Two substorm intensifications compared: Onset, expansion, and global consequences

Abstract

Abstract. We present observations of two sequential substorm onsets on May 15, 1996. The first event occurred during persistently negative IMF B z, whereas the second expansion followed a northward turning of the interplanetary magnetic field (IMF). While the first onset remained localized, the second event led to a major reconfiguration of the magnetotail. The two very different events are contrasted, and it is suggested that the IMF direction controls the evolution of the expansion phase after the initial onset. Magnetic field modeling and field-aligned mappings are used to find the high-altitude source region of the auroral features and currents giving rise to ground magnetic disturbances: It is shown that the auroral brightening is related to processes near the inner edge of the plasma sheet but that the initial field-aligned currents couple to the midtail region. Ground magnetograms show an abrupt, large-scale weakening of the electrojet during the recovery phase. This event is followed by eastward drifting omega bands in a double-oval configuration. During that period, the Geotail plasma data show oscillations at <100 km/s amplitude. We argue that both these features are connected with the global tail evolution as the neutral line ceases to be active and reforms in the distant tail.

MHD simulation of the magnetotail during the December 10, 1996, substorm

“…For example, the growth of a plasma instability in the inner or midmagnetotail has been suggested to start the global reconfiguration, at a time when the current sheet has become sufficiently thin and intense during the growth phase [Pulkkinen et al, 1998a].…”

Pseudobreakup and substorm onset: Observations and MHD simulations compared

“…The IMF was steadily southward for about an hour, after which it turned northward. Ground magnetograms from the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) showed two activations of almost equal intensity (~300 nT) at 0623 UT and at 0706 UT [see Pulkkinen et al, 1998]. Whereas the first activation remained localized both latitudinally and longitudinally, the second event led to widespread electrojet activityø The quasi-AL index shown in the second panel of Figure 1 shows the two activations somewhat delayed; this is caused by the strong westward electrojet in the morning sector, which masked the initial onset in the midnight sector.…”

Strong induction effects during the substorm on 27 August 2001

“…The thick red line shows a smoothed dependence turned to the north and the termination of the Poynting flux transfer explains the observed sharp increase in U PC and I R1 . Pulkkinen et al (1998) merely suggested that an impaired decrease of the geomagnetic activity after the external source was switched off (IMF northward turning) could be explained by the continuing energy input due to inductive electric fields~∂B/∂t. In the substorm under study in this paper, the magnetic activity after the northward turning was still increasing (Fig.…”