Spectral diagnostics of cool flare loops observed by SST: I. Inversion of the Ca II 8542 \r{A} and H$\beta$ lines

TL;DR

This study uses spectral inversion techniques on SST observations of cool flare loops to determine plasma parameters, revealing high electron densities and microturbulence, which support the loop model of stellar superflares.

Contribution

It introduces a non-LTE spectral inversion method for cool flare loops observed off-limb, providing new insights into their plasma conditions and supporting stellar flare models.

Findings

High electron densities around 2 x 10^{12} cm^{-3}

Large microturbulence (~25 km/s)

Consistent plasma parameters with other studies

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 \r{A} and the hydrogen H$\beta$ 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 \r{A} and H$\beta$ lines yields two similar solutions, both indicating high electron densities around $2 \times 10^{12}$ cm$^{-3}$ and relatively large microturbulence around 25 kms$^{-1}$. These are in reasonable agreement with other independent studies of the same or similar events. In particular, the high electron densities in the range $10^{12} - 10^{13}$ cm$^{-3}$ 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.