The Energy Distribution of Nanoflares at the Minimum and Rising Phase of Solar Cycle 24

Abstract

The energy distribution of weak emission events (nanoflares) in the solar corona measured for two stages of solar cycle 24, at the minimum and at the beginning of the rise in solar activity, is presented. Our study is based on data from two instruments, TESIS/CORONAS-PHOTON (for the cycle minimum; 2009) and AIA/SDO (the rising phase, 2010–2011), for which we have applied a unified event detection algorithm. The database collected by us comprises more than 105 flares. For all events we have measured the flux in the EUV spectral range and determined the thermal energy located in the range from 1023 to 1026 erg and distributed according to a power law: N(E)dE ∼ N−αdE. The index of the power-law distribution α in all of the cases studied has turned out to be more than two (α = 2.2–2.9). This means that the integrated energy of nanoflares increases when passing to weaker events. This scenario argues for the model of coronal heating by nanoflares. The index α reaches its maximum at the cycle minimum and then drops, implying a decrease in the fraction of weak events. This may be because part of the energy is redistributed in favor of large flares. The total energy of nanoflares in the range 1023–1026 erg has turned out to be lower than the energy losses of the solar corona through radiation by a factor of 30. For the coronal heating to be explained by nanoflares, their distribution with the same power-law index must extend at least to 1021 erg.