The orientation of birkeland current sheets in the dayside polar region and its relationship to the IMF

Abstract

Vector magnetic field observations made with the three‐axes magnetometer on the Triad satellite have been used to study the orientation of magnetic disturbances in the dayside polar region. These measurements were all made over the southern polar region and recorded at McMurdo, Antarctica. These disturbances are transverse to the main geomagnetic field and may be interpreted as being caused by field‐aligned Birkeland current sheets consistent with Maxwell's equations. The current sheets in the regions usually associated with the morning and afternoon auroral regions are most often aligned in the geomagnetic east‐west direction. The amplitudes of these ‘south auroral’ currents are larger in the morning than in the afternoon when the interplanetary magnetic field (IMF) is directed toward the sun (By < 0, Bx >0) and larger in the afternoon when the IMF is directed away (By > 0, Bx <0). This is the systematically reversed relationship between Birkeland current intensities and IMF found in the northern auroral zone by McDiarmid et al. (1977). The Birkeland current sheets in the region associated with the southern cusp are also aligned in the geomagnetic east‐west direction during periods of negative Bz. During periods of By <0 and Bx > 0 the Birkeland current flow in the region of the southern cusp is predominantly away from the ionosphere in contrast to the downward flow into the northern cusp as determined earlier (e.g., McDiarmid et al., 1978b; Iijima et al., 1978). The cusp Birkeland current flow directions appear to reverse for By > 0 and Bx < 0. From a search of the Triad data set, some rare examples of magnetic disturbances with a large north‐south (noon‐midnight) component have been discovered in the polar cap near noon. These always occurred during periods of northward IMF. These disturbances have been interpreted as being caused by sun‐aligned Birkeland current sheets. The intensities of these currents in a few cases studied here are correlated with the positive amplitude of Bz and are comparable to the cusp and auroral currents studied earlier (0.1–0.8 A/m). The fluxes of electrons associated with sun‐aligned polar arcs measured with the Isis 2 satellite (Ismail et al., 1977) are sufficiently intense to produce upward flowing sun‐aligned Birkeland current sheets, but the origin of the downward flowing currents is not clear. These sun‐aligned current sheets may be associated with the rare occurrence of multiple‐cell convective flow patterns in the polar cap (Burke et al., 1979; Spiro et al., 1979).