## 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

*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 *λ*5.22/Si XIII *λ*5.68. 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.

Key words: atomic processes / radiation mechanisms: non-thermal / Sun: flares / Sun: X-rays, gamma rays

*© ESO, 2011*