On the generation of magnetic dips ahead of advancing dipolarization fronts

Abstract

Dipolarizing flux bundles transport magnetic flux to the inner and dayside magnetosphere, heat the plasma sheet, and provide a seed population to the radiation belt. The magnetic perturbation ahead of them, often referred to as a dipolarization front (DF), is asymmetric with a small Bz dip followed by a sharp Bz enhancement. The Bz dip is thought to be generated from dawnward currents carried by DF-reflected ions; after reflection, these earthward moving ions gyrate clockwise and contribute to dawnward diamagnetic currents ahead of the front. Using observations of hundreds of DFs, we investigate this hypothesis. We find that the depth of the Bz dip as a function of the front azimuth depends on DF propagation speed and ambient plasma density. These statistical signatures support the hypothesis that the Bz dip is caused by ion reflection and suggest that secondary currents carried by these reflected ions can reshape the front significantly.