Dielectronic satellite lines and double layers in solar flares
Abstract
Context. Particle acceleration during solar flares results in departures of the distribution of particle energies from the Maxwellian distribution. Apart from the highenergy tail, the bulk of the distribution was recently also found to be significantly affected, due, e.g., to the presence of double layers.
Aims: We investigate the influence of several proposed nonMaxwellian distribution functions on the Xray flare line spectra. The distribution functions considered are sharply peaked and include the ndistribution, the moving Maxwellian distribution, and the distribution formed in strong double layers in the flaring plasma.
Methods: Synthetic Si xiidSi xiv spectra involving allowed and dielectronic transitions at 5  6 Å are calculated numerically. The parameters chosen for the calculations correspond to the impulsive phase of solar flares, as inferred by previous authors.
Results: The Si xiid λ5.56/Si xiii λ5.68 and Si xiid λ5.82/Si xiii λ5.68 ratios depend on the relative number of electrons at energies corresponding to the formation of the Si xiid lines. Therefore, these ratios increase with the increasing narrowness of the peak of the electron distribution function. The highest ratios are achieved for the distribution formed in double layers, while the moving Maxwellian distribution is less likely to reproduce the observed enhancement of Si xiid intensities. However, the ratio of the allowed Si xiv λ5.22/Si xiii λ5.68 transitions depends on the ionization equilibrium. This ratio is very small for the doublelayer distribution. Combination of the doublelayer distribution with a Maxwellian distribution with the same mean energy significantly enhances this ratio, while keeping the Si xiid intensities sufficiently increased to explain the characteristics of the observed spectra.
Conclusions: These results support the presence of double layers in the plasma during impulsive phase of solar flares.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 February 2013
 DOI:
 10.1051/00046361/201220501
 Bibcode:
 2013A&A...550A..60D
 Keywords:

 Sun: Xrays;
 gamma rays;
 Sun: flares;
 atomic processes;
 radiation mechanisms: nonthermal