Introduction
Dimmings are regions of the solar surface where emission falls to a background level, that comparable to coronal holes. These regions are often associated with flaring regions, CME's and EIT waves.
We often see a flare event which will produce an emission drop in the concavities of sigmoid like structures (B.J.Thompson et al.), this region of decreased emission is what is known as a dimming region. After a short time the region will regain it's original composure and an average solar emission again. The structures show evidence of pre-existing magnetic structures, maybe as the foot-points of a flux rope. The dimmings are associated with a decrease in coronal density.
These structures have been observed since 1983, they were first observed by Skylab and named, “transient coronal holes” (Rust et al.), but our understanding of their cause and structure is weak, there have been associations with various phenomena including CME's, EIT waves and Flares. Few dimming catalogs have been produced and no comprehensive ones for the period 25th of March 1997 through to the 16th of June 1998.
We present a catalog of Extreme Ultraviolet Imagine Telescope (EIT) wave related dimmings observed over the period beginning 1997 March 24 (beginning of high-cadence observations) until 1997 December 30. We analyze ninety four EIT wave events and look for associated dimmings, in 195 Å EIT images. Dimmings are identified in plain (logarithmically enhanced) images and difference images, whereby previous images are subtracted to highlight enhancements and depletions in emission. The dimmings are recorded in a catalog composed of two sections; a visual section and a tabulated section. The table includes the time at which the dimmings are first observed in EIT observations, the source location and a dimming rating which quantifies the intensity and size of individual dimmings. The table also includes related EIT wave, Coronal Mass Ejection (CME) and flare data when available. Eighty four out of a possible ninety four EIT waves have associated dimmings. The dimmings are classified into two groups; those created by an appreciable drop in quiet sun coronal emission and those created by the displacement/loss of overlying bright loops revealing the background corona. None of the dimmings have emissions lower than that of local coronal hole emission, forty eight dimmings showed evidence of related CME activity and seventy showed associated flare activity. The largest recorded, line-of-sight, dimming was 0.03 Å, where Å is the line-of-sight area of the solar disk.