Large‐scale Birkeland currents in the dayside polar region during strongly northward IMF: A new Birkeland current system

Abstract

Stable, well‐defined patterns of transverse magnetic disturbances have been observed in the polar regions that persist during periods of strongly positive Bz (≥ 5 nT) and that increase in amplitude as Bz increases. This has been determined from an examination of magnetic field data acquired during 146 orbits of MAGSAT over the south polar regions during November 1979 to January 1980 and supplemented by four consecutive orbits of TRIAD over the north polar region in July 1977. The characteristics of these polar disturbances include the following: (1) they occur at latitudes poleward of the Region 1 Birkeland current system at daytime magnetic local times (0600 MLT through noon to 1800 MLT); (2) the spatial distribution along the dawn‐dusk direction resembles the “W”‐shaped distribution of electric fields observed in the polar cap during periods of positive Bz (Burke et al. [1979]); (3) these patterns show remarkable stability, showing little change from orbit to orbit, up to seven orbits of MAGSAT (equivalent to a period of 10 hours); and (4) the magnitude of the peak disturbance, ΔB, correlates with a “complementary” magnetospheric transmission function of the form: ϵ* = (By² + Bz²)½ cos θ/2, where θ is the angle between the positive z axis and the interplanetary magnetic field (IMF). If the magnetic disturbances are interpreted in terms of Birkeland currents, they flow downward on the duskside and flow away on the morningside (identical to the cusp current flow reported by Iijima and Potemra [1976b]). During periods of negative By the region of morningside (upward flowing) currents is much larger than the eveningside (downward flowing) current region in the southern hemisphere. The density of the currents in the smaller spatial region is larger than the density of the currents in the larger region. This pattern systematically reverses during periods of positive By. We interpret these observations as evidence for a large‐scale, stable Birkeland current system in the polar region that is associated with merging on field lines in the geomagnetic tail. This current system intensifies and is more stable as Bz becomes more northward (reminiscent of the behavior of the Region 1 current system with increasing southward values of Bz). The new stable polar cap current system described here is referred to as the “NBZ” Birkeland current system for “northward Bz” and is important because of its relationship to a variety of other northward By phenomena such as polar cap auroral arcs (“theta aurora”) and multicell convective flow patterns. The existence of these stable NBZ currents and the correlation of their amplitudes with the IMF substantiate the fact that energy continues to flow to the earth's polar regions during periods of strongly northward IMF.