On the force balance around dipolarization fronts within bursty bulk flows

Abstract

[1] We study the force balance surrounding the arrival of dipolarization fronts within bursty bulk flows near substorm onset by comparing curvature force densities and total pressure gradient force densities ahead of and behind the fronts using three inner Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes separated along the Xgsm and the Zgsm directions. Curvature force density estimates are obtained by field line modeling utilizing the Z separation of the probes and the self-similar structure of the front over short distances. A dipolarization front is a boundary between the energetic particle population in the flow burst magnetic flux bundle and the ambient colder plasma ahead of the front. Force density imbalance is found ahead of and behind the front. Ahead of the front, decrease in tailward pressure gradient force results in earthward flow acceleration. Behind the dipolarization front, even though the radius of field line curvature increases, the curvature force density increases even further, mostly due to the increase in the magnetic field magnitude. Thus, plasma acceleration at and immediately after the dipolarization front can be explained by the resultant increased curvature force density.