The relationship between j × B and ∇·P_e in the magnetotail plasma sheet: Cluster observations

Abstract

In this paper we report an extension to Henderson et al.'s (2006) case study, in which we investigate the generality of the anticorrelation between the ∇·?_e and j × B terms in Ohm's law observed in the plasma sheet. Further current sheet crossings are investigated which occurred on 17 August 2003 and 24 September 2003. Data from this period are particularly relevant as the interspacecraft separations are at their smallest for the Cluster mission. We confirm the generality of the observed anticorrelation and observe that the contributions to the electric field were mainly in the direction normal to the neutral sheet. A simple magnetohydrostatic treatment is used to explain the correlation and directional organization. The treatment is able to explain the anticorrelation and how the relative contributions to the electric field from the ∇·?_e and j × B terms in Ohm's law may be linked to the temperature ratio of the different plasma constituents as well as their spatial scales. In the examples reported here, the scale length over which the electron pressure changed l_e was smaller than the scale over which the ion pressure changed l_i, with l_e/l_i being between ∼0.1 and ∼0.4.