SPECTRAL AND IMAGING OBSERVATIONS OF A WHITE-LIGHT SOLAR FLARE IN THE MID-INFRARED

Abstract

We report high-resolution observations at mid-infrared wavelengths of a minor solar flare, SOL2014-09-24T17:50 (C7.0), using Quantum Well Infrared Photodetector cameras at an auxiliary of the McMath-Pierce telescope. The flare emissions, the first simultaneous observations in two mid-infrared bands at 5.2 and with white-light and hard X-ray coverage, revealed impulsive time variability with increases on timescales of ∼4 s followed by exponential decay at ∼10 s in two bright regions separated by about . The brightest source is compact, unresolved spatially at the diffraction limit ( at ). We identify the IR sources as flare ribbons also seen in white-light emission at 6173 Å observed by SDO /HMI, with twin hard X-ray sources observed by Reuven Ramaty High Energy Solar Spectroscopic Imager , and with EUV sources (e.g., 94 Å) observed by SDO /AIA. The two infrared points have nearly the same flux density ( f ν , W m −2 Hz) and extrapolate to a level of about an order of magnitude below that observed in the visible band by HMI, but with a flux of more than two orders of magnitude above the free–free continuum from the hot (∼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 cm −3 . We tentatively interpret this emission mechanism as predominantly free–free emission in a highly ionized but cool and rather dense chromospheric region.