Compound streams, magnetic clouds, and major geomagnetic storms

Data from ISEE 3, Helios A, and Helios B were used to identify the components of two compound streams and to determine their configurations. In one case, ejecta containing a magnetic cloud associated with a disappearing quiescent filament were interacting with a corotating stream. In the second case, ejecta containing a magnetic cloud associated with a 2B flare were overtaking ejecta from a different source. Each of these compound streams produced an unusually large geomagnetic storm, on April 3, 1979, and on April 25, 1979, respectively. The largest geomagnetic storm in the period 1968-1986, which occurred on July 13, 1982, was associated with a compound stream. Thirty geomagnetic storms with A(p) greater than 90 occurred between 1972 and 1983, and there are interplanetary magnetic field and plasma data for 17 of these events. The data suggest that most large geomagnetic storms are associated with compound streams and/or magnetic clouds.