Millions of weak flares in the Sun's atmosphere can heat its corona to millions of degrees.
The outermost layer of the earth's atmosphere is the coldest, but the opposite is true on the sun. The solar corona, the outer layer of the star's atmosphere, is hot to a million degrees, and at its peak reaches two million, while the lower parts of the sun's atmosphere are heated a thousand times fainter, to about 5500 ° C.
The problem of overheating of the solar corona has remained unresolved for over a century. The influx of energy is associated with the action of acoustic waves propagating in the plasma, or with the reconnection of the lines of force of the solar magnetosphere. In an article published in the journal Nature Astronomy, Shin-nosuke Ishikawa and colleagues from Japan and the United States are developing an alternative version.
Instead of powerful hypersonic waves and magnetic reconnections, energy for heating the solar corona can be supplied by small, short, but numerous, "nanoflares", which cause fluctuations in the star's magnetic field in the plasma. These flares are reminiscent of the usual explosive outbursts that we register from time to time, but millions of times fainter. Each of them individually is too small for modern instruments to see them against the stormy background of the Sun. However, the authors of the article note that it is possible to notice some signs of the action of such "nanoflares".
So, scientists write about the data obtained in 2014 during the FOXSI-2 suborbital experiment, when a small launch vehicle raised an X-ray solar telescope to a height, which for 6, 5 minutes observed different regions of the solar disk, after which it returned to the ground.
There were no active flares on the Sun during this period, but the instrument recorded 15 high-energy X-ray photons that could have been emitted by plasma heated to a mind-boggling 10 million degrees. Models explaining plasma heating by the action of large-scale processes can hardly explain the appearance of a plasma section heated to such a temperature, scientists say. But the accidental action of a group of close "nanoflares", the energy of which was then scattered over the solar corona, may well manifest itself in this way.
The next experiment of the FOXSI series is planned for the summer of 2018. Perhaps then we will receive new data that will allow us to put an end to the long debate around the problem of heating the solar corona.