Sun-as-a-star analysis of simulated solar flares

Context. Stellar flares have an impact on habitable planets. To relate the observations of the Sun with those of stars, one needs to use a Sun-as-a-star analysis, that is, to degrade the resolution of the Sun to a single point. With the data of the Sun-as-a-star observations, a simulation of solar flares is required to provide a systemic clue for the Sun-as-a-star study. Aims. We aim to explore how the Sun-as-a-star spectrum varies with the flare magnitude and location based on a grid of solar flare models. Methods. Using 1D radiative hydrodynamics modeling and multi-thread flare assumption, we obtained the spectrum of a typical flare with an enhancement of chromospheric lines. Results. The Sun-as-a-star spectrum of the H alpha line shows enhanced and shifted components, which are highly dependent on the flare magnitude and location. The equivalent width Delta EW is a good indicator of energy release. The bisector method can be used to diagnose the sign of the line-of-sight velocity in the flaring atmosphere. For both H alpha and H beta lines, the Sun-as-a-star spectrum of a limb flare tends to be wider and shows a dip in the line center. In particular, we propose two quantities to diagnose the magnitude and location of the stellar flares. Besides this, caution must be taken when calculating the radiation energy, since the astrophysical flux-to-energy conversion ratio is dependent on the flare location.