Initial results from a tracking receiver direction finder for whistler mode signals

A new apparatus combining a tracking receiver and a direction‐finding signal processor for observation of VLF whistler mode signals is described. The system is expected to achieve greater bearing accuracy than can be obtained from a conventional goniometer and is usable on a wider variety of signals than can be processed by most other techniques. Convenience in data output has been emphasized. In July–August 1975 the new tracker/direction finder (TR/DF) was successfully field tested at Roberval, Canada (L ≃ 4). Observations were made on natural whistlers and emissions and on signals from the Siple, Antarctica, VLF transmitter. As a test of accuracy on arrival bearing of whistler mode signals, the TR/DF output was compared with results from a goniometer operating from the same set of crossed loop and vertical antennas. The two systems agreed within ≃10° on several‐minute averages of bearings of both Siple transmitter pulses and natural signals. Effects of multipath propagation and polarization error have been identified. On the basis of the initial data sets it is found that whistler mode signal paths in the outer plasmasphere have ionospheric exit points that can frequently be resolved to within a region 30–40 km on a side. The apparent whistler mode signal path endpoints are relatively close to the ionospheric projections of the field lines along which the wave energy propagated, within 50–100 km (in the north‐south direction). Bearings of Siple transmitter signals show an apparent concentration to the southwest of the receiving station.