Dependence of the Nanoflare Occurrence Rate on the Solar Cycle Phase

Abstract

We have used the SDO/AIA observational data obtained in the 171 Å channel to investigate the dependence of the occurrence rate of solar nanoflares observed in the vacuum ultraviolet spectral range on the solar cycle phase. For this purpose, we have processed more than 30 000 images obtained over the period from 2011 to 2020 inclusive that encompasses almost completely the 24th solar cycle and the beginning of the 25th cycle. To search for nanoflares, we have used the widespread method based on the selection of events that exceed the background by \(5\sigma\) or more, which we have improved for a higher-quality removal of artifacts associated with charged particles. As a result, we have shown that in the periods of a high solar activity the number of nanoflares approximately doubles compared to their number at the solar cycle minimum. The change in the number of nanoflares is generally consistent with the change in the number of normal flares, although it occurs with a considerably lower amplitude. We have not detected any influence of the solar cycle on the energy distribution of nanoflares, nor have we detected any difference between large and small nanoflares, although we assume that it can exist. We also conclude that particle-associated artifacts are the main obstacle to the studies of nanoflares, and further progress is impossible without improving the methods of their removal.