Another View of the EIT Wave Phenomenon

Abstract

This paper investigates three different EIT waves. The events occurred on 1997 November 3 at 10:31 UT, on 1998 January 27 at 22:19 UT, and on 1998 June 13 at 15:23 UT and were observed with EIT on board the Solar and Heliospheric Observatory (SOHO) using the 195 Å filter that contains an Fe XII line. The three events show the following similar structures:1. A flare in an active region.2. A bright front that can lie at the same location for several hours.3. A first dimming located between the bright front and the flare.4. A second dimming located between the bright front and a magnetic dipole located in the opposite hemisphere.5. A third dimming located in an active region located in the opposite hemisphere.6. Brightenings or dimmings of transequatorial loops that connect the flaring active region to an active region located in the opposite hemisphere. We propose that the appearance of the above features is strongly related to magnetic field topology. Most of the EIT waves are associated with a coronal mass ejection (CME). Moreover, the presence of a flare is not a necessary condition to produce an EIT wave since only eruptive flares that are associated with CMEs are associated with a wave. Therefore, we suggest that the EIT-wave phenomenon is more closely related to the magnetic field evolution involved in CMEs than to wave propagation driven by solar flares.