Asymmetries in Fe ix 17.1 nm and Fe xii 19.5 nm Line Profiles as Possible Signatures of Obscuration Dimming in Sun-as-a-star Spectra

Abstract

We report the possible detection of obscuration dimming, inferred from spectral line asymmetries in Sun-as-a-star extreme ultraviolet spectra. By analyzing asymmetries in the Fe ix 17.1 nm and Fe xii 19.5 nm line profiles obtained from the Solar Dynamics Observatory/Extreme Ultraviolet Variability Experiment during coronal dimming events from 2011 to 2014, we identify two candidate obscuration dimming events associated with M9.3- and X5.4-class flares, respectively. Both events demonstrated prominent irradiance reductions, characterized by greater decreases in blue-wing emission relative to the red wing. These line-profile asymmetries may be attributed to cooler filament material ejected during the flares, moving along the line of sight and partially obscuring hotter coronal emission. The possible obscuration dimming accounted for roughly 18% of the total irradiance loss and persisted for approximately 5–6 hr. Doppler velocity analysis of line profiles enabled us to infer possible motion trends of filament eruptions, which may represent the core regions of coronal mass ejections (CMEs). For event 1, the filament eruption possibly experienced deceleration with partial fallback toward the solar surface or deflection away from the Sun–Earth line, consistent with minimal geomagnetic activity. In contrast, event 2 showed potential acceleration toward Earth, correlating with a significant geomagnetic storm. Our results suggest that the obscuration dimming method presented in this study may serve as a useful diagnostic for constraining CME properties and informing space weather forecasts. In addition, this observational approach may provide a pathway toward identifying stellar CMEs and assessing whether they display the typical three-part structure seen in solar CMEs.