Observational signatures of electron-driven chromospheric evaporation in a white-light flare

Abstract: We investigate observational signatures of explosive chromospheric evaporation during a white-light flare (WLF) that occurred on 2022 August 27. Using the moment analysis, bisector techniques, and the Gaussian fitting method, red-shifted velocities of less than 20 km/s are detected in low-temperature spectral lines of Ha, C I and Si IV at the conjugated flare kernels, which could be regarded as downflows caused by chromospheric condensation. Blue-shifted velocities of about 30-40 km/s are found in the high-temperature line of Fe XXI, which can be interpreted as upflows driven by chromospheric evaporation. A nonthermal hard X-ray (HXR) source is co-spatial with one of the flare kernels, and the Doppler velocities are temporally correlated with the HXR fluxes. The nonthermal energy flux is estimated to be at least (1.3+-0.2)*10^10 erg/s/cm^2. The radiation enhancement at Fe I 6569.2 A and 6173 A suggests that the flare is a WLF. Moreover, the while-light emission at Fe I 6569.2 A is temporally and spatially correlated with the blue shift of Fe XXI line, suggesting that both the white-light enhancement and the chromospheric evaporation are triggered and driven by nonthermal electrons. All our observations support the scenario of an electron-driven explosive chromospheric evaporation in the WLF.