The study of bulk plasma motions and associated electric fields in the plasmasphere by means of whistler-mode signals

Abstract

Whistler-mode waves propagating to ground stations along geomagnetic-field-aligned paths provide powerful tools for investigating bulk motions of the magnetospheric plasma and thus the corresponding convection electric fields. Natural whistlers from lightning as well as signals from very low frequency (VLF) transmitters have been employed. The whistler method emphasizes measurement of temporal variations in the frequency versus time, or dispersion, properties of whistlers, while the transmitter method focuses upon measurement of the phase and group paths of fixed-frequency signal propagation. The methods depend upon wave properties that are sensitive to inhomogeneities in the geomagnetic field, and thus provide information on what are essentially cross- L plasma motions in a frame of reference rotating with the Earth. In addition, whistler data on the duskside plasmasphere bulge have been used to estimate values of the radial convection electric field (GSE Y direction) near dusk. In this topical review we discuss the development, beginning in the 1960s, of the whistler and transmitter methods, as well as a few of their geophysical applications. Whistlers have provided substantial new information on the spatially and temporally structured manner in which convection electric fields penetrate the plasmasphere, one example being the still unexplained reversal from inward to outward of the post-midnight radial flow direction following temporally isolated substorms. Whistlers have also been useful in identifying the plasmaspheric drifts associated with quiet-day electric fields of ionospheric dynamo origin and in showing that the E y (duskward), component of the convection electric field in the outer plasmasphere is substantially larger near dusk than it is near mid-night. Whistler-mode signals from transmitters have been found to be a powerful means of tracking cross- L motions in the plasmasphere near L=2.5 while separately identifying the effects of interchange fluxes with the ionosphere on the signal phase and group paths.