Magnetospheric electric fields deduced from drifting whistler paths.

Results of a study in which the amplitude of the E-W component E sub w of the convection electric field in the nightside magnetosphere has been inferred from the observed cross-L motions of whistler ducts within the plasmasphere, and several ducts distributed over 1 to 2 earth radii in L space and over plus or minus deg 15 around the longitude of the Eights, Antarctica, whistler station have been tracked simultaneously. The method appears capable of resolving fluctuations in E sub w with period T equal to approximately 15 min and rms amplitude as low as 0.05 mV/m. For variations with T greater than 1 hour the method has a sensitivity of the order of 0.01 mV/m. Three case studies are presented, two of which illustrate convection activity associated with relatively isolated substorms. In these two cases E sub w reversed from westward to eastward for a period following the decay of substorm bay activity. In the third case the substorm bay activity was prolonged, and E sub w remained westward and at enhanced levels until local dawn. Evidence was found that, at least in a limited longitudinal sector, perturbing substorm E sub w fields can penetrate deep within the plasmasphere. In two of the case studies comparisons of E sub w and the interplanetary magnetic-field theta component show evidence of a possible relation based on brief (less than or equal to 1 hour) southward excursions but not on long preceding southward events.