Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apjs..112..221j&link_type=abstract
Astrophysical Journal Supplement v.112, p.221
Astronomy and Astrophysics
Astronomy
16
Line: Profiles, Stars: Flare, Sun: Chromosphere, Sun: Corona, Sun: Flares
Scientific paper
We report on a successful program that used the Hamilton echelle spectrograph and the coude auxiliary telescope at Lick Observatory to take spectra of solar flares. Our observations consist of high-resolution ( lambda / delta lambda ~ 48,000) spectra covering the entire optical region from approximately 3800 to 9000 A 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 relatively large (GOES class M7.7) solar flare event. Our sequence of observations began before flare maximum and continued for more than 1 hr. We present our high signal-to-noise spectra and compare them with similar stellar flare observations. We find that the hydrogen-emitting layers in flares on the Sun differ markedly from those in flares on dMe 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 and H beta line profiles appear to be asymmetric as a result of absorption by chromospheric material expanding upward into the corona. We also examine solar flare model atmospheres synthesized with the non-LTE code MULTI and find that our observations can be generally understood in terms of equilibrium models of electron-beam-- and X-ray--heated chromospheres in equilibrium with coronal loops in which the pressure is rather high; however, there remain marked differences between the theoretical predictions and our observations, implying that substantial refinement of the models is in order. Several photospheric lines show flare enhancements as well. The temporal behavior of these line enhancements is identical to that of the chromospheric lines, but there is no indication that significant flare heating penetrates to continuum formation depths.
Basri Gibor
Hawley Suzanne L.
Johns-Krull Christopher M.
Valenti Jeff A.
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