ULTRAVIOLET OBSERVATIONS OF THE PREIMPULSIVE PHASE IN A SOLAR-FLARE - ENHANCED TURBULENCE AND HEATING

We present the unique UV spectra of the 1974 January 21 flare obtained from Skylab before the onset of an impulsive hard X-ray burst. The observations show that the UV emission lines were much enhanced and their line profiles much widened, indicating large nonthermal turbulence in the transition region plasmas before the impulsive phase. This shows that the transition region plasmas were already being excited and heated before the sudden energization of electrons as manifested in the emission of the impulsive hard X-rays. The excitation of the transition region plasma could be due to the dissipation of enhanced current in the preflare loop. This excitation continued until some parameters that characterized the current flow were exceeded and some plasma instabilities occurred that accelerated the electrons which produced the hard X-ray emission. It appeared that the energetic particles were not the major heating agent of the transition region plasma in this flare as is often assumed for many other flares. The heating of the flaring plasmas to high temperatures started before the impulsive phase as shown by the presence of the Fe xxi emission. The absence of the blueshifted component in the Fe xxi line profiles before, during, and after the impulsive phase indicated that chromospheric evaporation was not an important energy transport mechanism in this flare.