YOHKOH SOFT X-RAY SPECTROSCOPIC OBSERVATIONS OF THE BRIGHT LOOP-TOP KERNELS OF SOLAR FLARES

摘要

Observations of solar flares by the Soft X-ray Telescope (SXT) on board Yohkoh frequently show strongly enhanced brightenings near the tops of the magnetic loops containing hot plasma. The Yohkoh Bragg Crystal Spectrometer (BCS) cannot normally make observations of these loop-top sources in the absence of contamination by the legs and the feet of the loops since it has no spatial resolution. We have overcome this limitation by using the solar limb as an occulting edge in a sequence of similar flares that occurred over an interval of ～10 hr near the west limb on 1992 November 24. The progressive occultation by the limb restricts the line of sight to higher and higher altitudes during this sequence, with the final event showing only a compact source of the type often found at loop tops. BCS observations in Fe XXV, Ca XIX, and S XV show that electron temperatures and nonthermal velocities in these compact sources are similar to those quantities determined for disk flares in previous studies. As with disk flares, the nonthermal line broadening persists late into the decay phase of the flaring isolated loop tops. Our results favor mechanisms for nonthermal-velocity generation that are either independent of height or place the source near the apex of the flaring loop. In addition, there may be a temporal relationship between the hard X-ray emission and the nonthermal velocity, which suggests a possible association between the primary energy release of the flare, the nonthermal-velocity generation mechanism, and the loop top.