And Cme-related Coronal Euv Dimming Observations

Coronal Mass Ejections (CMEs) are large, magnetized plasma structures ejected from the solar corona into the heliosphere. CMEs, especially halo CMEs, can cause interplanetary shocks and drive geomagnetic disturbances representing one of the major sources of catastrophic space weather events. Despite the extensive observational and theoretical studies on CMEs, the physical origin and initial development are still not well understood. CMEs are accompanied by various types of phenomena, like flares, eruptive prominence, solar radio burst and coronal dimming. Study of these features can help us understand the onset of the CMEs and its dynamic evolution. Coronal dimmings associated with CMEs are of special interest to us in this study. Two aspects of this phenomena are investigated in this study, based on the EUV observation of coronal dimmings.1. The relevance of EUV dimmings and halo CMEsThe EIT 195A observations of CMEs are used to determine the relevance of coronal dimmings and halo CMEs. The events of halo CMEs examined in this study are observed by SOHO/LASCO and taken from the CDAW catalogue from 1996 to 2008. Dimming events are identified by using running-difference images or difference images. We found strong relevance of halo CMEs with the coronal dimming events, with up to 93.3% of the front-side halo CME onsets associated with the EIT 195 dimming events. The relevance is much greater than the backside-limb halo CMEs, for which up to 54.9% of the CMEs are shown to be associated with dimming events. We also found that the relevance of slower halo CMEs to dimmings is weaker.2. Study of coronal dimming with SOHO/SUMERWe studied two dimming events with SUMER observations. Both of them are limb events, identified by the EIT 195? observations. For the first event, dated on 1997 July 16, the dimmings are observed at 3 different emission lines with different instruments (Ne VIII (630,000K):SUMER; Mg IX (1MK):CDS; Fe XII (1.5MK):EIT), but no dimming occurs at line Si X (1.3MK,CDS).The reason why dimmings occur at spectral lines emitted from 1MK and 1.5MK plasmas and not at 1.3MK remains unanswered, which will be studied in further works.For the 2004 May 23 event, we found dimmings occurred at 2 different lines formed at the same temperature observed with SUMER. Both of the lines are Mg IX (1MK), they have different intensities and different intensity changes, but their general profiles are similar. Compared with EIT 195? (1.5MK) observation, we found the 1.5 MK dimming at about 2 hours later than the 1MK dimming. It is indicated that the dimming evolves temporally from the colder plasmas to the hotter plasmas, and the details of the dimming is affected by the formation mechanism of spectral lines.