Hot Onsets of Solar Flares

Abstract:

The study of the localized plasma conditions before the impulsive phase of a solar flare can help us understand the physical processes that occur leading up to the main flare energy release. We have studied the GOES soft X-ray data in flare precursors, prior to the hard X-ray onset, and find evidence that high-temperature plasma (10-15 MK) already has come into existence at these earliest times. We have confirmed the generality of this result for all classes of flares and checked the GOES isothermal temperatures for four events against RHESSI spectra. The use of AIA imagery and a limb-occultation flare sequence establish that these "hot onset" sources occur in or near the chromosphere. The absence of hard X-rays implies heating without particle acceleration. There is no indication of a gradual increase of temperature in the hot onset intervals, implying heating time scales below the temporal resolution of the data.