NuSTAR Observation of Energy Release in Eleven Solar Microflares

TL;DR

This study analyzes eleven solar microflares using NuSTAR, revealing properties similar to larger flares, such as impulsive profiles and high-energy excesses, and introduces a correction method for low-livetime spectral data.

Contribution

First comprehensive analysis of multiple microflares with NuSTAR, including a new correction method for low-livetime spectral measurements.

Findings

Microflares show impulsive time profiles and high-energy excess.

Spectral analysis indicates additional high-temperature plasma or accelerated particles.

Introduces a correction method for NuSTAR gain in low-livetime conditions.

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.