Non-Maxwellian Diagnostics from SDO/EVE Spectra of an X-class Flare

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Nugget
Number: 317
1st Author: Jaroslav Dudik
2nd Author:
Published: 19 February 2018
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Introduction

Particle acceleration is a common process in solar flares. Typically, high-energy power-law tails are observed at energies higher than a few keV in the bremsstrahlung (continuum) radiation. However, electrons at these energies are also involved in formation of high-temperature flare lines, i.e., in the bound-bound emission of highly ionized ions. For example, Fe XXV denotes a helium-like iron ion, one with two electrons; typically an ionization equilibrium theory, in which collisional stripping balances recombinations, determines the relative abundances of the different ionic states. In a tenuous plasma such as the corona, this differs strongly from the well-known Saha equlibrium.

Both the ionization and excitation of an ion are dominated by collisions with free electrons. The presence of high-energy electrons strongly influences the rates of both ionization and excitation (e.g., Ref [1], [2]). The inverse processes of recombination and collisional de-excitation are also affected. This means that the intensities of flare lines should also be influenced by the high-energy electrons, and in turn, the presence of these high-energy tails could in principle be derived also from observations of the flare emission lines, such as those of the Fe XVIII-XXIV ionic states of iron.


Figure 1: The X5.4 flare of 2012 March 07. Panels (a) and (b) show the GOES X-ray flux and its derivative, respectively. The vertical blue line shows the period selected as for obtaining the background EVE spectrum, while the red vertical line denotes the flare peak. Panels (c)-(e) show the evolution of the flare, as observed by the 94 Â channel of the Atmospheric Imaging Assembly (AIA) instrument onboard the SDO. From (Ref. [4]).
Figure 2: Results of the diagnostics of the kappa-distributions over the early and impulsive phase of the flare until the peak at 00:25 UT (left) and during the onset of the gradual phase starting from 00:25 UT (right). Observed line ratios are shown by diamonds together with the respective error-bars, while the theoretical ratios are plotted by full lines, where black stands for Maxwellian (kappa equal to infinity), green for kappa=5, orange for kappa=3, and red for kappa=2. Asterisks and triangles stand for the predicted ratios given the DEMs derived from a range of Fe X-XXIV lines. From (Ref. [4]).
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