Connections between plasma sheet transport, Region 2 currents, and entropy changes associated with convection, steady magnetospheric convection periods, and substorms

Here we describe how energy-dependent magnetic drift in the presence of a pressure gradient in the direction of the drift leads to a divergence of perpendicular particle flux, and that this violates conservation of flux tube particle content. We address how, within the plasma sheet, this divergence of particle flux should be expected to lead simultaneously to the divergence of perpendicular current that drives the Region 2 (R2) current system and to significant violation of entropy conservation. The modeling results of Wang et al. (2004b) show that the above violation of entropy conservation due to magnetic drift, when taken together with magnetic field stretching, offers a resolution to the pressure crises question. On the basis of our argument that the same energy-dependent magnetic drift effect leads to the perpendicular divergence that drives the R2 field-aligned current system and the violation of entropy conservation, we suggest that the existence of the R2 current system can by itself be viewed as a signature of violation of entropy conservation. Finally, we propose that observational evidence suggests that an enhanced rate of entropy reduction and R2 currents resulting from particle divergence within the vicinity of the Harang reversal may be a critical aspect of the substorm expansion phase.