Sun-as-a-star Study of an X-class Solar Flare with Spectroscopic Observations of CHASE

Sun-as-a-star spectroscopic characteristics of solar flares can be used as a benchmark for the detection and analyses of stellar flares. Here, we study the Sun-as-a-star properties of an X1.0 solar flare using high-resolution spectroscopic data obtained by the Chinese $\mathrm{H} \alpha$ Solar Explorer (CHASE). A noise reduction algorithm based on discrete Fourier transformation is first employed to enhance the signal-to-noise ratio of the space-integral $\mathrm{H} \alpha$ spectrum with a focus on its typical characteristics. For the flare of interest, we find that the average $\mathrm{H} \alpha$ profile displays a strong emission at the line center and an obvious line broadening. It also presents a clear red asymmetry, corresponding to a redshift velocity of around $50 \ \mathrm{km \ s^{-1}}$ that slightly decreases with time, consistent with previous results. Furthermore, we study how the size of the space-integral region affects the characteristics of the flare Sun-as-a-star $\mathrm{H} \alpha$ profile. It is found that although the redshift velocity calculated from the $\mathrm{H} \alpha$ profile remains unchanged, the detectability of the characteristics weakens as the space-integral region becomes large. An upper limit for the size of the target region where the red asymmetry is detectable is estimated. It is also found that the intensity in $\mathrm{H} \alpha$ profiles, measured by the equivalent widths of the spectra, are significantly underestimated if the $\mathrm{H} \alpha$ spectra are further averaged in the time domain.