HAMILTON ECHELLE SPECTROSCOPY OF THE 1993 MARCH 6 SOLAR-FLARE

We report on a successful program that used the Hamilton echelle spect rograph and the coudie auxiliary telescope at Lick Observatory to take spectra of solar flares. Our observations consist of high-resolution (lambda/delta lambda similar to 48,000) spectra covering the entire op tical region from approximately 3800 to 9000 Angstrom in each exposure . These are the first time-resolved high-resolution optical spectra of this type obtained for a solar flare. On 1993 March 6 we observed a r elatively large (GOES class M7.7) solar flare event. Our sequence of o bservations began before flare maximum and continued for more than 1 h r. We present our high signal-to-noise spectra and compare them with s imilar stellar flare observations. We find that the hydrogen-emitting layers in flares on the Sun differ markedly from those in flares on dM e stars, though the total energy emitted in various emission lines can be rather similar. We also find that the amount of energy released in the optical emission lines is similar to that emitted in soft X-rays. We find evidence for Stark broadening in the Balmer lines for members lower in the series than reported in earlier studies. This appears to have occurred because the optical depth in the Balmer lines is lower than in previously reported flares. Early in the flare, the H alpha an d H beta line profiles appear to be asymmetric as a result of absorpti on by chromospheric material expanding upward into the corona. We also examine solar flare model atmospheres synthesized with the non-LTE co de MULTI and find that our observations can be generally understood in terms of equilibrium models of electron-beam- and X-ray-heated chromo spheres in equilibrium with coronal loops in which the pressure is rat her high; however, there remain marked differences between the theoret ical predictions and our observations, implying that substantial refin ement of the models is in order. Several photospheric lines show flare enhancements as well. The temporal behavior of these line enhancement s is identical to that of the chromospheric lines, but there is no ind ication that significant flare heating penetrates to continuum formati on depths.