Energy Distribution of Microevents in the Quiet Solar Corona

Abstract

Recent imaging observations of EUV line emissions have shown evidence for frequent flarelike events in a majority of the pixels in quiet regions of the solar corona. The changes in coronal emission measure indicate impulsive heating of new material to coronal temperatures. These heating or evaporation events are candidate signatures of ‘‘ nanoflares ’’ or ‘‘ microflares ’’ proposed to interpret the high temperature as well as the very existence of the corona. The energy distribution of these microevents reported in the literature differ widely, and so do the estimates of their total energy input into the corona. Here we analyze the assumptions of the different methods, compare them by using the same data set, and discuss their results. We also estimate the different forms of energy input and output, keeping in mind that the observed brightenings are most likely secondary phenomena. A rough estimate of the energy input observed by EIT on the SOHO satellite is of the order of 10% of the total radiative output in the same region. It is considerably smaller for the two reported TRACE observations. The discrepancy can be explained by flare selection and different thresholds for flare detection. There is agreement on the slope and the absolute value of the distribution if the same methods are used and a numerical error is corrected. The extrapolation of the power law to unobserved energies that are many orders of magnitude smaller remains questionable. Nevertheless, these microevents and unresolved smaller events are currently the best source of information on the heating process of the corona. Subject headings: Sun: activity — Sun: corona — Sun: flares — Sun: X-rays, gamma rays