Magnetospheric electric field properties deduced from simultaneous balloon flights

Nearly 400 hours of ionospheric electric field data have been obtained by a series of four simultaneous balloon flights from six sites located between the plasmapause and the polar cap. Analyses of the long-term averages of these data indicate that magnetospheric electric fields and bulk plasma flow are turbulent with a scale size less than about 100 km in the ionosphere, the average equatorial electric field strength increases with increasing Kp and is independent of altitude to within about a factor of 2 from L=4 to 23, and its direction varies with a typical diurnal pattern. Short-term averages of these data show that this diurnal pattern is associated with magnetic bays, in that the east-west component of the ionospheric electric field in a rotating field of reference becomes westward about an hour before a bay and remains so throughout the night, while the meridional component switches from poleward to equatorward at the onset of the bay near local midnight. A previous model of the magnetospheric electric field variation during a bay is reinforced and extended by the present observations. In this model, bays are triggered deep within the magnetosphere in the equatorial plane by an instability associated with a radial plasma density gradient. This gradient is established by the convective flow stemming from a westward electric field that exists for about an hour before the onset of the bay. The equatorward component of the ionospheric electric field that develops at the onset of the bay drives Hall currents that are largely responsible for the ground-observed magnetic variations. The electric fields implied by this model are in agreement with balloon observations in the auroral zone on a total of 17 nights, during 10 of which negative bays occurred.