Observations of Thomson scattering from a loop-prominence system

TL;DR

This paper presents observations of a solar flare's coronal structures using polarimetric data, identifying Thomson scattering in hot plasma and estimating the associated mass and filling factor.

Contribution

It provides the first direct estimation of coronal mass from Thomson scattering observations using HMI polarimetry.

Findings

Thomson scattering observed in hot plasma above flare loops.

Estimated coronal mass of approximately 8 x 10^14 grams.

Filling factor of the scattering source is near unity.

Abstract

We describe observations of the white-light structures in the low corona following the X8.2 flare SOL2017-09-10, as observed in full Stokes parameters by the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. These data show both bright loops and a diffuse emission region above them. We interpret the loops as the white-light counterpart of a classical loop-prominence system, intermediate between the hot X-ray loops and coronal rain. The diffuse emission external to the loops is linearly polarized and has a natural interpretation in terms of Thomson scattering from the hot plasma seen prior to its cooling and recombination. The polarimetric data from HMI enable us to distinguish this contribution of scattering from the HMI pseudo-continuum measurement, and to make a direct estimation of the coronal mass in the polarized source. For a snapshot at 16:19~UT, we estimate a mass $8 \times 10^{14}$~g. We further conclude that the volumetric filling factor of this source is near unity.