The geomagnetic indices used to quantify the magnitude of geomagnetic storms are highly correlated with solar wind speed and the strength of the southward component (Bs) of the IP magnetic field. These parameters are in general enhanced during the passage of IP CMEs. We next summarize the studies that demonstrate the new paradigm linking CMEs with IP and geomagnetic disturbances [e.g., Kahler, 1992; Gosling, 1993].
Geomagnetic storms are often preceded by abrupt increases in the
northward component of the earth's field, called sudden
commencements, which are well correlated with IP shocks. Since
these shocks are driven by fast CMEs, we infer that it is the CME
that causes the sudden commencement storms. Gosling et al.
[1991] demonstrated this statistically using bidirectional events
observed by the International Sun-Earth Explorer-3 (ISEE-3)
spacecraft as proxies for CMEs. They found that all but one of
the largest storms from 1978--1982 was associated with the
passage of bidirectional electron events and/or shocks. Such
storms are most geoeffective when both a fast CME and its shock
pass over the earth; in this case the disturbance is encountered
head-on where the flow parameters are maximized. However, Gosling
et al. note that the association between the bidirectional events
and storms is much reduced for the more frequent smaller storms.
In a related study Zhao et al. [1993] identified all ISEE-3
periods which had strong Bs fields 
10
T for durations
3 hr, and found that 78% of these periods were
associated with one or more CME proxies.
Compression and draping of magnetic field lines in the leading edge of the CME and of the ambient IP field are prime causes of strong southward fields [e.g., Tsurutani et al., 1992; Gosling, 1993]. Draping alone can enhance Bs over the ecliptic field components. If, in addition, the strong field in the leading edge of the CME is southward, a severe storm at the earth can ensue. Crooker et al. [1992] emphasize that the semiannual effect, caused by the periodic interaction between the earth's dipole and IP magnetic fields, will also enhance Bs fields due to compression and draping of the shock sheath fields in the ecliptic.
Good associations have been found between storms and other IP proxies of CMEs [cf., Webb, 1993]. Strong correlations were found between geomagnetic disturbances and large, sustained values of Bs in magnetic clouds at 1 AU. Compound streams, which are formed by faster streams overtaking slower ones, often involve magnetic clouds and can be associated with storms. Burlaga et al. [1987] found that more than half of the 17 largest storms were associated with a compound stream or magnetic cloud or both.
He(A) events, considered a good CME proxy, observed by the Interplanetary Monitoring Platform (IMP) spacecraft were well correlated with geomagnetic storms [ Borrini et al., 1982]. Since the main phase storm intensity was much greater for shocks with He(A) than for shocks without He(A), we again infer that it is CMEs that influence storm development. Finally, isolated filament disappearances are associated with sudden commencements and storms [ Joselyn and McIntosh, 1981; Wright, 1990]. Wright [1990] compared IP disturbances associated with disappearing filaments with geomagnetic parameters, and found that filament-related disturbances led to significant increases in sudden commencements, geomagnetic indices, and occurrences of large Bs, with delays from onset of 2.5--5 days.