The characteristics of field-aligned currents associated with equatorial plasma bubbles as observed by the CHAMP satellite

Abstract

Field-aligned currents (FACs) generate magnetic deflections perpendicular to the ambient Earth magnetic field. We investigate the characteristics of FACs associated with equatorial plasma bubbles (EPBs) as deduced from magnetic field measurements by the CHAMP satellite. Meridional magnetic deflections inside EPBs show a clear hemispheric anti-symmetry for events observed before 21:00 LT: inward in the Northern Hemisphere and outward in the Southern Hemisphere. When an eastward electric field is assumed the magnetic signature signifies a Poynting flux directed downward along the magnetic field lines. This means that FACs are driven by a high-altitude equatorial source. Such a scheme cannot be drawn as strictly from our observations after 22:00 LT, possibly because of a westward turning of the electric field inside EPBs and/or a decay of EPBs later at night. The perpendicular magnetic deflection is tilted by 40 degrees from the magnetic meridional plane in westward direction, which implies that the depleted volume of EPBs, as well as the FAC structure, is tilted westward by 40 degrees above the magnetic equator. The peak-to-peak amplitude of the FAC density is found to range typically between 0.1-0.5 mu A/m(2). The field-aligned sheet current density and the diamagnetic current strength show no correlation.