The relation between hard X-ray and transition-region line emission in solar flares

Abstract

Observational evidence suggests that both the hard X-ray and ultraviolet emission from the impulsive phase of flares result from an electron beam. We present the results of model calculations that are consistent with this theory. The impulsive phase is envisioned as occurring in many small magnetically confined loops, each of which maintains an electron beam for only a few seconds. This model successfully matches several observed aspects of the impulsive phase. The corona is heated to less than 2 × 106 K, maximum enhanced emission occurs in lines formed near 105 K, and there is only slight enhancement between 105 and 2 × 106 K. The slope of the observed relationship between hard X-ray and Ov 1371 A emission is also matched, but the relative emission is not. The calculations indicate that UV emission lines formed below a temperature of about 105 K will arise predominantly from the chromospheric region heated by the electron beam to transition region temperatures. Emission lines formed at higher temperatures will be produced in the transition region. This should be detectable in density-sensitive line ratios. To account successfully for the impulsive UV emission, the peak temperature in the impulsively heated loops must remain below about 2 × 106 K. Thus our model implies that the impulsive heating takes place in different loops from the hotter gradual phase emission.