Impulsive extreme-ultraviolet and hard X-ray emission during solar flares

Abstract

Impulsive solar bursts of hard X-rays observed with moderate spectral resolution and high time resolution by the OGO S satellite were compared with concurrent enhancements of extreme- ultraviolet (EUV) radiation observed by means of sudden frequency deviations. The impulsive EUV flux rises and decays, with a time structure most like that of the lowest-energy hard X-rays observed-the 10-20 keV X-rays-but slower than hard X-rays with photon energy greater than 32 keV. These results are interpreted in terms of the dominance of 10-25 keV electrons in exciting the high-density impulsive EUV source. The impulsive EUV emission varies with the central meridian distance of the flare, with a peak in EUV intensity for flares near the central meridian and a marked decline in intensity near the solar limb. These observations are consistent with a model of the impulsive phase in which the EUV burst originates in sources embedded deep in a cool absorbing atmosphere, including fine spatial structure smaller than an arcsec, and in which both the electron precipitation efficiency and the EUV radiation efficiency are small. Subject headings: Sun: flares - Sun: X-rays - ultraviolet: general