Effect of Anomalous Cross-Field Diffusion on the Field-Aligned Current Generation

Abstract

First, on the basis of the satellite observations of the broadband electrostatic noise and the Alfvén wave noise in the auroral magnetosphere, within a frame of the quasi-linear theory we show that the proton (anomalous cross-field) diffusion coefficient averaged over the flux tube is likely to reach a significant fraction of the Bohm rate at least in the disturbed periods. Second, as an extension of recently proposed model that a pair of the region 1 and region 2 field-aligned currents (FACs) can be generated as a result of "natural distortion" of the hot plasma torus (HPT) in the magnetosphere under the influence of the usually prevailing convection with twin vortex cells (when the interplanetary magnetic field (IMF) is southward), we study the possible effect of the anomalous cross-field diffusion on the large-scale FACs resulting from the HPT polarization. In the magnetosphere, the hot (≥ 1 keV) plasma particles are primarily distributed in a magnetic shell which is connected to two ovals of diffuse auroras on the northern and southern polar ionospheres. Such hot plasma population is called the hot plasma torus (HPT). The numerical calculations specifically show that primary modification of the FAC pattern by this effect is the appearance of the triple FAC structure (i.e., upward FAC zone sandwiched between two downward FAC zones) in the midnight sector, which can be identified from the satellite observations at least during substorm periods. As a direct consequence of formation of such a triple FAC structure, the Harang discontinuity appears in the electric potential pattern. Thus the formation of the Harang discontinuity can be understood as a result of the HPT polarization by both effects of magnetic drift and anomalous cross-field diffusion.