A possible explanation of how the upper part of the Sun’s atmosphere can be thousands of times hotter than its surface is described in a paper published online in Nature Astronomy this week. The study describes the detection of very hot solar plasma above an active region of the Sun that exhibits no eruptive activity, suggesting the presence of nanoflares.
The upper part of the solar atmosphere - the solar corona - is several million degrees Kelvin hotter than the visible solar surface, the photosphere. Determining the mechanism behind this temperature disparity, and how the solar corona becomes so hot, is one of the main objectives of solar physics. Solar eruptions (flares) inject a great amount of energy in the corona, but they are too infrequent to maintain its temperature. However, as solar flares become less energetic, they become more frequent, so it has been suggested that a large number of very faint flares (nanoflares) could be enough to heat the corona. But, until now, these explosions have been too small to be detected with current instruments.
Here, Shin-nosuke Ishikawa and colleagues analyse X-ray measurements of active regions of the Sun taken by the FOXSI-2 sounding rocket. They identify very energetic X-rays, a signature of plasma heated at more than 10 million Kelvin, over a region that does not show any visible flaring activity. The authors conclude that such heated plasma could be generated only by the action of nanoflares.
These observations open the way to more targeted X-ray observational campaigns by future instruments, like the NuSTAR spacecraft or the next launches of FOXSI. Increasing the number of observations of these heating events will allow a better understanding of the role that nanoflares play in coronal heating.