Observations of Solar Flare Doppler Shift Oscillations with the Bragg Crystal Spectrometer on Yohkoh

摘要

Oscillations in solar coronal loops appear to be a common phenomenon.Transverse and longitudinal oscillations have been observed with both theTransition Region and Coronal Explorer and Extreme Ultraviolet ImagingTelescope imaging experiments. Damped Doppler shift oscillations have beenobserved in emission lines from ions formed at flare temperatures with theSolar Ultraviolet Measurements of Emitted Radiation Spectrometer. Theseobservations provide valuable diagnostic information on coronal conditions andmay help refine our understanding of coronal heating mechanisms. I haveinitiated a study of the time dependence of Doppler shifts measured duringflares with the Bragg Crystal Spectrometer (BCS) on Yohkoh. This Letter reportsthe detection of oscillatory behavior in Doppler shifts measured as a functionof time in the emission lines of S XV and Ca XIX. For some flares, both linesexhibit damped Doppler shift oscillations with amplitudes of a few km/s andperiods and decay times of a few minutes. The observations appear to beconsistent with transverse oscillations. Because the BCS observed continuouslyfor almost an entire solar cycle, it provides numerous flare data sets, whichshould permit an excellent characterization of the average properties of theoscillations.