Evaluation of the region 1 field-aligned current from the low-latitude boundary layer using the 1989 Tsyganenko model

On the basis of the 1989 Tsyganenko magnetic field model, the field-aligned current (FAC) density distribution in the low-latitude boundary layer (LLBL) is numerically evaluated by assuming the entry of magnetosheath particles into the LLBL. The calculated FAC density/intensity profiles are consistent with observations. This FAC generation arises from the divergence of the magnetic drift current density carried by LLBL particles, and the current intensity is then increased with their kinetic energy density. The FAC generation occurs at the inner edge of the LLBL whenever sheath particles penetrate into the magnetosphere, regardless of the entry process. It is also emphasized that the LLBL FAC production is an inevitable consequence of the formation of the magnetopause, because the magnetopause currents act to shield the magnetic field originating from the currents inside the closed region of the magnetosphere, causing the LLBL inner edge to intersect the magnetic drift paths. This simple situation can be illustrated by calculating the distribution of the flux tube volume in the Tsyganenko model

Distortion of the nightside boundary of the "firmly-closed" region in the 1996 Tsyganenko magnetic field model

This paper proposes that the outer boundary of the "firmly-closed" region should be represented by field lines with the adiabaticity parameter, K, equal to unity at the equator, where K^2 is the ratio between the radius of the field-line curvature and the Larmor radius of an ion with 1 keV of energy. Just outside the boundary where K = 1, plasma particles (primarily ions) can be nonadiabatically accelerated in the presence of the dawn-to-dusk electric field. An inwardly convecting flux tube will attain the maximum content of nonadiabatically accelerated particles when it passes the K - 1 boundary. Thus, the K = 1 boundary outlines the region of the plasma population with a maximum content of nonadiabatically accelerated particles. In addition, the field lines with K = 1 are shown to have a minimum field strength of roughly 1 nT at the equator. From this fact, a field line with k < 1 may not be considered as being "firmly-closed" in the sense that such a field line may easily merge with an interplanetary field line. The outer boundary of the nightside firmly-closed region in the Tsyganenko model has an IMF Bz dependence that is consistent with observations. Moreover, this boundary is found to be "distorted", favoring the generation of region 1 field-aligned currents

Latitudinal structure of the nightside region 1 field-aligned current observed from the EXOS-D satellite

The average pattern of region 1 field-aligned current (FAC) of IIJIMA and POTEMRA (J. Geophys. Res., 81, 2165, 1976a; J. Geophys. Res., 83, 599, 1978) spans 2-3° in magnetic latitude. Using the magnetic field data acquired with the EXOS-D satellite, we notice that sharp variations of magnetic field perturbation, showing the high current densities of several micro-amperes per squared meter at the ionospheric height, are included in the region 1 current zone. In this paper, we focus our analysis on a sharp gradient of magnetic field change within 1° in latitude. In the 20-04 MLT sector, the thin current sheet as an important part of the region 1 FAC system is often found to be just equatorward of the boundary FAC system (FuKUNism et al, J. Geophys. Res., 98, 11235, 1993). A statistical survey of more than 200 satellite\u27s crossings of the region 1 zone (in the 17-05 MLT range) shows that about 60% of the identified region 1 current systems have the latitudinal structure of a thin (on average, 0.5°) current sheet with current intensity greater than one third of the total FAC intensity of the region 1 system

Distortion of the outer boundary of the closed region in the Tsyganenko magnetic field model

Using the Tsyganenko magnetic field model (TSYGANENKO, Planet. Space Sci., 37, 5, 1989) we make an attempt to determine the outer boundary of the closed region when the interplanetary magnetic field (IMF) is southward. As a simple magnetic field model including the effect of IMF B_z<0, the B_z component of a constant value of minus a few nanoTeslas is added to the magnetic field in the Tsyganenko model with low K_p values. In this paper, if the magnetic field strength, B, is not less than 2 nT in the whole range of a field line (namely the minimum B along a field line is greater than 2 nT), this field line is judged to be "firmly" closed. The firmly closed field lines are thought to be definitely closed as long as the fluctuation amplitude of B_z (around its average level) in the interplanetary (solar wind) magnetic field is less than 2 nT. The outer boundary of the firmly closed region is then constituted by field lines with the minimum B of 2 nT. This boundary is found to be close to (just inside of) the open-closed boundary, which can be determined with accuracy of 0.01° in latitude of the foot point of a field line. It is found that a circle with the center at a latitude of about 85° on the midnight meridian can be fitted to the outer boundary of the firmly closed region, as it is projected to the ionosphere. Interestingly this circle coincides with a typical auroral circle; the auroral circles are those delineating the poleward boundary of the quiet auroral belt, which were earlier identified from the statistical analysis of satellites\u27 auroral images by MENG et al. (J. Geophys. Res., 82, 164, 1977). Importantly we find that the outer boundary of the firmly closed region is "distorted" on the nightside in the sense that the ionospheric projection of the average magnetic drift velocity of a plasma with isotropic pressure is not parallel to the boundary; more specifically, that of an isotropic ion fluid has an equatorward component on the duskside boundary and a poleward one on the dawnside boundary, respectively. This kind of the boundary distortion may be one of the possible causes of the generation of the nightside region 1 field-aligned current, which has been first suggested by HRUSKA (J. Geophys. Res., 91, 371, 1986) and recently, further studied by YAMAMOTO and INOUE (Proc. NIPR Symp. Upper Atmos. Phys., 11, 106, 1998)

Quasi-steady production of region 1 and region 2 field-aligned currents

Quasi-steady production of the hot plasma torus (HPT) in the magnetosphere and simultaneous generation of the region 1 and region 2 field-aligned currents (FACs) are numerically simulated for the case of southward interplanetary magnetic field. The magnetosphere-ionosphere coupling is treated in the two-dimensional electrostatic simulation model, and particularly incorporated are effects of the following processes : 1) nonadiabatic acceleration of ions incident on the tail plasma sheet (including particle acceleration at the magnetic reconnection), 2) plasma escape, along the open field lines, to the interplanetary space, and 3) anomalous cross-field diffusion of plasma particles. Under the condition that the open-closed boundary is distorted by the two-cell convection, the region 1 FAC (as well as the region 2 current) is steadily generated by a pressure-gradient mechanism. (Convection-distortion here means that the center of the circle fitted to the open-closed boundary (projected onto the ionospheric plane) is shifted antisunward so that this center is positioned at a latitude (on the midnight meridian) lower than that of the (averaged) ionospheric projection of the path of a magnetically drifting particle in the magnetosphere.) The simulations show that the HPT can be maintained as long as continual particle injection and energization take place, which implies that the primary energy source for the HPT and FACs is the particles nonadiabatically accelerated near the nightside injection boundary