NuSTAR Observation of Eleven Solar Microflares

Abstract:

This work presents eleven microflares observed by the Nuclear Spectroscopic Telescope ARray (NuSTAR), representing the first time that a sizable number of these events have been examined collectively. NuSTAR’s direct focusing optics give it a dramatic increase in sensitivity over indirect imagers in the hard X-ray (HXR) range. HXR emission in solar flares originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, both of which are diagnostic of flare energy release. Therefore, NuSTAR solar observation campaigns can give unique insight into the energetics of faint microflares, including those that were unobservable with previous-generation HXR instruments. We discuss the temporal, spatial, and energetic properties of all eleven microflares 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 found to most likely originate from additional higher-temperature plasma volumes in 10/11 microflares, and from an accelerated particle distribution in the last. Finally, we introduce an observed variation in the NuSTAR gain unique to the extremely low-livetime (< 1%) regime, and establish a correction method to be used in future NuSTAR solar spectral analysis.