YOHKOH OBSERVATIONS OF THE CREATION OF HIGH-TEMPERATURE PLASMA IN THEFLARE OF 16-DECEMBER-1991

Yohkoh observations of an impulsive solar flare which occurred on 16 D ecember, 1991 are presented. This flare was a GOES M2.7 class event wi th a simple morphology indicative of a single flaring loop. X-ray imag es were taken with the Hard X-ray Telescope (HXT) and soft X-ray spect ra were obtained with the Bragg Crystal Spectrometer (BCS) on board th e satellite. The spectrometer observations were made at high sensivity from the earliest stages of the flare, are continued throughout the r ise and decay phases, and indicate extremely strong blueshifts, which account for the majority of emission in Ca XIX during the initial phas e of the flare. The data are compared with observations from other spa ce and ground-based instruments. A balance calculation is performed wh ich indicates that the energy contained in non-thermal electrons is su fficient to explain the high temperature plasma which fills the loop. The cooling of this plasma by thermal conduction is independently veri fied in a manner which indicates that the loop filling factor is close to 100%. The production of 'superhot' plasma in impulsive events is s hown to differ in detail from the morphology and mechanisms appropriat e for more gradual events.