Understanding the Ly\(\alpha \) 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 Å, 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 Å, which is significantly broader than the distance of Si iii line at 1206 Å 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 Å, 1600 Å, and 1700 Å 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 Å in active regions, whereas in the quiet region and prominence, their correlation coefficients are about 55%.