Large‐scale characteristics of field‐aligned currents associated with substorms

Abstract

Characteristics of field-aligned currents have been determined during a large number of substorms from the magnetic field observations acquired with the Triad satellite. The statistical features of field-aligned currents include the following: (1) The large-scale regions of field-aligned currents determined previously by the authors (Iijima and Potemra, 1976a) persist during all phases of substorm activity, namely, region 1, located near the poleward boundary of the field-aligned current region, and region 2, located near the equatorward boundary. Field-aligned currents flow into region I on the morningside and away from region 1 on the eveningside. The current flow in region 2 is reversed to region I at any given local time except in the Harang discontinuity region (42000-2400 MLT), where the flow patterns are more complicated. (2) During active periods (IAL[ > 100 -•) the average latitude width of regions I and 2 increases by 20-30%, and the centers of these regions shift equatorward by 2o-3 ø with respect to the quiet time values. (3) The current density in region I is statistically larger than the current density in region 2 at all local times except during active periods and in the midnight to morning local time sector. In this region, where the westward electrojet is most active, the current density in region 2 can exceed the current density in region 1. (4) During relatively quiet conditions ([ALl < 100 '•) the largest field-aligned current densities occur in two areas of region I near noon (near 41030 MLT and 41300 MLT) with an average value of 41.6 t•A/m •. During active periods (IAL] > 100-•) the regions of peak current density shift toward the nightside (the region near 1030 MLT shifts to 40730 MLT, and the region near 41300 MLT shifts to •-, 1430 MLT), and the average current density increases to •2.2 t•A/m •'. (5) The average total amount of field-aligned current flowing into the ionosphere always equals the current flow away from the ionosphere during a wide range of quiet and disturbed conditiong. The average total current during quiet periods is 42.7 X 10 • A and during disturbed periods is 45.2 X 10 • A. (6) A three-region pattern of'field-aligned current flow persists in the Harang discontinuity region (42000-2400 MLT) during undisturbed and disturbed periods, when the westward auroral electrojet does not intrude into this sector. This flow pattern consists of an upward flowing field-aligned current surrounded to the north and south by downward flowing currents. During periods when the westward auroral electrojet has intruded deeply into the evening sector the Triad magnetometer data exhibit complicated and fine-structured variations indicating the presence of complex field-aligned currents in this sector. (7) The alignment of current sheets is generally along the boundary of the auroral oval (rather than in the east-west direction), but noticeable distortions of this alignment occur during very disturbed periods. The alignment of fieldaligned currents is different in region I and region 2 durin...

Empirical modeling of the quiet time nightside magnetosphere

“…It is a region characterized by an exponential decrease of electron energy density with decreasing radial distance and is generally located at geocentric distances of ~6 to 8 RE in the nightside. It is implicitly assumed by a number of researchers that the inner edge of the plasma sheet is associated with an earthward decrease of the total plasma pressure and theories on the closure of the large scale region 2 field-aligned current system [lijima and Potemra, 1978] have been built upon this assumption (see, e.g., the review by Mauk and Zanetti, 1987). As pointed out by Mauk and Zanetti [1987], the observed plasma pressure shows a persistent earthward increase rather than a decrease in the geocentric distances usually ascribed to be the inner edge of the plasma sheet.…”

Seasonal effects on auroral particle acceleration and precipitation

“…Interestingly, there are two auroral regions that show the least seasonal variations in the energy of precipitating electrons. These regions, one in the dusk sector but equatorward of the oval and one in the dawn sector but poleward of the oval, approximately map to the statistical laxge-scale downward field-aligned currents [Iijima and Potemra, 1978]. It is likely that auroras seen in these regions are associated with ion precipitation, which is less affected by field-aligned potential drops.…”

Role of the magnetospheric and ionospheric currents in the generation of the equatorial scintillations during geomagnetic storms

“…With the advance in our knowledge about magnetosphere, the electrodynamics relation between the solar wind, magnetosphere and ionosphere are realized through the field-aligned currents. As was shown by Iigima and Potemra (1978), the field-aligned currents are closely connected with the auroral electrojets and the disturbance polar (DP) systems. They also showed that the location and intensity of the FAC are defined by the B z and B y components of the IMF.…”

“…Numerous works present the experimental and theoretical relations found by their authors between the solar wind parameters and auroral ionosphere. The studies of Zmuda and Armstrong (1974), Iigima and Potemra (1978), Foster et al (1989) and other authors lead to the conclusion that the field-aligned currents (FAC) constitute a major interconnection between the magnetosphere and polar ionosphere. Intensive ionospheric currents during geomagnetic storms and disturbances can change the quiet ionosphere parameters.…”