Spectral diagnostics of cool flare loops observed by SST: I. Inversion of the Ca II 8542 Å and H lines
arXiv:1909.07356 · doi:10.3847/1538-4357/ab4426
Abstract
Flare loops form an integral part of eruptive events, being detected in the range of temperatures from X-rays down to cool chromospheric-like plasmas. While the hot loops are routinely observed by the Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA), cool loops seen off-limb are rare. In this paper we employ unique observations of the SOL2017-09-10T16:06 X8.2-class flare which produced an extended arcade of loops. The Swedish 1-m Solar Telescope (SST) made a series of spectral images of the cool off-limb loops in the Ca II 8542 Å and the hydrogen H lines. Our focus is on the loop apices. Non-LTE spectral inversion is achieved through the construction of extended grids of models covering a realistic range of plasma parameters. The Multilevel Accelerated Lambda Iterations (MALI) code solves the non-LTE radiative-transfer problem in a 1D externally-illuminated slab, approximating the studied loop segment. Inversion of the Ca II 8542 Å and H lines yields two similar solutions, both indicating high electron densities around cm and relatively large microturbulence around 25 kms. These are in reasonable agreement with other independent studies of the same or similar events. In particular, the high electron densities in the range cm are consistent with those derived from the SDO's Helioseismic and Magnetic Imager white-light observations. The presence of such high densities in solar eruptive flares supports the loop interpretation of the optical continuum emission of stars which manifest superflares.
arXiv comment16 pages, 9 figures; accepted for publication in The Astrophysical Journal on September 11, 2019
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