Spectral and Imaging Observations of a White-light Flare in the Mid-Infrared

TL;DR

This study presents high-resolution mid-infrared observations of a solar flare, revealing impulsive emission features, spatial correlations with white-light and X-ray sources, and suggesting a free-free emission mechanism in the chromosphere.

Contribution

First simultaneous mid-infrared imaging of a solar flare at 5 and 8 microns, linking IR sources with white-light and X-ray emissions, and proposing a new interpretation of IR emission mechanisms.

Findings

IR emission is impulsive and spatially correlated with white-light and X-ray sources.

IR flux levels are below visible but above coronal free-free emission.

IR emission likely originates from dense, cool chromospheric regions via free-free processes.

Abstract

We report high-resolution observations at mid-infrared wavelengths of a minor solar flare, SOL2014-09-24T17T17:50 (C7.0), using Quantum Well Infrared Photodetector (QWIP) cameras at an auxiliary of the McMath-Pierce telescope. The flare emissions, the first simultaneous observations in two mid-infrared bands at $5\ \mu$m and $8\ \mu$m with white-light and hard X-ray coverage, revealed impulsive time variability with increases on time scales of $\sim 4$~s followed by exponential decay at $\sim$10 s in two bright regions separated by about 13$"$. The brightest source is compact, unresolved spatially at the diffraction limit ($1.3"$ at $5\ \mu$m). We identify the IR sources as flare ribbons also seen in white-light emission at 6173~\AA~observed by SDO/HMI, with twin hard X-ray sources observed by RHESSI, and with EUV sources (e.g., 94~\AA) observed by SDO/AIA. The two infrared points have closely the same flux density ($f_\nu$, W/m$^2$Hz) and extrapolate to a level about an order of magnitude below that observed in the visible band by HMI, but with a flux more than two orders of magnitude above the free-free continuum from the hot ($\sim$15 MK) coronal flare loop observed in the X-ray range. The observations suggest that the IR emission is optically thin, this constraint and others suggest major contributions from a density less than about $3 \times 10^{13}$~cm$^{-3}$. We tentatively interpret this emission mechanism as predominantly free-free emission in a highly ionized but cool and rather dense chromospheric region.