The morphology of average solar flare time profiles from observations of the Sun's lower atmosphere

Abstract

We study the decay phase of solar flares in several spectral bands using a method based on that successfully applied to white light flares observed on an M4 dwarf. We selected and processed 102 events detected in the Sun-as-a-star flux obtained with SDO/AIA images in the 1600~Å and 304~Å channels and 54 events detected in the 1700~Å channel. The main criterion for the selection of time profiles was a slow, continuous flux decay without significant new bursts. The obtained averaged time profiles were fitted with analytical templates, using different time intervals, that consisted of a combination of two independent exponents or a broken power law. The average flare profile observed in the 1700~Å channel decayed more slowly than the average flare profile observed on the M4 dwarf. As the 1700~Å emission is associated with a similar temperature to that usually ascribed to M dwarf flares, this implies that the M dwarf flare emission comes from a more dense layer than solar flare emission in the 1700~Å band. The cooling processes in solar flares were best described by the two exponents model, fitted over the intervals t1=[0, 0.5]$t_{1/2}$ and t2=[3, 10]$t_{1/2}$ where $t_{1/2}$ is time taken for the profile to decay to half the maximum value. The broken power law model provided a good fit to the first decay phase, as it was able to account for the impact of chromospheric plasma evaporation, but it did not successfully fit the second decay phase.

arXiv comment8 pages, 10 figures, accepted for publication in MNRAS

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