Origin of interplanetary southward magnetic fields responsible for major magnetic storms near solar maximum (1978–1979)

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Abstract
The origins of the interplanetary southward Bz which cause the 10 major (Dst < −100 nT) magnetic storms detected during the 500 days of study (August 16, 1978, to December 28, 1979) of the Gonzalez and Tsurutani (1987) work are examined in detail. A full complement of ISEE 3 plasma and field data, an 11‐station AE index and the near‐equatorial Dst index, are used in this analysis. It is found that the origins of the interplanetary southward Bz events are quite varied. If it is defined that the Bz event which leads to Dst < −100 nT is “the cause” of the storm, then one of the storm intensifications is caused by shock compression of preexisting southward interplanetary magnetic fields, four (or five) are related to driver gas magnetic fields, one (or two) are caused by shocked kinky heliospheric current sheets, two (or three) by turbulence or waves behind interplanetary shocks, and one possibly by draped fields associated with a noncompressive density enhancement event (without a shock or a high‐speed stream). In simplistic terms, four (or five) storms are caused by driver gas fields, four by shocked (sheath) fields, and one possibly by high‐intensity draped fields. In actuality, many of the interplanetary southward Bz and corresponding magnetic storm (Dst) structures are more complex than stated above. At least four of the interplanetary events have both major sheath and driver gas southward Bz events. In two storms, sheath southward Bz features led to Dst reaching levels of −90 nT prior to driver gas southward Bz features; the following driver gas fields then caused Dst to exceed our storm criteria of ≤ −100 nT. In two other cases, sheath Bz features led to magnetic storm onsets (Dst < −100 nT); the following driver gas southward Bz features cause further storm intensifications. The above magnetic storms therefore displayed two‐stage development characteristics. The results of this study indicate the equal importance of both sheath fields or draped fields and driver gas fields for the generation of major geomagnetic storms. Because of the importance of the sheath fields the intensity and duration of geomagnetic storms cannot be predicted by solar observations of active regions alone. Tang et al. (1988) will address this topic in detail.