Sequence of SOHO/EIT images showing a shock wave running across the solar disk. The shock wave originated in the vicinity of a flaring solar active region; the flare began on 1997 April 7 at 13:59 UT, in conjunction with the flare, a large "halo" coronal mass ejection (CME) (see LASCO image http://sohowww.nascom.nasa.gov/gallery/LASCO/las015.gif and the special page on the 1997 April 7-9 solar-terrestrial event, http://umbra.nascom.nasa.gov/eit/cme/april7/index.html . EIT recorded these images in emission lines of Fe XII at 195 A; this ion is formed at temperature of about 1.5 million degrees. Since the shock is relatively difficult to see in the original images, displayed here are running difference images, i.e. each image shows the difference from the previous image. The speed at which the the shock wave runs across the solar disk was estimated at 1.5 million km/h - supersonic even for the Sun!