Spectral Diagnostics of Cool Flare Loops Observed by the SST. I: Inversion of the Ca ii 8542 Å and Hβ Lines

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 hot loops are routinely observed by the Solar Dynamics Observatory's Atmospheric Imaging Assembly, 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 made a series of spectral images of the cool off-limb loops in the Ca ii 8542 Å and the hydrogen Hβ lines. Our focus is on the loop apices. Non-local thermal equilibrium (non-LTE; i.e., departures from 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 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 Hβ lines yields two similar solutions, both indicating high electron densities around 2 × 1012 cm−3 and relatively large microturbulence around 25 km s−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 1012–1013 cm−3 are consistent with those derived from the Solar Dynamics Observatory'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.