NuSTAR Observation of Energy Release in Eleven Solar Microflares

Abstract

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 eleven 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 peaktimes in higher energy HXRs. For two events where 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 consistent 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.

Accepted for publication in ApJ

References in corpus (5)

• RHESSI Microflare Statistics II. X-ray Imaging, Spectroscopy & Energy Distributions
• The Thermal Properties of Solar Flares Over Three Solar Cycles Using GOES X-ray Observations
• Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA
• NuSTAR hard X-ray observation of a sub-A class solar flare
• NuSTAR Observation of a Minuscule Microflare in a Solar Active Region

Cited by in corpus (12)

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• Multi-wavelength observations by XSM, Hinode and SDO of an active region. Chemical abundances and temperatures
• The First Survey of Quiet Sun Features Observed in Hard X-Rays With NuSTAR
• Multiple electron acceleration instances during a series of solar microflares observed simultaneously at X-rays and microwaves
• On the faintest solar coronal hard X-rays observed with FOXSI
• First joint X-ray solar microflare observations with NuSTAR and Solar Orbiter/STIX
• Detecting non-thermal emission in a solar microflare using nested sampling
• Thermal Evolution of an Active Region through Quiet and Flaring Phases as Observed by NuSTAR XRT, and AIA
• Quiet Sun impulsive events observed with NuSTAR during solar minimum
• Evolution of Elemental Abundances During B-Class Solar Flares: Soft X-ray Spectral Measurements with Chandrayaan-2 XSM