Coincident POLAR/UVI and WIND observations of pseudobreakups

Abstract

Using POLAR/UVI global images, we have identified a period of successive minor auroral activations during which WIND was making a perigee pass (X ~ - 11 R(E)). These auroral brightenings are interpreted to be pseudobreakups due to the lack of global expansion. Large magnetic field fluctuations and high earthward ion velocity moments measured by the WIND spacecraft show a nearly one-to-one correspondence with the auroral intensifications. Analysis of the plasma parameters indicates that there is no difference in the behavior of the plasma during pseudo-breakups as compared to substorm expansive phase onset. Inspection of the ion distribution functions during high velocity moment events reveals the presence of a two component plasma. The particles contributing to the large mean velocities are energetic ions with energies from ~2 to 27 keV. We conclude that pseudobreakups are the ionospheric signature of high velocity moment events.