Helium Abundance in High‐Temperature Solar Flare Plasmas

Abstract

In the present work we use emission from dense plasmas late in the decay phase of flares, while the plasmas are still hot [(2-4)×106 K], to determine the absolute helium abundance in flaring solar corona. To achieve this, we make use of intensity ratios of H I and He II lines and of intensity ratios between the He II lines and the bremsstrahlung continuum radiation emitted by the local postflare plasmas. These ratios are very sensitive to the electron temperature, which is measured by using two different techniques: intensity ratios from lines emitted by heavier elements and from the line width of H I and He II lines. The latter method is made possible by the low atomic weight of H and He and by the high temperature of the plasma, which causes the widths of these lines to exceed 0.6 Å (He) and 1.1 Å (H). Such values significantly exceed the broadening due to nonthermal mass motions, so line widths can provide accurate temperature measurements. The average helium absolute abundance was found to be 12.2%+/-2.4%.