It is generally accepted that the fast interplanetary manifestations of cor
onal mass ejections (ICMEs) are the major solar drivers of many space weath
er phenomena, including large, non-recurrent geomagnetic storms and solar e
nergetic particle events. High-quality synoptic observations of the solar c
orona, as carried out by the EIT and LASCO experiments on SOHO, provide nea
r real-time imaging of CMEs from the base of the corona out to a projected
distance of 30R(circle dot) (R-circle dot is the solar radius). The average
characteristics of CMEs observed by LASCO are similar to those from earlie
r coronagraphs. Most CMEs travel with approximately constant speed through
the LASCO field of view. However, a significant number accelerate as they m
ove outward, and some fast events have been observed to decelerate. LASCO h
as observed a great deal of internal structure in many CMEs. Concave-outwar
d structures that are interpreted as magnetic flux ropes are observed in ap
proximately one third of all events. Complex structures are often observed
in the trailing portions of CMEs. Halo CMEs, many of which are very faint,
can be routinely observed with LASCO. Extreme-ultraviolet (EW) images of th
e low corona and solar disk, as recorded by EIT, reveal a complex web of CM
E signatures. These include large-scale waves in the inner corona associate
d with CMEs, extended regions of depleted EW intensity, and bright arcades
that form following the occurrence of a CME. The combination of LASCO and E
IT imagery allows the source regions of halo CMEs to be identified, and can
be used to help predict the occurrence of geomagnetic disturbances several
days in advance. The most important parameters in determining the geoeffec
tiveness of an ICME are the magnetic field strength and direction, and the
speed of the disturbance. The most intense geomagnetic storms are associate
d with strong and persistent southward fields, either within the ICME itsel
f or in the compressed sheath of solar wind plasma ahead of it. (C) 2001 El
sevier Science Ltd. All rights reserved.