Diagnostics of non-thermal distributions in solar flare spectra observed by RESIK and RHESSI

Abstract

Context. During solar flares an enormous amount of energy is released, and the charged particles, like electrons, are accelerated. These non-thermal electrons interact with the plasma in various parts of solar flares, where the distribution function of electrons can therefore be non-Maxwellian. Aims. We focus on the non-thermal components of the electron distribution in the keV range and analyse high-energy resolution X-ray spectra detected by RESIK and RHESSI for three solar flares. Methods. In the 2-4 keV range we assume that the electron distribution can be modelled by an n-distribution. Using a method of line-intensity ratios, we analyse allowed and satellite lines of Si observed by RESIK and estimate the parameters of this n-distribution. At higher energies we explore RHESSI bremsstrahlung spectra. Adopting a forward-fitting approach and thick-target approximation, we determine the characteristics of injected electron beams. Results. RHESSI non-thermal component associated with the electron beam is correlated well with presence of the non-thermal n-distribution obtained from the RESIK spectra. In addition, such an n-distribution occurs during radio bursts observed in the 0.61-15.4 GHz range. Furthermore, we show that the n-distribution could also explain RHESSI emission below ∼5 keV. Therefore, two independent diagnostics methods indicate the flare plasma being affected by the electron beam can have a non-thermal component in the ∼2-5 keV range, which is described by the n-distribution well. Finally, spectral line analysis reveals that the n-distribution does not occupy the same location as the thermal component detected by RHESSI at ∼10 keV.

The bound-bound and free-free radiative losses for the nonthermal distributions in solar and stellar coronae

“…The intensities of the allowed Si xiii and Si xiv line were also reproduced. Kulinová et al (2011) shows that the diagnostics of the parameter n from both line and continuum spectra yield consistent results. The value of parameter n is time-dependent, with highest detectable values of n ≈ 11.…”

“…In solar and possibly stellar physics, departures from Maxwellian distribution can occur at low plasma densities in combination with strong temperature or density gradients (e.g., Scudder & Olbert 1979a,b;Roussel-Dupré 1980;Shoub 1983;Owocki & Scudder 1983;Dufton et al 1984;Ljepojevic & MacNiece 1988;Scudder 1992;Pinfield et al 1999), in the impulsive phase of solar flares (Seely et al 1987;Dzifčáková & Kulinová 2010;Kulinová et al 2011) or in situations, where the the average particle energies are not held fixed, but can change by about an order of magnitude (Collier 2004), e.g., due to dynamic coronal heating.…”

The non-Maxwellian continuum in the X-ray, UV, and radio range

“…These processes commonly result in the formation of power-law, high-energy tails. However, changes in the bulk of the distribution function were also reported during flares (Seely et al 1987;Kulinová et al 2011). …”

“…Another parametric distribution is the n-distribution, invoked to explain the observed ratios of dielectronic to allowed lines during solar flares (Seely et al 1987;Kulinová et al 2011). These works report increased intensities of the X-ray Fe or Si dielectronic satellite lines with respect to allowed lines of a nearby He-like ion of the same element.…”

“…This increase occurs only during the impulsive phase of the observed flares, coincides with the presence of electron beams inferred from radio bursts , and most importantly, cannot be explained by emission from a multithermal Maxwellian plasma . Moreover, Kulinová et al (2011) combined the X-ray observations made by the RESIK spectrometer (Sylwester et al 2005) with the RHESSI (Lin et al 2002) observations of energies down to 4 keV to show that not only the presence of n-distributions increase the goodness-of-fit in terms of χ 2 , but that the emission measure associated with the plasma with n-distribution dominates over the contribution from the thermal component. In the range of a few keV, which corresponds to the excitation energy of the dielectronic satellites, the n-distribution can be mimicked by the Maxwellian distribution with a velocity drift (Karlický et al 2012).…”