M. Ozaki

Convective generation of “giant” undulations on the evening diffuse auroral boundary

Convective generation of “giant” undulations on the equatorward boundary of an evening diffuse aurora is numerically simulated. A giant undulation is defined as a waveform having the crest-to-trough amplitude comparable to the wavelength. The two-dimensional electrostatic particle code is used for studying the motion of magnetospheric plasma perpendicular to the geomagnetic field. According to the simulation results by Yamamoto et al. [1993], the giant undulation is a manifestation of the Kelvin-Helmholtz (K-H) waves arising from the polarization of “an arc sheet” (dense plasma population of ionospheric origin) in the magnetosphere, which is assumed to be located just equatorward of the region of proton diffuse aurora. In this previous simulation, initially, irregularities are given evenly over the entire azimuthal length of the arc sheet so that the resulting undulations are periodic. The present simulation deals with a different situation, that a K-H wave starts growing from local irregularities on the polarized arc sheet. The simulation results show that the disturbance propagates both westward and eastward (relative to the background flow), forming a series of K-H waves along the arc sheet. As a consequence, giant undulations with spatially varying amplitudes are developed on the equatorward boundary of a diffuse aurora, which is located just poleward of the arc sheet. A series of giant undulations convectively produced in the simulation is remarkably similar to some auroral images photographed from the Defense Meteorological Satellite Program (DMSP) satellites. In addition, we show the direct evidence for the presence of an arc sheet, probably associated with the giant undulations, which is provided by the measurements of precipitating particles from the DMSP F7 satellite crossing the equatorward boundary of the diffuse aurora; the high-density (∼10 cm−3) ions with energy of a few hundred electron volts are detected near the edge of the region of energetic (∼10 keV) ion precipitation. The primary cause of the formation of that arc sheet is thought to be escape of the oxygen and hydrogen ions from the topside ionosphere, due to the transverse acceleration by the ion-cyclotron waves.

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

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].

On the limitation of the current sheet approximation in estimation of the northward B z associated field-aligned currents

This paper calls attention to the limitation of the current sheet approximation, which has commonly been used in deducing the field-aligned currents (FACs) from the field-perpendicular components of magnetic field disturbances detected from low-altitude/midaltitude (below a few Earth radii) satellites. We focus our study on the current system involved in the northward interplanetary magnetic field associated FAC (so-called northward Bz (NBZ)). Assuming a numerical model for the total system of the region 1, region 2, and NBZ FACs, plus the associated ionospheric currents, the dusk-to-dawn and sunward components of the resulting magnetic disturbance δBx and δBy are calculated. In contrast to the commonly adopted hypothesis that the field-perpendicular magnetic disturbances as observed from the low-altitude (a few hundred kilometers) satellites are attributed primarily to the FACs, it is found that the ionospheric currents can make a significant contribution to those disturbances. Particularly, in the central part of the region of NBZ FACs, δBx from the ionospheric currents dominates δBx from the FACs. Next, for a simple discussion on the limit of the current sheet approximation we consider the magnetic disturbances at altitudes higher than several hundred kilometers where the contribution (to δBy) from the ionospheric currents can be neglected. It is found that in general, the FAC intensities based on the current sheet model are significantly different from those actually given in the FAC distribution. Specific points to be noted are as follows: (1) On the dusk-dawn lines passing the central part of the NBZ region the intensity of NBZ under the current sheet approximation is considerably smaller than the actual value, and the intensity ratio between region 1 and NBZ can be overestimated, by >50%, under that approximation. (2) On the dusk-dawn lines near the edges (but just inside) of the NBZ region, the current sheet model breaks down even qualitatively; the current sheet model cannot infer the right locations of region 1 and NBZ FACs. (3) On the dusk-dawn lines just outside of the NBZ region the so-called “W”-shaped profile in δBy, which can commonly be interpreted as a signature of the presence of NBZ FACs, emerges in the region where actually no NBZ FAC exists.