Coordinated soft X-ray and Hα observations of solar flares

Abstract

We have analyzed a unique set of four solar flares for which coordinated soft X-ray Ca XIX and Hα spectral observations were obtained during the impulsive phase. In all events, a blue asymmetry (indicative of upflows) was observed in the coronal Ca XIX line during the soft X-ray rise phase. In all events, a red asymmetry (indicative of downflows) was observed simultaneously in chromospheric Hα at spatial locations associated with enhanced flare heating. Combining the velocity data with estimates of the evaporated mass based on soft X-ray emission measure, we demonstrate that the impulsive phase momentum of upflowing soft X-ray plasma equalled that of the downflowing Hα plasma, to within an order of magnitude. This equality supports the explosive chromospheric evaporation model of solar flares — the only model that predicts equal momentum content in upflowing coronal and downflowing chromospheric plasmas.