VLF REMOTE-SENSING OF THE AURORAL ELECTROJET

We investigate a new potential technique to determine the position of the auroral electrojet from ground-based VLF amplitude and phase measu rements. The chief advantage of this technique over conventional groun d magnetometer measurements is that it can provide data on a continent al scale with a small number of receiving stations and with a minimum of data processing. At the edge of the amoral zone, where the electroj et current system flows, high-energy (E > 300 keV) precipitating elect rons cause local electron density enhancements in the ionosphere which cause phase and amplitude perturbations in VLF waves propagating in t he Earth-ionosphere waveguide. Continuous measurements of the amplitud e and phase of signals from the Omega North Dakota VLF transmitter wer e made in Nome, Alaska. Using a two-dimensional model of VLF propagati on which accounts for ionospheric disturbances caused by the electron precipitation associated with the electrojet, the amplitude and phase signatures of electrojet incursion across each propagation path were p redicted. Seventeen nights of simultaneous VLF amplitude and phase dat a and ground magnetometer data were examined and catalogued based on t he degree of temporal correlation between the two data sets and the de gree to which the VLF events matched the propagation simulations. Of t he nights exhibiting activity, more than 60% exhibited excellent corre lation between the magnetometer and VLF events, and the majority of th ese showed good agreement with the model results. An additional estima te of the electrojet position was provided for one of the studied nigh ts by field-aligned current measurements from the Freja satellite. A c omparison of these independent means of determining the electrojet pos ition shows that they are in good agreement for the night examined.