Pseudobreakup and Substorm Growth-Phase in the Ionosphere and Magnetosphere

We present observations made in space and on the ground during the growth phase and the onset of a substorm on August 31, 1986. About 20 min after the epsilon parameter at the magnetopause had exceeded 10(11) W, magnetic field dipolarization with an increase of energetic particle fluxes was observed by the AMPTE Charge Composition Explorer (CCE) spacecraft at the geocentric distance of 8.7 R(E) close to magnetic midnight. The event exhibited local signatures of a substorm onset at AMPTE CCE and a weak wedgelike current system in the midnight sector ionosphere. However, it did not lead to a full-scale substorm expansion, as determined by several ground-based instruments, nor did it produce large particle injections at geostationary orbit. Only after another 20 min of continued growth phase the entire magnetosphere-ionosphere system could apparently allow the onset of a regular substorm expansion. The initial activation is interpreted in the present paper as a ''pseudobreakup.'' We examine the physical conditions in the near-Earth plasma sheet using spacecraft observations and analyze the development in the ionosphere using ground-based magnetometers and electric field observations from the STARE radar. We find that the main observable differences between pseudobreakups and ordinary breakups are the strength and consequences. Furthermore, it is shown that ionospheric activity at the time of a pseudobreakup is not necessarily as localized in longitude as generally believed.