Solar Flare Doppler Blueshifted Soft X‐Ray Emission and Hard X‐Ray Bursts

Abstract

Hard X-ray emission and the dynamics of soft X-ray emitting plasma are among the most immediate manifestations of solar flare energy release. To understand better the early impulsive phase of solar flare energy release, we have examined Ca XIX and Fe XXV soft X-ray resonance line spectra that exhibit strong blue-wing asymmetries from 32 flares. These spectra were obtained with the Bragg Crystal Spectrometer on the Japanese Yohkoh spacecraft, which is up to 10 times more sensitive than previously flown crystal spectrometers. The increased sensitivity allowed the comparison of Doppler blueshifted X-ray emission to hard X-ray emission observed with the Compton Gamma Ray Observatory's Burst and Transient Source Experiment, the Yohkoh Wide Band Spectrometer, and Yohkoh Hard X-Ray Telescope. We found that (1) hard X-ray burst light curves and resonance line blue-wing light curves are similar for most flares that exhibit strong blueshifts; (2) the time evolution of the intensity ratio of the blue wing to the peak unshifted spectral component resembles higher energy hard X-ray emission more closely than the blue-wing light curve alone; (3) though many flares had blue-wing/resonance peak ratios that resembled or peaked at the same time as the hard X-ray burst, nearly one-third of the flares exhibited ratio curves that either were nonzero before the detection of burst hard X-rays or peaked before the hard X-ray burst peaked—a few of these flares exhibited small blueshifted line profiles before the detection of hard X-ray emission; (4) a few flares with strong blueshifts had little or no detectable emission above 30 keV; and (5) the time derivative of the resonance peak emission usually resembles the blue-wing light curve. The implication of these results for current flare models is discussed.