Spontaneous substorm onset during a prolonged period of steady, southward interplanetary magnetic field

We document a clear case of spontaneous substorm onset under conditions of steady southward interplanetary magnetic field on January 22, 1993. The substorm occurred at 0415 UT with an intensification at 0455 UT during an interval of convection bay activity but had most of the features associated with substorms. These features include high-latitude and midlatitude magnetic bays at ground magnetometers, and magnetic field compression and energetic particle injection at geosynchronous altitude. In addition, a plasmoid release was observed in the distant tail plasma sheet by the Geotail spacecraft ata GSM position (-96, 1, -6) R(E). We conclude that spontaneous-onset substorms occur occasionally even during steady convection bay events. Such substorms and ''classical'' substorms progress in a very similar manner, Irrespective of possible differences in their onset mechanism.