Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Japan

We present ground-based, plasma sheet, and magnetosheath observations of two subsequent short-duration (10-20 min) increases of the postmidnight westward electrojet on November 28, 1995. Appearing as though small (150-200 nT) substorms, they were not accompanied by any substorm expansion onset signatures. Auroral breakup, worldwide Pi2 pulsations, and the corresponding plasma sheet activity, such as fast flows, current disruption, and plasmoid generation, were all observed only at the recovery of the second electrojet increase. These convection bays were associated with the equatorward expansion of the auroras and simultaneous magnetic variations in the polar cap and middle latitudes. Growth phase signatures of the lobe field increase and tailward stretching of magnetic field were also observed in the plasma sheet. Bursty bulk flows in the plasma sheet seem to be quenched at the onset of first convection bay and did not resume until the auroral breakup which concluded the second convection bay. A point of interest of this event was the "incomplete" convection/current system with a well-developed dawn vortex in the absence of well-defined dusk vortex; instead, a complicated transient activity dominated over the afternoon-dusk local time sector. We interpret this asymmetry either in terms of the magnetopause encounter with the edge of the solar wind driver, i.e., strong southward IMF, which hits only the dawn part of the magnetosphere, or with an extremely slant interplanetary discontinuity. This unique configuration was inferred from observations of uncorrelated strong southward B_z events by the Wind and IMP 8 spacecraft in the dusk and dawn magnetosheath, respectively, as well as from the directional analysis of the interplanetary discontinuities which form the edges of these structures. We suggest that interaction of the magnetosphere with very slant solar wind discontinuities may bring various specific features to magnetospheric and ionospheric dynamics that have not been reported.

J. Geophys. Res., 103, No. A10, 23593-23609, Oct. 1998