The FOXSI solar sounding rocket campaigns

Grefenstette

et al. 2021

ApJ

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Solar flares are explosive releases of magnetic energy. Hard X-ray (HXR) flare emission originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, giving insight into flare energy release. The Nuclear Spectroscopic Telescope ARray (NuSTAR) utilizes direct-focusing optics to attain much higher sensitivity in the HXR range than that of previous indirect imagers. This paper presents 11 NuSTAR microflares from two active regions (AR 12671 on 2017 August 21 and AR 12712 on 2018 May 29). The temporal, spatial, and energetic properties of each are discussed in context with previously published HXR brightenings. They are seen to display several "large flare" properties, such as impulsive time profiles and earlier peak times in higher-energy HXRs. For two events where the active region background could be removed, microflare emission did not display spatial complexity; differing NuSTAR energy ranges had equivalent emission centroids. Finally, spectral fitting showed a high-energy excess over a single thermal model in all events. This excess was consistent with additional higher-temperature plasma volumes in 10/11 microflares and only with an accelerated particle distribution in the last. Previous NuSTAR studies focused on one or a few microflares at a time, making this the first to collectively examine a sizable number of events. Additionally, this paper introduces an observed variation in the NuSTAR gain unique to the extremely low livetime (<1%) regime and establishes a correction method to be used in future NuSTAR solar spectral analysis.

Section: Introductionmentioning

confidence: 99%

NuSTAR Observation of Energy Release in 11 Solar Microflares

Duncan

Grefenstette

et al. 2021

ApJ

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“…However, 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, although GOES class A microflares could be observed 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 (Krucker et al 2014;Glesener et al 2016Glesener et al , 2017Wright et al 2017;Kuhar et al 2018;Hannah et al 2019;Athiray et al 2020;J. T. Vievering et al 2020, in preparation).…”

Section: Introductionmentioning

confidence: 99%

Accelerated Electrons Observed Down to <7 keV in a NuSTAR Solar Microflare

Krucker

Duncan

et al. 2020

ApJL

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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 thicktarget 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×10 27 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.

“…In recent years, new instruments have begun to demonstrate the dramatically increased sensitivity available via direct HXR focusing as opposed to RHESSI's indirect imaging method , with the first two flights of the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket (Krucker et al 2014;Glesener et al 2016) and occasional solar pointings by the Nuclear Spectroscopic Telescope Array (NuSTAR) astrophysics spacecraft (Harrison et al 2013;Grefenstette et al 2016). Focusing HXR instruments, with their larger effective areas and drastically reduced detector backgrounds, can measure flares of smaller temperatures, brightnesses, and total energies than those available to indirect imagers.…”

Section: Introductionmentioning

confidence: 99%

NuSTAR Hard X-Ray Observation of a Sub-A Class Solar Flare

Krucker

Hannah

et al. 2017

ApJ

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We report a Nuclear Spectroscopic Telescope Array (NuSTAR) observation of a solar microflare, SOL2015-09-01T04. Although it was too faint to be observed by the GOES X-ray Sensor, we estimate the event to be an A0.1 class flare in brightness. This microflare, with only ∼5 counts s −1 detector −1 observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), is fainter than any hard X-ray (HXR) flare in the existing literature. The microflare occurred during a solar pointing by the highly sensitive NuSTAR astrophysical observatory, which used its direct focusing optics to produce detailed HXR microflare spectra and images. The microflare exhibits HXR properties commonly observed in larger flares, including a fast rise and more gradual decay, earlier peak time with higher energy, spatial dimensions similar to the RHESSI microflares, and a high-energy excess beyond an isothermal spectral component during the impulsive phase. The microflare is small in emission measure, temperature, and energy, though not in physical size; observations are consistent with an origin via the interaction of at least two magnetic loops. We estimate the increase in thermal energy at the time of the microflare to be 2.4×10 27 erg. The observation suggests that flares do indeed scale down to extremely small energies and retain what we customarily think of as "flare-like" properties.