The Harang Reversal and the Interchange Stability of the Magnetotail

Abstract:

During the substorm growth phase the overall convection, electric currents, and aurora structures in the nightside ionosphere often change very gradually over prolonged (> 1 hr) periods, and therefore the magnetosphere-ionosphere (M-I) system can be considered to be in a quasi-steady state. For the source region of a downward field-aligned current (FAC) the cross-tail current, which is carried mostly by ions, closes with a FAC, which is carried by electrons moving away from the ionosphere. Thus both ions and electrons accumulate there and accordingly, the plasma pressure increases. In the source region of an upward FAC, in contrast, the reduction of plasma pressure is expected. Since the plasma pressure, more precisely the entropy parameter (pVr), is most critical for the interchange stability of the magnetotail, this simple assessment raises a fundamental question about the magnetotail dynamics, that is, how the magnetotail remains to be steady. In this study we argue that if the magnetosphere is in a steady state, those expected increase and decrease in plasma pressure need to balance with the change due to the plasma transport by convection. This requirement, along with the condition for the interchange stability, leads to the conclusion that the associated pattern of convection has a structure that is presumably the magnetospheric counter part of the Harang reversal. More specifically, for the dusk convection cell, the convection flow is directed azimuthally westward in the source region of the downward R2 current, whereas it is directed sunward in the source region of the upward R1 current. We verify this idea by examining a quasi-steady magnetotail modeled by the RCM-Dungey code. Using equi-potential contours as a reference we also suggest that auroral arcs mapped to the equator tend to be oriented in the east-west and Sun-Earth direction if they are located in the premidnight R2 and R1 currents, respectively.