Magnetospheric electric field measurements with balloons

Abstract

Two balloons have been flown from Fort Churchill, Canada, in August 1968 to measure horizontal and vertical electric fields. It is shown theoretically and experimentally that horizontal upper atmospheric electric fields are generally equal to one-second, several hundred kilometer averages of the horizontal ionospheric electric field. Analyses of thirty-five hours of data obtained during quiet and active periods yield the following conclusions. The average equatorial electric field strength was 0.6 mv/m during a quiet period and 1.0 mv/m during an active period. During magnetic bays, ground-based magnetometers principally measured Hall currents whose variations depended on conductivity changes as much or more than on electric field variations. For a period of about 30 minutes before the onset of a negative bay near local midnight, the ionospheric electric field pointed westward, indicating radially inward flow in the equatorial plane. This result is interpreted in terms of a model of the magnetospheric substorm in which bays are triggered deep within the magnetosphere by an instability associated with a radial plasma density gradient established by convective flow. The average electric field in the dayside ionosphere was about an order of magnitude smaller than that at night, suggesting that the dayside magnetosphere tended to corotate with the earth. Equipotential contours of the electric field in the equatorial plane are constructed based on this and other hypotheses.