Electrostatic field and ion temperature drop in thin current sheets: A theory

The observational evidence presented by Liang et al. (2009) showed that a neutral sheet-pointing electrostatic field frequently arises in the late growth-phase current sheet in the magnetotail. In this paper, we elaborate on the suggestion that this electric field is associated with the thinning of the current sheet to the ion scale at which the electron and ion current sheets begin to separate. The attendant effect of a decreasing ion temperature, also interpreted in terms of a thinning current sheet, suggests that a cold plasma population is involved. We review existing theories of "charged" Harris sheet that can produce electrostatic fields and show that they cannot explain the observations for various reasons. A particular problem is the over shielding of the electrostatic field by the cold population embedding the current sheet. We argue that this problem stems from not treating the cold plasma as a separate population from the hot plasma forming the thin current sheet (TCS). We show that if the cold population is treated as external to the TCS and behaving in a largely MHD manner, the resultant solution yields an electrostatic field and ion temperature drop consistent with the observations.