Solar energetic particle events and coronal mass ejections: New insights from SOHO

The scientific payload of the Solar and Heliospheric Observatory (SOHO), launched in December 1995, enables comprehensive studies of the Sun from its interior, to the outer corona and solar wind. In its halo orbit around the Lagrangian point of the Sun-Earth system the Comprehensive Suprathermal and Energetic Particle Analyzer (COSTEP) measures in-situ energetic particles in the energy range 44 keV/particle to >53 MeV/n. Although solar activity was at minimum, COSTEP has detected from mid December 1995 until end of July 1997, 30 solar energetic particle (SEP) events, including both gradual and impulsive type SEPs. These minimum phase SEP events are unique in the sense that their associated solar source phenomena can be investigated in detail without interference by other simultaneous solar events as is usually the case at times around solar activity maximum. Simultaneous observations of the solar corona are provided by the Large Angle Spectroscopic Coronagraph (LASCO) and the Extreme Ultraviolet Imaging Telescope (EIT). From the correlated SOHO observations we find a one to one correspondence of SEP events with coronal mass ejections (CMEs). Most of the SEP events were associated with west-limb CMEs, some with halo CMEs that later passed the SOHO spacecraft and with Moreton-like disturbances in the lower solar atmosphere as observed by EIT. A number of SEP events had associated solar source events at positions on the solar disk which were widely separated in heliolongitude from the nominal footpoints of the interplanetary magnetic field lines which connected SOHO to the Sun. The CMEs' speeds and sizes varied greatly in the individual SEP events including many slow (similar to 300 km/s) CMEs. Some of the particle/CME events had no association to type II radio bursts indicative for coronal shocks neither to X-ray flares. Many SEP events were detected at sector boundaries of the interplanetary magnetic field (IMF) suggesting a magnetic connection to coronal streamers at the Sun as supported by LASCO observations of mass ejections at the base of helmet streamers. Energetic particle and LASCO white-light observations yield evidence that CMEs often lead to large-scale disturbances of the Sun's corona, probably affecting at times areas all around the Sun.