Nonthermal Velocity Evolution in the Precursor Phase of a Solar Flare

Abstract

We present observations of two solar flares occurring in Active Region 7590 on 1993 October 3 using data from the Yohkoh spacecraft. The hard X-ray bursts from the two flares occurred within an 18 minute interval, with the soft X-ray emission having a shorter separation of ≈5 minutes. Both flares occurred within one Yohkoh orbit, and hence we have continuous coverage of the soft X-ray line broadening at the peak of the first flare, reducing to the active region level of 33.5 km s^-1 in S XV (66.1 km s^-1 in Ca XIX) and then increasing to the peak in the second flare. The rise above the active region background level begins 11 minutes before the start of the second flare as defined by the start of the hard X-ray emission. During this extended rise time of V_nt, there is no increase in the light curves or the electron temperature. We suggest that this increase is an indicator of turbulent changes in the active region prior to the flare that are related to the flare trigger mechanism.