Satellite studies of magnetospheric substorms on August 15, 1968. 1. State of the magnetosphere

The sequence of events occurring throughout the magnetosphere during a substorm has not been precisely determined. This paper introduces a collection of papers that attempts to establish this sequence for two substorms on August 15, 1968. Data from a wide variety of sources are used, the major emphasis being changes in the magnetic field. In this paper we use ground magnetograms to determine the onset times of two substorms that occurred while the Ogo 5 satellite was inbound on the midnight meridian through the cusp region of the geomagnetic tail (the region of rapid change from taillike to dipolar field). We conclude that at least two worldwide substorm expansions were preceded by growth phases. Probable beginnings of these phases were at 0330 and 0640 UT. However, the onset of the former growth phase was partially obscured by the effects of a preceding expansion phase around 0220 and a possible localized event in the auroral zone near 0320 UT. The onsets of the corresponding expansion phases were 0430 and 0714 UT. Further support for these determinations is provided by data discussed in the subsequent notes.