Small Fluctuation of Coronal X-Ray Intensity and a Signature of Nanoflares

If small energy events (nanoflares) contribute to the heating of the solar corona, they may be too small to be recognized as independent events. We create a simple histogram of the X-ray intensity fluctuation around the mean intensity and find that the histogram consists of a central Gaussian component and a wing component. The Gaussian component corresponds to random fluctuation around the mean intensity. The width of this component becomes broader with increasing intensity and is larger than the predicted photon noise distribution. We suggest that nanoflares produce the observed fluctuation of the X-ray intensity. The energy of nanoflares is estimated to be less than 1022 ergs to explain the observed fluctuation for active regions. It is estimated that if the energy of nanoflares is 1020 ergs, then these nanoflares must occur at a rate of 106 s-1 in a single active region (~1020 cm2) to supply the required energy (~5 × 107 ergs cm-2 s-1) to maintain the corona.