Synthetic Spectra of the Fe VIII – Fe XVI Emission Lines for Electron Non-Thermal Distributions

et al. 2011

A&A

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.

Section: Diagnostics Of the N-distributionmentioning

confidence: 99%

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

Kulinová

Kašparová

et al. 2011

A&A

“…Another type of nonthermal distribution is the n-distribution (e.g., Seely et al 1987;Dzifčáková & Tóthová 2007) …”

Section: The N-distributionsmentioning

confidence: 99%

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

Dudík

Karlický

et al. 2011

A&A

Context. The radiative-loss function is an important ingredient in the physics of the solar corona, transition region, and flares. Aims. We investigate the radiative losses due to the bound-bound transitions and bremsstrahlung for nonthermal κ-and n-distributions. Methods. The bound-bound radiative losses are computed by integrating synthetic spectra. An analytical expression is derived for nonthermal bremsstrahlung. The bremsstrahlung is computed numerically using accurate values of the free-free Gaunt factor. Results. We find that the changes in radiative-loss functions due to nonthermal distributions are several times greater than the errors due to the missing contribution of the free-bound continuum or errors in atomic data. For κ-distributions, the radiative-loss functions are in general weaker than for Maxwellian distribution, with a few exceptions caused by the behavior of Fe. The peaks of the radiativeloss functions are in general flatter. The situation is opposite for n-distributions, for which the radiative-loss functions have higher and narrower peaks. Local minima and maxima of the radiative-loss functions may also be shifted. The contribution from bremsstrahlung only changes by a few percent except in the extreme nonthermal case of κ = 2. Stability analysis reveals that the X-ray loops are stable against the radiatively-driven thermal instability.

“…The original modification of CHIANTI 5.2 developed by Dzifčáková (2006b) was used previously for computation of line intensities for the nonthermal κ-and n-distributions (e.g., Dzifčáková & Tóthová 2007;Dzifčáková & Mason 2008). This modification can be used for computation of line intensities for any kind of distribution, i.e., also for the composed np-distributions.…”

Section: Synthetic Spectramentioning

confidence: 99%

“…The resulting space-angle integrated distribution can be described by the n-distribution (Dzifčáková & Karlický 2008). Its deviation from the Maxwellian distribution depends on the ratio Article published by EDP Sciences A111, page 1 of 5 of the electron drift velocity to the electron thermal velocity and can be expessed by a free parameter n. The possible presence of n-distributions during impusive phase of the flare was first shown by Seely et al (1987) and their effect on the ionization equilibrium and line spectra was subsequently studied by, e.g., Dzifčáková (1998Dzifčáková ( , 2000 and Dzifčáková & Tóthová (2007). analyzed the X-ray line spectra observed by CORONAS-F/RESIK (Sylwester et al 2005).…”

Section: Introductionmentioning

confidence: 99%

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

Homola

Dudík

2011

A&A

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 temperaturesensitive lines Si XIV λ5.22/Si XIII λ5.68. Because this ratio can be fitted with a thermal distribution with higher temperature, the highenergy 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.