Helium resonance lines in the flare of 15 June, 1973

Abstract

Time sequences of He i and He ii resonance line intensities at several sites within the flare of 15 June, 1973 are derived from observations obtained with the Naval Research Laboratory's Slitless Spectroheliograph on Skylab. The data are compared with predictions in six model flare atmospheres based on two values for the heating rate and three for the flux of photoionizing coronal X-rays and EUV. A peak ionizing flux more than 103 times that in the quiet Sun is indicated. For most conditions in flare kernels the He ii Lα and Lβ lines are found to be formed by collisional excitation, thereby contributing to the local cooling of the plasma at temperatures above 6 × 104 K. Emission in the higher Lyman lines is generally the result of a mixture of collisional excitation at these temperatures and photoionization and recombination at temperatures near 2.5 × 104 K. We discuss implications for the common practice of deriving stellar coronal fluxes from He ii 1640 A fluxes assuming dominance of the recombination mechanism.