NuSTAR X-ray Observations of a Minuscule Microflare from a Repeatedly Microflaring Active Region

Highly frequent, small flares are thought to contribute to heating the Sun's atmosphere, particularly in active regions. This impulsive energy release would heat plasma to at least 10 MK and accelerate electrons, producing weak thermal and non-thermal signatures that could be observed by a very sensitive X-ray telescope. No such solar telescope exists (yet) so we use Nuclear Spectroscopic Telescope Array (NuSTAR), an astrophysical X-ray telescope, with focusing optics imaging spectroscopy providing a unique sensitivity for observing the Sun above 2.5 keV. We present an overview of several microflares from the recently emerged active region AR12721 on 2018 September 9-10. Using NuSTAR's imaging spectroscopy and the Solar Dynamics Observatory's Atmospheric Imaging Assembly's (SDO/AIA) EUV imaging capabilities we can analyse the temporal, spatial, and spectral evolution of these microflares, determining the energy release and associated heating of the solar atmosphere. All microflares from AR12721 were below GOES A1 equivalent level and the heated coronal loops were all visible in an Fe XVIII proxy channel derived from SDO/AIA channels. In particular, we present our recently published analysis of the weakest microflare from AR12721 (Cooper et al. 2020 ApJL 893 2) finding it to be one of the smallest active region X-ray flares on record, with material heated up to 7MK and a thermal energy of just 1e26 erg.