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