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Solar active regions contain a broad range of temperatures, with the thermalplasma distribution often observed to peak in the few millions of kelvin.Differential emission measure (DEM) analysis can allow instruments with diversetemperature responses to be used in concert to estimate this distribution.NuSTAR HXR observations are uniquely sensitive to the highest-temperaturecomponents of the corona, and thus extremely powerful for examining signaturesof reconnection-driven heating. Here, we use NuSTAR diagnostics in combinationwith EUV and SXR observations (from SDO/AIA and Hinode/XRT) to construct DEMsover 170 distinct time intervals during a five-hour observation of analternately flaring and quiet active region (NOAA designation AR 12712). Thisrepresents the first HXR study to examine the time evolution of thedistribution of thermal plasma in an active region. During microflares, we findthat the initial microflare-associated plasma heating is dominantly heating ofmaterial that is already relatively hot, followed later on by broader heatingof initially-cooler material. During quiescent times, we show that the amountof extremely hot (>10 MK) material in this region is significantly (~3 ordersof magnitude) less than that found in the quiescent active region observed inHXRs by FOXSI-2 (Ishikawa et al. 2017). This result implies there can beradically different high-temperature thermal distributions in different activeregions, and strongly motivates future HXR DEM studies covering a large numberof these regions.

Thermal Evolution of an Active Region through Quiet and Flaring Phases as Observed by NuSTAR XRT, and AIA