Field‐aligned currents in the south polar cusp and their relationship to the interplanetary magnetic field

Abstract

The large‐scale field‐aligned currents (FAC) as reported earlier from the Triad magnetometer data received at College, Alaska (lijima and Potemra, 1976a, b), are also observed in the south polar region. From the analysis of vector magnetometer data of more than 230 passes of Triad that were recorded at McMurdo, Antarctica, the principal characteristics of the cusp region FAC in the dayside south polar region that have been determined are as follows: (1) The cusp region FAC (defined as the currents appearing poleward of, and adjacent to, the region 1 FAC of Iijima and Potemra (1976a)), are most often observed in the midday sector of ∼0900–1500 MLT (magnetic local time) in the south polar region. These currents generally flow into the ionosphere in the postnoon sector (∼1200–1500 MLT) and flow away from the ionosphere in the prenoon sector (∼0900–1200 MLT). The demarcation line between the prenoon cusp region FAC and the postnoon cusp region FAC scatters statistically around the magnetic noon meridian. At a given MLT the flow of the cusp region FAC is the opposite of the flow of the region 1 FAC. (2) During weakly disturbed polar geomagnetic conditions (|;AL| < 100 nT) the cusp region FAC are statistically confined to the invariant latitude range of ∼75°‐80°. This area shifts equatorward and spreads in longitude (toward ∼0800 MLT and ∼1500 MLT) during enhanced polar geomagnetic conditions. (3) The average intensity of the cusp region FAC in the south polar region during the period from the end of March to August 1974 and during weakly disturbed polar geomagnetic conditions (|AL| < 100 nT) is ∼0.16 A/m. (4) In the south polar region the prenoon cusp region FAC (flowing away from the ionosphere) are observed about twice as often during periods when the interplanetary magnetic field is directed toward the sun (By < 0) as during periods when the interplanetary magnetic field is directed away from the sun (By > 0). Conversely, the postnoon cusp region FAC (flowing into the ionosphere) are observed about twice as often during periods when the interplanetary magnetic field is directed away from the sun (By > 0) as during periods when the By is negative. In contrast, the region 1 FAC show less of a systematic occurrence tendency which depends upon the polarity of the interplanetary magnetic By component. (5) There is no special preference of the polarity of the interplanetary magnetic Bz component associated with the occurrence of the cusp region FAC. We suggest that the region 1 FAC and the cusp region FAC have basically independent generators, probably in the magnetosphere, and that the cusp region FAC may be directly related to regions of the magnetosphere which mark the transitions of plasma and magnetic fields from the magnetosheath type to the internal‐to‐the‐magnetosphere type. The more intense cusp region FAC are associated with increases of Bz in the southward direction, as determined for the northern hemisphere (Iijima and Potemra, 1976b). But the larger number of events studied here show a wide spread of FAC intensities for a given Bz, and there is not a close correlation between the intensities and Bz. There is no close correlation between the FAC intensities and the By component.