Investigations of turbulent and directed motions in solar flares

Abstract

During the past solar maximum, spectrally resolved observations obtained with the P78-1, Hinotori, and Solar Maximum Mission (SMM) satellites have revealed strongly asymmetric line profiles as well as symmetrically broadened (in excess of thermal broadening) line profiles during the impulsive phase of most solar flares. These line profiles are the result of emissions from plasmas which have high upflow bulk velocities in the flaring loop added to the emission from the plasma which has a relatively low bulk velocity. The large observed symmetric broadening is the result of random ionic or turbulent motions within the flaring loop. Most observations have been made in soft X-rays of the 10 7 K and hotter plasma, although, similar effects have been noted in UV emissions. The importance of these observations is clear: the soft X-ray emission represents the thermal response of the rapidly heated flare plasma, and a proper understanding of these emissions necessarily constraints the interpretation for the heating source. For example, plasma heated by fast electrons or thermal conduction fronts may produce different directed and turbulent motions which could be detected in the observed X-ray spectra. We present a new method for fitting these asymmetric spectra which we have applied to over 40 flares observed with the Bent Crystal Spectrometer (BCS) on board the SMM. Correlations between turbulent and directed motions are discussed and we infer the temperature of the upflowing component separately from the stationary component from ratios of Ca xix and Fe xxv line intensities.