The ionization equilibrium and flare line spectra for the electron distribution with a power-law tail

¹ Astronomical Institute of the Academy of Sciences of the Czech Republic, Fričova 298, 251 65 Ondřejov, Czech Republic
e-mail: elena@asu.cas.cz
² Faculty of Mathematics Physics and Informatics, Comenius University, Mlynská dolina F2, 842 48 Bratislava, Slovakia
e-mail: homola@fmph.uniba.sk, dudik@fmph.uniba.sk

Received: 12 April 2011
Accepted: 1 June 2011

Abstract

Context. Electron energy spectra exhibiting a high-energy tail are commonly observed during solar flares.

Aims. We investigate the influence of the high-energy tail and thermal or nonthermal plasma bulk on the ionization equilibrium of Si and Si flare line spectra.

Methods. We construct a realistically composed distribution that reflects the fits to RHESSI observations. We describe the high-energy tail by a power-law distribution and the bulk of the electron distribution by either the Maxwellian or n-distribution. The shape of this composed distribution is described by three parameters: the ratio of the plasma bulk density to the density of the high-energy tail, the power-law index of the high-energy tail, and the parameter n, which describes the bulk of the distribution.

Results. Both the plasma bulk and the high-energy tail change the ionization equilibrium. The relative ion abundances are sensitive to the shape of the plasma bulk, but are much less sensitive to the high-energy tail. The high-energy tail increases the ratio of temperature-sensitive lines Si XIV λ5.22/Si XIII λ5.68. Because this ratio can be fitted with a thermal distribution with higher temperature, the high-energy tail influences the temperature diagnostics from flare lines. The high-energy tail has only a small effect on the ratio of the satellite-to-allowed Si XIId/Si XIII lines, which are dominantly sensitive on the shape of the plasma bulk. This enables us to perform an accurate diagnostic of the parameter n describing the plasma bulk.

Conclusions. The realistically composed distribution is able to explain the observed features of the RESIK X-ray flare line spectra.