ocr: A. shock wave running across the solar disk as observed by EIT on 7 April 1997. This sequence of run- ning difference images, which enhances changes in the solar atmosphere, is the "ground-track" of the large halo CME of 7 April 1997 shown before, which caused a signiticant amount of geomagnetic activily in northern regions all over the world on 10-71. April 1997.
