Closure of Ion-Pressure-Driven Magnetospheric Electric Currents Through the Ionosphere: Implications for Global Electric Fields

Energetic neutral atom (ENA) images of H+ and O+ ions 10-60 keV/nuc from the HENA camera on the IMAGE spacecraft reveal that the storm-time ring current is often asymmetric. This implies an azimuthal gradient in the ion pressure that will drive field-aligned currents into and out of the ionosphere. Kinetic modeling of the ring current indicates that the closure of this `'partial ring current'' through the ionosphere will in turn produce electric fields in the magnetosphere. Such electric fields are strong and are distorted from the classical picture of just a solar wind plus corotational electric field. This in turn implies that the transport of ring current ions may be dominated by localized regions of fast ExB drifts, as is required to explain the temporal and spatial dependence of the asymmetry observed in the HENA images. We are now able to calculate the global electric currents throughout the magnetosphere that are driven by the ion pressure gradients. Where those currents enter the ionosphere, we can calculate the intensity of the Region 2 field-aligned current (FAC) system. The ion pressure (P) is deduced from the H+ and O+ ion intensities extracted by inversion of the HENA images (assuming pitch-angle isotropy). Then the electric current intensity (J) may be calculated from the force-balance relation JxB=∇ P using the Euler potential representation of J introduced by Roelof [Adv. Space Res., 9(12), 195, 1989]. For storm main phases (e.g., 4 October 2000), the computed FAC into the ionosphere has been compared with the FAC measured directly by magnetometers on the Iridium low-altitude satellites. We have also analyzed periods during which the partial ring current builds up, and we find a consistent increase in the FAC pattern. We expect that further aspects of this kind of magnetosphere-ionosphere coupling may be revealed by comparison with EUV images from the IMAGE spacecraft of plasmasphere erosion in response to the electric fields that are generated by this process.