Understanding the Lyα Emission Observed by the Solar Disk Imager Aboard the Advanced Space-based Solar Observatory

Abstract: The H I Lyman-alpha (Ly$\alpha$) emission, with a wavelength of 1216 \r{A}, is the brightest solar ultraviolet (UV) line. However, comprehensive observations of the Ly$\alpha$ emission line across the full solar disk remain limited. As part of the ASO-S mission, the Solar Disk Imager (SDI) has successfully captured full-disk images in the Ly$\alpha$ band. Gaussian fitting of SDI's spectral response function (SRF) yields a full width at half maximum (FWHM) of approximately 85 \r{A}, which is significantly broader than the distance of Si III line at 1206 \r{A} and the Ly$\alpha$ line. Thus, the emission contribution of Si III to the SDI Ly$\alpha$ passband needs to be considered. For flares, in practice, we calculated the integrated intensity ratio $I$(Si III)/$I$(Ly$\alpha$) by analyzing spectral observations from the SOLSTICE instrument. It yields values between 1.7% and 14.6%. Empirically, the ratio is proportional to the SXR flux. Further analysis of spectral data from the SUMER instrument reveals that the ratio $I$(Si III)/$I$(Ly$\alpha$) is approximately 0.5% for prominences, 0.7%--0.9% for the inner disk, and 1.4%--1.9% close to the limb. These findings suggest that $I$(Si III)/$I$(Ly$\alpha$) is minimal for prominences and the inner disk, and the varying ratios across regions align with the center-to-limb variation of the Si III and Ly$\alpha$ lines. Additionally, we compared Ly$\alpha$ image intensity with 304 \r{A}, 1600 \r{A}, and 1700 \r{A} observations from AIA, as well as H$\alpha$ from CHASE, in multiple regions (a prominence region, two active regions, and a quiet region). A relatively higher correlation of about 85% is found between Ly$\alpha$ and 304 \r{A} in active regions, whereas in the quiet region and prominence, their correlation coefficients are about 55%.