Sun-as-a-star Analysis of H$α$ Spectra of a Solar Flare Observed by SMART/SDDI: Time Evolution of Red Asymmetry and Line Broadening

Abstract

Stellar flares sometimes show red/blue asymmetries of H$α$ line, which can indicate chromospheric dynamics and prominence activations. However, the origin of asymmetries is not completely understood. For a deeper understanding of stellar data, we performed a Sun-as-a-star analysis of H$α$ line profiles of an M4.2-class solar flare showing dominant emissions from flare ribbons by using the data of the Solar Dynamics Doppler Imager onboard the Solar Magnetic Activity Research Telescope at Hida Observatory. The Sun-as-a-star H$α$ spectra of the flare show red asymmetry of up to $\sim$95 km s$^{-1}$ and line broadening of up to $\sim$7.5 Å. The Sun-as-a-star H$α$ profiles are consistent with spectra from flare regions with weak intensity, but they take smaller redshift velocities and line widths by a factor of $\sim$2 than those with strong intensity. The redshift velocities, as well as line widths, peak out and decay more rapidly than the H$α$ equivalent widths, which is consistent with chromospheric condensation model and spatially-resolved flare spectra. This suggests that as a result of superposition, the nature of chromospheric condensation is observable even from stellar flare spectra. The time evolution of redshift velocities is found to be similar to that of luminosities of near-ultraviolet rays (1600 Å), while the time evolution of line broadening is similar to that of optical white lights. These H$α$ spectral behaviors in Sun-as-a-star view could be helpful to distinguish whether the origin of H$α$ red asymmetry of stellar flares is a flare ribbon or other phenomena.