RELATIVE TIMING OF SOFT-X-RAY NONTHERMAL LINE BROADENING AND HARD X-RAY-EMISSION IN SOLAR-FLARES

The time development of both hard X-ray emission and soft X-ray nonthe rmal line widths is important for an understanding of energy transport in the flaring solar corona. In this Letter, we investigate the relat ionship between the temporal behavior of these two phenomena for a num ber of flares detected by instruments on the Yohkoh spacecraft. We exa mine 10 flares, all occurring within 30 degrees of the limb, using dat a from the Bragg Crystal Spectrometer (BCS) and the hard X-ray telesco pe (HXT). We find that the nonthermal velocity either (1) exhibits a m aximum prior to the first significant burst of hard X-rays or (2) is a lready decaying from an earlier unobserved maximum at the time of the first significant burst of hard X-rays, The decay of the nonthermal ve locity as it proceeds from its observed maximum shows little evidence for a direct association with individual hard X-ray bursts. These obse rvations suggest that the nonthermal broadening may be a direct conseq uence of the flare energy release process rather than a by-product of the energy deposition. In addition, the attainment of a maximum in the nonthermal line width very early in the flare is more indicative that plasma turbulence is the source of the observed broadening rather tha n hydrodynamic flows, such as chromospheric evaporation.