Large‐scale characteristics of field‐aligned currents associated with substorms
Authors: Iijima T. , Potemra T. A. 
Published: February 2008

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 1 on the morningside and away from region 1 on the eveningside. The current flow in region 2 is reversed to region 1 at any given local time except in the Harang discontinuity region (∼2000–2400 MLT), where the flow patterns are more complicated. (2) During active periods (|AL| ≥ 100 γ) the average latitude width of regions 1 and 2 increases by 20–30%, and the centers of these regions shift equatorward by 2°–3° with respect to the quiet time values. (3) The current density in region 1 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 (|AL| < 100 γ) the largest field‐aligned current densities occur in two areas of region 1 near noon (near ∼ 1030 MLT and ∼ 1300 MLT) with an average value of ∼1.6µA/m². During active periods (|AL| ≥ 100 γ) the regions of peak current density shift toward the nightside (the region near 1030 MLT shifts to ∼0730 MLT, and the region near ∼1300 MLT shifts to ∼1430 MLT), and the average current density increases to ∼2.2 µA/m². (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 conditions. The average total current during quiet periods is ∼2.7 × 106 A and during disturbed periods is ∼5.2 × 106 A. (6) A three‐region pattern of field‐aligned current flow persists in the Harang discontinuity region (∼2000–2400 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 field‐aligned currents is different in region 1 and region 2 during active periods. The different behavior of field‐aligned currents in region 1 and 2 during substorms actively suggests that they are controlled by different source regions in the magnetosphere or ionosphere. The region 1 field‐aligned currents map to the outermost part of the magnetosphere and magnetotail region, whereas the region 2 currents map to regions of the plasma sheet closer to the earth.