The average auroral zone electric field

Four hundred and seventy-eight hours of electric field data obtained by the flight of 32 balloons between L = 5.4 and L = 8.2 have been averaged to determine mean properties of the auroral zone electric field. When it is mapped into the equatorial plane, the average E × B flow has a sunward component at all local times and produces a convection pattern consistent with that expected for flow around a rotating obstacle. The average auroral zone electric field strength increases by a factor of about 2 as Kp increases from 0 to 6, but the larger-scale convection pattern is not greatly affected by the level of magnetic activity. The average electric field variation with Kp is small compared to that caused by local turbulent fluctuations, and so local field strengths can be large during quiet periods or small during magnetic activity. The average electric field magnitude is enhanced by a factor of about 2 when the interplanetary magnetic field has a southward component; this finding provides further evidence in favor of the generation of the auroral zone electric field by magnetic field reconnection. The y component of the interplanetary magnetic field causes the northern auroral zone electric field strength to be larger than average near local dawn, when By > 0, and larger than average near local dusk, when By < 0. These behaviors are understood in terms of a model in which the two-cell convection pattern is rotated about the sun-earth line by an amount that depends on the sign and magnitude of By. This model and the data suggest either that important parallel potential drops exist along auroral zone magnetic field lines or that simple conjugacy between the two hemispheres does not exist along such field lines.