An Attempt to Detect Coronal Mass Ejections in Lyman-alpha Using SOHO Swan

In this study, the possibility that coronal mass ejections CMEs may be observed in neutral Lyman-alpha emission was investigated. An observing campaign was initiated for SWAN Solar Wind ANisotropies, a Lyman-alpha scanning photometer on board the Solar and Heliospheric Observatory SOHO dedicated to monitoring the latitude distribution of the solar wind from its imprints on the interstellar sky background. This was part of SOHO Joint Observing Program JOP 159 and was an exploratory investigation as it was not known how, or even if, CMEs interact with the solar wind and interstellar neutral hydrogen at this distance approximately 60 and 120 Rs. The study addresses the lack of methods for tracking CMEs beyond the field-of-view of current coronagraphs 30 Rs. In our first method we used LASCO white-light coronagraphs on SOHO, and EIT, an extreme ultraviolet imaging telescope also on SOHO, to identify CME candidates which, subject to certain criteria, should have been observable in SWAN. The criteria included SWAN observation time and location, CME position angle, and extrapolated speed. None of the CME candidates that we discuss were identified in the SWAN data. For our second method we analyzed all of the SWAN data for 184 runs of the observing campaign, and this has yielded one candidate CME detection. The candidate CME appears as a dimming of the background Lyman-alpha intensity representing approximately 10 of the original intensity, moving radially away from the Sun. Multiple candidate CMEs observed by LASCO and EIT were found which may have caused this dimming. Here we discuss the campaign, data analysis technique and statistics, and the results.