On the nature of EIT waves, EUV dimmings and their link to CMEs

Abstract

EIT waves and extreme-ultraviolet (EUV) dimmings attract particular attention as they frequently accompany Coronal Mass Ejections (CMEs). We present several examples of EIT waves and EUV dimmings with particular morphologies previously unreported in the literature. We report for the first time an EIT wave in the Fe XV (284 A) bandpass of the SOHO/EIT instrument. The observations of this event confirm previous results that an EIT wave is a purely coronal phenomenon that does not propagate in the transition region plasma. Two EIT wave events initiated close to the solar limb are investigated, thus per- mitting us to see simultaneously the wave and the magnetic configuration of the CME. These observations suggest that EIT wave can be regarded as a bimodal phenomenon. The wave mode represents a wave-like propagating disturbance. Its char- acteristic features are propagation of a bright front to large distances from dimming sites and quasi-circular appearance. The eruptive mode is the propagation of a dimming and of an EIT wave as a result of successive opening of magnetic field lines during the CME lift-off. It can be identified by noting the expansion of a dimming and the appearance of another dimming ahead of a bright front. We reveal the temperature structure of the EUV dimmings that appeared after the classical EIT wave event on May 12, 1997, using differential emission measure (DEM) maps obtained through the analysis of images in four EIT bandpasses. The part of the CME mass contained in the low corona observed by the EIT is estimated to be about 10 15 g. It appears that around 50% of this total CME mass in the low corona is contained outside of transient coronal holes. It is shown that at present it is difficult to reconcile all the observational facts into a coherent physical model. In particular, the physical nature of the wave mode of EIT waves remains elusive.