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Sudden commencement events are examined in terms of available auroral-zone and low-latitude magnetic field, data, interplanetary plasma and magnetic field data, and magnetospheric electron flux and magnetic field data from the geostationary satellite ATS 1.

The problem of understanding the physical conditions preceding and causing the explosive or expansion phase of substorms has been studied for several years. In its most recent form this problem concerns questions of the existence of a growth phase prior to the explosive phase of a substorm and whether, if it exists, the growth phase current systems are fundamentally different from those during the expansion phase. The purpose of the present letter is to show that the resolution of different views on these questions may be achieved through examination of ionospheric electric field data obtained during isolated substorms.