THE PHYSICS OF THE HARANG DISCONTINUITY

Absent a source of energetic ions at the flanks of the tail, the westward gradient/curvature drift of E x B-convecting plasma results in the depletion of energetic ions from the dawnside of the plasma sheet. This dawnside depletion effect means that, on average, the duskside of the plasma sheet will have higher ion temperatures, pressures, and flux tube contents, and hence, stronger westward cross-tail drift current than the dawnside. The resulting cross-tail divergence of drift current must find closure by means of Birkeland currents connecting to the ionosphere. Tailward and poleward of the inner edge region, the divergence of cross-tail current requires upward current from the ionosphere. In the ionosphere, current closure requires electric fields that are directed toward the center of the upward current, i.e., directed equatorward on the poleward side, poleward on the equatorward side. This is precisely the nature of the Harang discontinuity. The region poleward of the Harang discontinuity maps well out into the plasma sheet and provided an eastward component of E x B drift to oppose the westward gradient/curvature drift of the ions. This helps keep the flow of plasma sheet ions directed toward the inner plasma sheet, rather than toward the dusk flank of the tail, a point originally made by Atkinson. The region equatorward of the Harang discontinuity maps close to the inner edge of the plasma sheet and results in westward E x B drift, increasing the westward flow of plasma azimuthally around the duskside of the inner magnetosphere and toward the dayside magnetopause. Although this scenario can be understood qualitatively, runs were carried out using the Rice convection model (RCM) to examine the ionospheric-magnetospheric coupling implications of this dawnside depletion effect. These runs confirm the above scenario, generally. They show that dawnside ion depletion results in a band of upward Birkeland current in the central auroral zone on the nightside, similar to what has been consistently observed by Iijima and Potemra and others. These currents modify the nightside, auroral electric field distribution to produce a strong reversal in the meridional electric field, similar to the classically observed Harang discontinuity. Inclusion of dawnside ion depletion also results in a major improvement in the agreement between the RCM and observations with regard to the latitudinal distribution of Birkeland currents. Finally, the dawnside depletion effect results in a reduction of plasma sheet pressures in the near-Earth, midnight sector of the plasma sheet. However, this reduction is significantly less than that suggested by Kivelson and Spence.