Origin and effects of electric fields during isolated magnetospheric substorms

Balloon-measured electric-field data for 5-hour periods during each of 19 substorms near local midnight between L=6.6 and 8.3 have been analyzed and averaged to show that the ionospheric electric field is made up of approximately equal contributions from a large-scale field and small-scale turbulence. The large-scale field in a nonrotating frame of reference develops a westward component during the initial phase of an isolated substorm and an equatorward component about an hour later at the onset of the explosive phase of the substorm. The origin of the westward electric-field component is related to the connection of southward-turning interplanetary magnetic-field lines with terrestrial magnetic-field lines at the dayside magnetopause, and an example of this causal relationship is presented. The E×B drift of the nightside magnetosphere in response to this westward electric field results in the equatorward drift of auroral arcs, thinning of the plasma sheet, development of the tail-like magnetic-field geometry on auroral-zone magnetic-field lines, and many observed ground magnetometer effects. After about an hour, this drift also carries the nightside magnetosphere into an unstable configuration in which instabilities deep within the magnetosphere trigger the explosive phase of the substorm. It is postulated that the equatorward component of the electric field that develops at this time is due to polarization of the ionosphere by Hall currents driven by the westward electric field. The fact that this polarization is absent before the explosive phase requires that field-aligned sheet currents flow into the ionosphere near the low-latitude boundary of the auroral zone and out of the ionosphere near its poleward edge during the initial phase of the substorm. The interruption of this current and the development of the westward Cowling current in the auroral electrojet are manifestations of the current reorganization occurring at the onset of the explosive phase. The poleward surge of auroral arcs and the thickening of the plasma sheet after the substorm explosion are not due to simple E×B drifts of the magnetosphere but may be related to the outward propagation of the instability that triggers the explosive phase of the substorm.