A Multi-instrument Analysis of the Thermal and Nonthermal Properties of a Faint, Occulted Microflare

Solar flares are energetic events made up of complex interactions among the photosphere, chromosphere, and corona. The most direct probe of these interactions is the observation of X-rays sourced from bremsstrahlung. Thermal emission from the hot plasma at the flare-loop footpoints often dominates the lower energy nonthermal emission from the accelerated particles, making it difficult to observe and characterize. However, the coronal emission no longer competes with the thermal emission from the footpoints during an occulted flare and provides a clearer view of the flare’s energy release and distribution.

This presentation showcases an observation of an occulted flare on March 18, 2023 by the Nuclear Spectroscopic Telescope ARray (NuSTAR), the Solar Dynamics Observatory (SDO), and Hinode. Together, these instruments provide a cohesive picture of the energy injection and subsequent plasma evolution of the flare’s coronal emission. The NuSTAR spectra are well-modeled by a single thermal component throughout the evolution of the flare and a steep nonthermal component during the impulsive phase, making this the first occulted flare from which NuSTAR observed nonthermal emission. The extreme ultraviolet (EUV) data from SDO and soft X-ray (SXR) data from Hinode are used to determine the flare and loop geometry and infer additional thermal properties of the plasma. The EUV and SXR data indicate a dominant presence of high temperature (8-10 MK) plasma with relatively little lower temperature plasma, agreeing with the spectral results from NuSTAR. Differential emission measure analyses utilizing all three instruments offer a more detailed look into the time evolution of the thermal emission and further confirms the presence of this high temperature population.