Global characteristics of northward IMF‐associated (NBZ) field‐aligned currents

Abstract

Global characteristics of large‐scale field‐aligned current (FAC) systems have been determined by analyzing the magnetic disturbances acquired with the MAGSAT satellite in the southern summer hemisphere during November 1979 through January 1980 for the three cases of the IMF Bz > By > 0, IMF By > Bz > 0, and IMF By > 0 and Bz < 0. Transverse magnetic disturbances having a half wavelength larger than 113 km at the MAGSAT altitudes in the range of ∼350 to ∼550 km were subjected to the present analysis. The global distribution was limited by the MAGSAT orbits over the south pole to latitudes above about 83° ML (magnetic latitude) on the dayside and above about 71°–72° ML on the nightside. The large‐scale FAC systems include the usual region 1 and region 2 systems and the recently found NBZ system. The NBZ system denotes the FACs which flow into and away from the polar cap ionosphere (poleward of the region 1 system) during periods of the IMF Bz > 0. The principal results of the observations include the following: (1) Global characteristics of large‐scale FAC systems consist of two distinctive parts, i.e., the dayside part and nightside part. (2) During periods of the IMF Bz > 0 the dayside part is characterized by a large NBZ system which dominates the entire polar cap and is flanked by a definite region 1 system. These NBZ and region 1 systems on the dayside become progressively asymmetric with respect to the noon‐midnight meridian as the IMF θ angle (measured from the positive z axis in the y‐z plane) rotates from near 0° (Bz > By > 0) toward near 90° (By > Bz > 0): The region 1 and NBZ systems on the prenoonside severely shrink (or almost vanish), and the NBZ and region 1 systems on the postnoonside largely expand across the polar cap and are greatly augmented; the current densities of these NBZ and region 1 systems tend to diminish from the noon toward dawn and dusk. Even when the IMF θ changes to large (>90°) angles by the Bz < 0 and By > 0, this asymmetric NBZ‐type system persists in the dayside polar cap along with an asymmetric region 1 system. (3) During periods of the IMF Bz > 0 the nightside part is characterized by a distinctive NBZ system which develops along with well‐defined region 1 and region 2 systems. For both cases of the IMF Bz > By > 0 and IMF By > Bz > 0 these NBZ and region 1 systems on the nightside exhibit the dawn‐dusk hemispheric asymmetries: The nightside NBZ system is composed of multiple current belts which run from the midnight toward both the dawn and the dusk (parallel with and poleward of the adjoining region 1 system), and their current densities tend to diminish from the midnight toward dawn and dusk; The early morningside region 1 system is confined to a smaller area, and the late eveningside region 1 system expands over a larger area. When the IMF changes to larger (>90°) angles by the Bz < 0 and By > 0, the nightside NBZ system almost vanishes from the polar cap, and well‐defined region 1 and region 2 are enhanced, dominating over the entire hemisphere on both the morningside and the eveningside. The conclusions from this study are the continuous transition in the FAC pattern (NBZ + region 1 + region 2 systems) from IMF Bz > 0 to IMF Bz < 0 and the disconnection between the dayside and the nightside parts of these FAC systems.