Solar Flare Composition and Thermodynamics From Resik X-Ray Spectra

Abstract

Previous estimates of the solar flare abundances of Si, S, Cl, Ar, and K from the RESIK X-ray crystal spectrometer on board the CORONAS-F spacecraft were made on the assumption of isothermal X-ray emission. We investigate the effect on these estimates by relaxing this assumption and instead determining the differential emission measure (DEM) or thermal structure of the emitting plasma by re-analyzing RESIK data for a GOES class M1.0 flare on 2002 November 14 (SOL2002-11-14T22:26) for which there was good data coverage. The analysis method uses a maximumlikelihood (Withbroe-Sylwester) routine for evaluating the DEM. In a first step, called here AbuOpt, an optimized set of abundances of Si, S, Ar, and K is found that is consistent with the observed spectra. With these abundances, the differential emission measure evolution during the flare is found. The abundance optimization leads to revised abundances of silicon and sulfur in the flare plasma: A(S) = 6.94 ± 0.06 and A(Si) = 7.56 ± 0.08 (on a logarithmic scale with A(H) = 12). Previously determined abundances of Ar, K, and Cl from an isothermal assumption are still the preferred values. During the flare's maximum phase, the X-ray-emitting plasma has a basically two-temperature structure, with the cooler plasma with approximately constant temperature (3-6 MK) and a hotter plasma with temperature 16 − 21 MK. Using imaging data from the RHESSI hard X-ray spacecraft, the emission volume of the hot plasma is deduced from which lower limits of the electron density N e and the thermal content of the plasma are given.

Soft X-ray emission in kink-unstable coronal loops

“…7, the emission light-curves show an impulsive initial growth followed by a slower decay. The decay phase is faster the higher the photon energy (and slower the lower the photon energy), as is the case for the solar flare light-curves measured by RHESSI and GOES in Sylwester et al (2014) for the 1−8 Å, 0.5−1 Å, 6−12 keV and 12−25 keV bands. Figure 7 shows that the highest X-ray flux obtained is of about 1.2 × 10 5 photons cm −2 s −1 in the 1−3 keV band, 2 × 10 4 photons cm −2 s −1 in the 3−6 keV band, 2 × 10 3 photons cm −2 s −1 in the 6−12 keV band, and 50 photons cm −2 s −1 in the 12−25 keV band.…”

Inverse First Ionization Potential Effects in Giant Solar Flares Found from Earth X-Ray Albedo with Suzaku/XIS

“…However, such a sophisticated spectral modeling is beyond the scope of this paper. We thus assumed the temperatures (kT ) of the 2T model to be 0.5 keV and 1.7 keV, based on previous studies showing a bimodal DEM with peak temperatures of those values (e.g., Sylwester et al 2014;Caspi et al 2014). As for the DEM model, we assumed kT max to be 2 keV, because the lines of our interest (lines from He-and H-like ions of Si, S, Ar, and Ca) are formed by thermal plasmas with a temperature range of 0.2-2 keV.…”

Numerical calculation of ion runaway distributions

“…Solar flares are thought to be initiated by reconnection in the corona [22], but the origin of the observed fast ion populations in flares is still not completely understood [23]. Both stochastic acceleration by waves and the direct acceleration of the particles by the electric field have been considered, and it appears likely that a combination of the two can be at work [1,24,25].…”