Magnetic field perturbations in the dayside cleft and their relationship to the IMF

Abstract

Magnetometers on the Isis 2 satellite have been used to study large‐scale perturbations in the geomagnetic field in the region of the dayside cleft (or cusp). Relatively large perturbations, in a plane perpendicular to the main field direction, are observed on most satellite passes; on the average the perturbations near noon are larger than those observed in the dawn and dusk sectors. The peak perturbations tend to lie in two relatively narrow ranges of direction, pointing approximately eastward or westward depending on the sign of By, the y component of the interplanetary magnetic field (IMF). In general, for this northern hemisphere data set, positive By corresponds to an eastward perturbation, and negative By to a westward perturbation. The invariant latitude of the perturbation is correlated with Bz, the z component of the IMF, while the magnitude of the perturbations is more strongly correlated with By than with Bz. The perturbations are discussed briefly in terms of the tilt of the field direction relative to a model field, and it is suggested that this is related to the direction of convection in the magnetosphere. On the basis of this interpretation the predominant convection direction in the cleft is approximately either eastward or westward.