A possible broadening mechanism of Hei 10 830 Åprofiles observed in a limb flare

Abstract

The Hei 10 830 Åspectra observed in the impulsive phase of the 2N/X20 solar limb flare of 16 August 1989 show unusually broad profiles, and most of the profiles are blue-shifted and/or blue asymmetric with full-width-at-half-maximum (FWHM) greater than 10 Å. Contrary to prominences, weak post-flare loops and surges, the observed profiles of the flare cannot be simulated by Doppler broadening with a uniform flaring atmosphere model. Calculations show that only when the electron density is greater than 1016 cm-3 can Stark broadening produce an obvious influence on the Hei 10 830 Åprofile. This indicates that it is not plausible that Stark broadening leads to the observed profiles even though they can be fitted by Stark broadening. Alternatively, an expanding flaring atmosphere can perfectly simulate all the observed Hei 10 830 Åprofiles. The expanding velocities are generally several tens of km s-1 and that corresponding to the widest profile is greater than 120 km s-1. Expanding velocity and other parameters vary with time and the observed area. This expanding flaring atmosphere model is a perspective candidate for line broadening. It is an indication of the lateral expansion resulting from rapid chromospheric heating in addition to the upward evaporation and downward condensation.