Seasonal dependence of high‐latitude electric fields

Abstract

The seasonal dependence of the high‐latitude electric field was investigated using Sondrestrom incoherent scatter radar data. Average E×B drifts were derived from 5 years of measurements centered around solar minimum. The electrostatic potentials that best fit the observed average electric field were calculated. It was found that the large‐scale convection pattern significantly changes with season. This change involves the overall shape of the convection pattern, as well as the electric field intensity, and thus the total dawn‐dusk potential across the polar cap. The cross polar cap potential drop is largest in fall, followed by winter, spring and summer. The small difference found between the summer and winter cross polar cap potential can be attributed to differing field‐aligned potential drops. In view of the well‐known relationship between field‐aligned currents and parallel potential drop, this is consistent with the observations that Birkeland currents are larger in summer than in winter. Changes in the overall shape of the convection pattern are consistent with the simple notion that the whole pattern is shifted toward the nightside as well as, to a lesser extent, toward the dawnside in summer as compared to winter. This assumption is based on the following observed effects: (1) The rotation of the overall convection pattern toward earlier local times with respect to the noon‐midnight direction is maximum for summer on the dayside. (2) On the nightside, the Harang discontinuity is typically located within the radar field of view (Λ = 67 to 82) in the winter averaged patterns, but it is equatorward of the field of view in summer. (3) The line that joins the dawn and dusk potential maxima is shifted toward the midnight sector in summer as compared to winter by about 5°. (4) In the dawn cell, the latitude of the convection reversal is the lowest during summer; in the dusk cell the latitude of the reversal is the lowest during winter. The shift in the antisunward direction is attributed to the dipole tilt angle variation, whereas the shift in the dawn‐dusk direction is attributed to the differing day‐night conductivity gradients.