Pressure and entropy changes in the flow-braking region during magnetic field dipolarization

Abstract

Changes in plasma moments and entropy during dipolarizations are studied using Time History of Events and Macroscale Interactions during Substorms observations made near the neutral sheet at 6-12 RE on the nightside. Plasma tube entropy (pV5/3) at the observation point is estimated for the data set of 147 dipolarizations using the formula of Wolf et al. (2006). Plasma pressure began to increase shortly before dipolarization and proceeded without considerable entropy change until the arrival of the dipolarization front, after which major entropy changes occurred. We have found that on average, postdipolarization plasma pressure changes very little from predipolarization plasma pressure at r = 10-12 RE and increases only slightly in the near-Earth region. Although the associated plasma tube entropy always decreases in the region downtail of r = 8 RE, this decrease becomes smaller closer to the Earth. The local entropy estimate pn -5/3 shows a large increase, however, suggesting ∼40% reduction in flux tube particle content after dipolarization. Our statistical results provide a constraint for dipolarization theories and support the bubble model of dipolarizations. Copyright 2010 by the American Geophysical Union.