Previous statistical investigations have revealed a relationship between storm sudden commencements (ssc) and magnetospheric substorm onsets. Little is known about the physical processes constituting this relationship. We used a comprehensive data set for a detailed case study. The ssc occurred on July 6, 1979, at 1930 UT. The substorm expansion phase started 5 min later. The event was preceded by a loading phase of more than 1.5 hours. The loading phase developed in three steps. During each step the cross-tail current sheet suddenly expanded earthward and intensified. The third step, at 1930 UT, coincided with the ssc. It was very likely caused by the interaction of ions with magnetohydrodynamic waves generated by the ssc. This step was followed at 1935 UT by the onset of the expansion phase accompanied by PiB magnetic pulsations, impulsive electron precipitation, and energetic ion injection at GEOS 2 orbit. The dipolarization of the geomagnetic field started 9 min after the expansion phase onset together with energetic electron injection and a decrease in the energetic ion flux.

Signatures of Birkeland currents at dipolarization, and a pressure anisotropy P∥ions>P⊥ions during the 4 min before dipolarization, indicate field-aligned processes consistent with partial diversion of the cross-tail current into the ionosphere. The observations are discussed in the framework of a model in which the ballooning mode instability (BMI) developing in the near-Earth plasma sheet is regarded as the trigger process for the expansion phase. Quantitative estimates of the instability criteria show that the necessary conditions for the BMI are fulfilled during the whole loading phase. An energetic ion pressure gradient, which is needed to drive the BMI, was observed during the interval 1931 UT (ssc) to 1944 UT (dipolarization). Signatures of the instability itself, in terms of regular ion pressure gradient variations, were recorded during the same time interval.

We conclude that the magnetosphere was potentially unstable for the BMI when the ssc occurred and that the interaction of the ssc with the magnetospheric particles expanded the cross-tail current sheet further earthward, thereby creating plasma conditions in which the BMI could grow. The instability started at 1931 UT, and the BMI in turn triggered the expansion phase at 1935 UT. ¿ American Geophysical Union 1993