Study of Time Evolution of Thermal and Nonthermal Emission from an M-class Solar Flare

We conduct a wide-band X-ray spectral analysis in the energy range of 1.5–100 keV to study the time evolution of the M7.6-class flare of 2016 July 23, with the Miniature X-ray Solar Spectrometer (MinXSS) CubeSat and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft. With the combination of MinXSS for soft X-rays and RHESSI for hard X-rays, a nonthermal component and three-temperature multithermal component—“cool” (T ≈ 3 MK), “hot” (T ≈ 15 MK), and “superhot” (T ≈ 30 MK)—were measured simultaneously. In addition, we successfully obtained the spectral evolution of the multithermal and nonthermal components with a 10 s cadence, which corresponds to the Alfvén timescale in the solar corona. We find that the emission measures of the cool and hot thermal components are drastically increasing more than hundreds of times and the superhot thermal component is gradually appearing after the peak of the nonthermal emission. We also study the microwave spectra obtained by the Nobeyama Radio Polarimeters, and we find that there is continuous gyrosynchrotron emission from mildly relativistic nonthermal electrons. In addition, we conducted a differential emission measure (DEM) analysis by using Atmospheric Imaging Assembly on board the Solar Dynamics Observatory and determined that the DEM of cool plasma increases within the flaring loop. We find that the cool and hot plasma components are associated with chromospheric evaporation. The superhot plasma component could be explained by the thermalization of the nonthermal electrons trapped in the flaring loop.