Aims: We aim to prove and diagnose the occurrence of nonthermal electron distributions in solar flare plasma using X-ray spectral observations.
Methods: An M4.9 flare on 2003 January 7/8 was observed with the RESIK instrument in the 3-6 Å wavelength range (2-4 keV) and with RHESSI at energies above 6 keV. The temporal behavior of RESIK flare spectra has been analyzed for two different types of velocity distributions - a thermal (Maxwellian) distribution and a nonthermal plasma distribution of free electrons. The Si XIV, Si XIII, and Si XIId satellite lines observed with RESIK in the 5-6 Å range were used to determine the degree of deviation from Maxwellian, and the equivalent non-Maxwellian pseudo-temperature, τ. The diagnostics presented are sensitive to the shape of the distribution in the energy range where the maximum of the electron distribution occurs (where the bulk of electrons reside) and does not include the influence of the shape of the high-energy tail of the distribution. Under the assumption of a Maxwellian distribution of electron velocities, the plasma temperature was determined from an emission measure (EM) loci analysis and a differential emission measure (DEM) analysis of RESIK spectra. The high-energy end of the flare radiative emission was investigated through RHESSI spectral analysis.
Results: The nonthermal analysis of RESIK spectra has shown that the largest deviations of the plasma electron distribution from Maxwellian appeared during the impulsive phase of the flare. The decay phase spectra had an almost isothermal character. The pseudo-temperature, τ, reached its maximum around the peak time of the soft and hard X-ray fluxes. The temporal behavior of the temperatures derived from the thermal analysis was similar to the behavior of the nonthermal pseudo-temperature. The values of the pseudo-temperature were consistent with the temperatures obtained in both thermal analyses, but lower than the temperatures derived from the slope of the RHESSI continua. In comparison with the synthetic isothermal or multithermal spectra, the nonthermal synthetic spectra fitted the observed Si XIId satellite lines much more closely (the error is less than 10%). The fluxes in the Si XIId satellite lines in isothermal or multithermal spectra have been underestimated by a factor of three or more in comparison to the observed fluxes. The value of this factor varies with time and it is different for the different satellite lines.
Conclusions: Evidence was found for considerable deviations of the distribution of free electrons from Maxwellian in the plasma during a solar flare. These occurred mainly during the flare impulsive phase and can be diagnosed using existing X-ray spectral observations.