Spectral characteristics of solar flares in different chromospheric lines and their implications

摘要

Using the spectral data of representative solar flares observed with the infrared detector system of the solar spectrograph at Purple Mountain Observatory, we study the spectroscopic characteristics of solar flares in the H alpha, the Ca II 8 542 angstrom, and the He I 10 830 angstrom lines in different phases and various locations of flares and discuss their possible implications coupled with space observations. Our results show that in the initial phase of a flare the H alpha line displays a red shift only with no wide wing. Large broadenings of the H alpha line are observed a few minutes after the flare onset within small regions of 3 - 5 '' in both disk and limb flares with and without nonthermal processes. Far wings similar to those of damping broadening appear not only in the H alpha line but in the He I 10 830 angstrom line as well in flares with nonthermal processes. Sometimes we even detect weak far-wing emission in the Ca II 8 542 angstrom line in disk flares. Such large broadenings are observed in both the footpoints and the flare loop-top regions and possibly result from strong turbulence and/or macroscopic motions. Therefore, the so-called nonthermal wing of the H alpha line profile is not a sufficient condition to distinguish whether nonthermal electrons are accelerated or not in a flare. The Ca II 8 542 angstrom line shows lower intensity in the loop-top regions and higher intensity in the parts close to the solar surface. Emissions larger than nearby continuum in the He I 10 830 angstrom line are detected only in small regions with strong X-ray emissions and avoid sunspot umbrae.