more turbulent the plasma heats due to friction and electric currents reaching the huge temperatures seen.
“Through numerical simulations, we show that the observed characteristic motion matches well what is expected from resonant absorption,” says Antolin.
Signatures of this motion, simulated on the left, were found in solar filaments in the Sun’s transition region, the region between the corona and photosphere, by NASA’s Interface Region Imaging Spectrograph (IRIS) and the Japan Aerospace Exploration Agency (JAXA)/MASA’s Hinode solar observatory.
“Now the work starts to study if this mechanism also plays a role in heating plasma to coronal temperatures,” says Bart De Pontieu, science lead for IRIS at Lockheed Martin Solar and Astrophysics Lab.