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 measurements. The chief advantage of this technique over conventional ground magnetometer measurements is that it can provide data on a continental scale with a small number of receiving stations and with a minimum of data processing. At the edge of the amoral zone, where the electrojet current system flows, high-energy (E > 300 keV) precipitating electrons cause local electron density enhancements in the ionosphere which cause phase and amplitude perturbations in VLF waves propagating in the Earth-ionosphere waveguide. Continuous measurements of the amplitude and phase of signals from the Omega North Dakota VLF transmitter were made in Nome, Alaska. Using a two-dimensional model of VLF propagation 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 predicted. Seventeen nights of simultaneous VLF amplitude and phase data and ground magnetometer data were examined and catalogued based on the degree of temporal correlation between the two data sets and the degree to which the VLF events matched the propagation simulations. Of the nights exhibiting activity, more than 60% exhibited excellent correlation between the magnetometer and VLF events, and the majority of these showed good agreement with the model results. An additional estimate of the electrojet position was provided for one of the studied nights by field-aligned current measurements from the Freja satellite. A comparison of these independent means of determining the electrojet position shows that they are in good agreement for the night examined.