Sequence of SOHO-EIT first difference images showing the intensity (density) enhancement and following rarefaction associated with a shock wave expanding across the solar disk from the site of the origin of a coronal mass ejection (CME), 1997 May 12. A halo CME was observed by SOHO-LASCO. These images were formed from the differences of successive images in the emission lines of Fe XII near 195 A; this ion is formed at temperatures of about 1.5 million degrees. The wave front travels at speeds of ~ 300 km/s, typical of a fast mode Alfvén shock in the lower solar corona.

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