Coronal X-ray activity preceding solar flares

Abstract

A comprehensive survey of Skylab S-054 soft X-ray images was performed to investigate the characteristics of coronal enhancements preceding solar flares. A search interval of 30 min before flare onset was used. A control sample was developed and tests of the statistical results performed. X-ray images with preflare enhancements were compared with high resolution Hα images and photospheric magnetograms.

The results are as follows: preflare X-ray enhancements were found in a statistically significant number of the preflare intervals, and consisted of one to three loops, kernels or sinuous features per interval. Typically, the preflare feature was not at the flare site and did not reach flare brightness. There was no systematically observed time within the preflare interval for the preflare events to appear and no correlation of preflare event characteristics with the subsequent flare energy. Gas pressures of several preflare features were calculated to be on the order of several dyne cm⁻², typical of active region loops, not flares. These results suggest that observations with both high spatial resolution and low coronal temperature sensitivity are required to detect these small, low pressure enhancements that preceded the smaller flares typical of the Skylab epoch. Hα brightenings were associated with nearly all of the preflare X-ray enhancements. Changing Hα absorption features in the form of surges or filament activations were observed in about half of the cases. These results do not provide observational support for models which involve preheating of the flare loop, but they are consistent with some current sheet models which invoke the brightening of structures displaced from the flare site tens of min before onset.