High-Cadence Radio Observations of an EIT Wave

Sensitive radio observations of the 1997 September 24 EIT wave show its velocity to be 830 km s−1. The wave first appears a short distance from the flare site, an d its trajectory projects back to the flare site at the peak of the im pulsive phase. The radio spectrum appears to be consistent with optically thin coronal emission rather than chromospheric emission. The observed radio brightness temperatures are consistent with the EIT fluxes if the temperature of the emitting gas is no t at the peak formation temperature of the Fe XII 195 ˚ A line, or if abundances are closer to photospheric than coronal. An important result is that no deceleration is obse rved during the 4 minutes that the wave is visible in the radio images: the discrepancy between EIT wave and Hα Moreton wave speeds requires that EIT waves slow substantially as they propagate, if they are the same disturbance.