Hard X-ray Spectroscopy of Six NuSTAR Microflares

Hard X-ray (HXR) emission in solar flares can originate from regions of high temperature plasma, as well as from non-thermal particle populations. Both of these sources of HXR radiation make solar observation in this band important for study of flare energetics. NuSTAR is the first HXR telescope with direct focusing optics, giving it a dramatic increase in sensitivity over previous indirect imaging methods. Here we present NuSTAR observation of six microflares from one solar active region during a period of several hours on May 29th, 2018. Spectral fitting of emission at each flare time shows excess high energy emission over an isothermal spectral component in all six flares. The most likely origin of this excess could be either additional volumes of high-temperature plasma, or non-thermally accelerated particles. For each event, characterization of this excess is presented, including determination of upper limits on the non-thermal emission possible in events where it is not directly observed.