A theory for generation of the paired region 1 and region 2 field‐aligned currents

Abstract

We present a new theoretical model for generation of a pair of region 1 and region 2 field‐aligned currents (FACs) under the condition of a southward interplanetary magnetic field. On the basis of the satellite observations it is assumed that the hot (≳1 keV) plasma particles are distributed in a magnetic shell connected to two ovals of diffuse auroras on the northern and southern polar ionospheres. The hot plasma population contained in this magnetic shell having several degrees of latitude in width is called the hot plasma torus (HPT). It is proposed that the region 1/region 2 FACs can be generated as a result of natural distortion of the HPT due to the solar wind convection. When the interplanetary magnetic field has a southward component, i.e., the IMF Bz is negative, the solar wind flow across open geomagnetic field lines gives rise to electric field convection patterns over the polar caps, which are modeled as twin vortex cells with antisunward flows in the center of the polar caps. The convection thus driven by the solar wind is referred to as the solar wind convection. If it were not for an E × B convection flow, the HPT would be shaped such that the HPT particles are contained in the “magnetic drift shells,” which are tangent to the averaged total magnetic drift velocity. In the presence of the solar wind convection, the configuration of the HPT will be deformed from the magnetic drift shells. Because of the distortion of the HPT, the pressure gradient in the HPT gains a component parallel to the magnetic drift. Therefore the HPT can be polarized because of oppositely directed magnetic drifts of the HPT electrons and protons: the high‐latitude and low‐latitude sides of the HPT on the eveningside are negative and positive, respectively, and the polarity is reversed on the morningside. The resulting pattern of large‐scale field‐aligned currents due to the polarization of the HPT is consistent with the observations of region 1 and region 2 FACs. Moreover, provided that the solar wind acts as a voltage generator in the interaction with the open field lines, as a long‐term characteristic of the paired region 1 and region 2 FACs we can obtain the relationship between the FAC intensity and the ionospheric conductivity: both the region 1 and region 2 intensities increase linearly with the Pedersen conductivity, while the proportionality constant for the region 2 FAC is smaller than that for the region 1 FAC. Our predicted relation for geomagnetic quiet conditions quantitatively agrees with the regression lines between the current intensities and the Pedersen conductivities obtained on the basis of Magsat satellite observations by Fujii and Iijima [1987].