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Using data from the Bragg Crystal Spectrometer (BCS) on the {\it Yohkoh\/} spacecraft, I have computed measures of the total intensity, centroid position, and line width for the resonance lines of Fe~XXV, Ca~XIX, and S~XV during the rise phase and after maximum for 190 solar flares. The difference between centroid positions early and late in each flare yields a measure of the line-of-sight velocity shift of the line centroids. Roughly 25\% of the flares show evidence for spatial shifts of the emitting plasma during the flare. For all three resonance lines in those flares that show only Doppler shifts, I find a trend in the average value of the centroid shift with distance from Sun center suggesting radial mass motions with characteristic velocities of 76, 63, and 58~km~s$^{-1}$ in the resonance lines of Fe~XXV, Ca~XIX, and S~XV, respectively. For each resonance line, I find no correlation between the centroid shift and the peak intensity, rise time, and total flare duration; and no correlation between the line width and the distance from Sun center, peak intensity, rise time, and total flare duration. Comparing data from different BCS wavelength channels, I find that the centroid shift in each channel is correlated with the centroid shifts in the other channels. Similarly, the line width in each channel is correlated with the line widths in the other channels. Exploratory model atmosphere calculations suggest that many of these properties can be explained by an electron-beam-heated flare model.
This work was supported by the Office of Naval Research.
