THE ULTRAVIOLET-SPECTRUM OF A 3B CLASS FLARE OBSERVED WITH SOLSTICE

An observation of the ultraviolet spectrum (1200-1800 Angstrom) during the impulsive phase of a very extended 3B-X3 class solar flare on 199 2 February 27 was obtained with the Solar-Stellar Irradiance Compariso n Experiment (SOLSTICE). This observation is combined with ground-base d H alpha, magnetogram, and microwave data as well as hard X-ray measu rements from the Ulysses spacecraft. This flare shows a dramatic enhan cement of lines formed in the solar transition region. The irradiance (emitted flux density from the entire solar disk) of the resonance lin es of C IV and Si IV increased by a factor of 12-13 during the impulsi ve phase of the flare. These irradiance enhancements are comparable wi th those measured during stellar flares. By taking into account the em itting flare area we infer that the radiance (specific intensity) of t he flaring plasma was at least a factor of 15,000 brighter than the av erage solar disk radiance just prior to the event. Assuming the flare site's initial radiance was that of a typical active region, it then m ust have brightened by a factor of at least 3400. Such enhancement far exceeds previous published values (e.g., OSO 8, Skylab, and SMM) and indicates that many observations were affected by limited dynamic rang e. Thus, the SOLSTICE observation may be the first measurement of the true UV enhancement during the impulsive phase of very bright solar fl ares. The Si III multiplet near 1295 Angstrom also shows remarkable en hancement, but other allowed lines of C II, Si III (1206 Angstrom), N V, and He II show more moderate enhancements, the weakest being H I Ly alpha, the irradiance of which increases only 6%. Some of the differe nces between the various enhancements are certainly caused by the timi ng of the observations since the scanning spectrometer observed differ ent spectral features over periods of 4 minutes. Other differences due to line formation processes are being investigated but are consistent with density effects in the line emission coefficients. The inferred Ly alpha radiance enhancement is consistent with current post-impulsiv e phase flare models. However, the formation of the C IV and Si IV lin es, formed during the impulsive phase of the flare, remains unknown. D uring the impulsive phase of the flare the strong transition region li nes are systematically redshifted by 50 km s(-1).