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 1992 February 27 was obtained with the Solar-Stellar Irradiance Comparison Experiment (SOLSTICE). This observation is combined with ground-based H alpha, magnetogram, and microwave data as well as hard X-ray measurements from the Ulysses spacecraft. This flare shows a dramatic enhancement of lines formed in the solar transition region. The irradiance (emitted flux density from the entire solar disk) of the resonance lines of C IV and Si IV increased by a factor of 12-13 during the impulsive phase of the flare. These irradiance enhancements are comparable with those measured during stellar flares. By taking into account the emitting flare area we infer that the radiance (specific intensity) of the flaring plasma was at least a factor of 15,000 brighter than the average solar disk radiance just prior to the event. Assuming the flare site's initial radiance was that of a typical active region, it then must 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 range. Thus, the SOLSTICE observation may be the first measurement of the true UV enhancement during the impulsive phase of very bright solar flares. The Si III multiplet near 1295 Angstrom also shows remarkable enhancement, 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 differences between the various enhancements are certainly caused by the timing of the observations since the scanning spectrometer observed different 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-impulsive phase flare models. However, the formation of the C IV and Si IV lines, formed during the impulsive phase of the flare, remains unknown. During the impulsive phase of the flare the strong transition region lines are systematically redshifted by 50 km s(-1).