Abstract
We report the detection of emission from a nonthermal electron distribution in a small solar microflare (GOES class A5.7) observed by the Nuclear Spectroscopic Telescope Array, with supporting observation by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The flaring plasma is well accounted for by a thick-target model of accelerated electrons collisionally thermalizing within the loop, akin to the "coronal thick-target" behavior occasionally observed in larger flares. This is the first positive detection of nonthermal hard X-rays from the Sun using a direct imager (as opposed to indirectly imaging instruments). The accelerated electron distribution has a spectral index of 6.3 ± 0.7, extends down to at least 6.5 keV, and deposits energy at a rate of ~2 × 1027 erg s-1, heating the flare loop to at least 10 MK. The existence of dominant nonthermal emission in X-rays down to <5 keV means that RHESSI emission is almost entirely nonthermal, contrary to what is usually assumed in RHESSI spectroscopy. The ratio of nonthermal to thermal energies is similar to that of large flares, in contrast to what has been found in previous studies of small RHESSI flares. We suggest that a coronal thick target may be a common property of many small microflares based on the average electron energy and collisional mean free path. Future observations of this kind will enable understanding of how flare particle acceleration changes across energy scales, and will aid the push toward the observational regime of nanoflares, which are a possible source of significant coronal heating.
Highlights
While large solar flares of GOES classes M and X garner the most public and scientific attention, small flares occur far more frequently
The recent advent of directly focusing hard X-rays (HXRs) instruments in the form of the Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft and the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket have enabled the observation of small microflares orders of magnitude fainter than those observed by Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
The nonthermal emission observed by NuSTAR and RHESSI emanates from the flare loop, not from its footpoints, akin to the “coronal thicktarget” flares studied by Veronig & Brown (2004), Veronig et al (2005), and Fleishman et al (2016)
Summary
While large solar flares of GOES classes M and X garner the most public and scientific attention, small flares occur far more frequently. The most advanced solar HXR spacecraft instrument, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft, was limited in its sensitivity to small flares due to its indirect imaging method, GOES class A microflares could be observed (Christe et al 2008; Hannah et al 2008). The recent advent of directly focusing HXR instruments in the form of the Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft and the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket have enabled the observation of small microflares orders of magnitude fainter than those observed by RHESSI We analyze the microflare’s thermal and nonthermal properties and compare these to larger flares
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