K? line emission during solar X-ray bursts

Abstract

Kα X-ray line emission from S, Ar, Ca and Fe is calculated for conditions likely to exist in solar flares. We consider both the non-thermal and thermal phases of flares as indicated by X-ray observations. Impulsive non-thermal events seen at the onset of a flare at photon energies > 20 keV generally give rise to small Kα line fluxes (<250 photons cm-2 s-1) on the basis of data presented by Kane and Anderson. The amount of S Kα radiation in particular depends sensitively on the lower-energy bound of the non-thermal electron distribution giving rise to the impulsive burst, offering a possible means of determining this. Thermal Kα emission is significant for only Fe ions. For S, Ar and Ca, the temperatures required for a sizeable number of electrons with energies greater than the K-ionization potential will also strip these elements to ionization stages too high for Kα transitions to be possible. Comparison of thermal Kα emission from iron during an intense solar flare leads to a very high emission measure on the basis of these calculations, but such a value seems to be compatible with an analysis of the 1–3 A continuum during the same event.