EVIDENCE FOR MULTIPLE EJECTA - APRIL 7-11, 1997, ISTP SUN-EARTH CONNECTION EVENT

Evidence is presented that the enhanced geomagnetic activity, on April 10-11, 1997, was caused by one of two ejecta that left the Sun at app roximate to 14 UT on April 7. This ejecta was not directly detected at the Earth. The evidence for this interpretation is based on WIND spac ecraft observations in the solar wind (SW). It is consistent with: (i) measured velocities of the coronal mass ejections from the SOHO coron agraph; (ii) the initial propagation speed of the shock generated in t his event, estimation from type II radio burst observations from the W AVES instrument on WIND, and (iii) the time profile of energetic ions observed by EPACT on WIND. This locally unobserved ejecta (moving at 6 00 to 700 kms(-1)) generated a fast shock which accelerated ions to se veral tens of MeV/amu. The inferred passage of the first ejecta close to Earth (on April 10 to 11) is based on the observation of an interpl anetary shock (IS) ahead of a field and plasma compressional region wh ere the draping of the SW flow and possibly the changes in the directi on of the IMF are consistent with a location northward of a faster eje cta. This ejecta was responsible for disturbed SW conditions including approximately ten hours of southward orientation of the interplanetar y magnetic field (IMF) and a ram pressure many times above normal. The slower moving ejecta was directed toward Earth and was observed with WIND from about 0550 until 1500 UT on April 11. It had a strong northw ard IMF and produced density enhancements which elevated the ram press ure to more than four times above normal.