Observations on the GEOS-1 satellite of whistler mode signals transmitted by the Omega navigation system transmitter in northern Norway

Whistler mode signals of 0.9-1.1 s duration at 10.2, 11.33, and 13.6 kHz launched by the Omega Navigation System transmitter located in northern Norway have been observed on GEOS 1. The signals were observed both inside and outside the plasmasphere in two regions of the magnetosphere, in December 1977 at 10-20° north of the equator on the morningside and in May 1978 10° south of the equator on the nightside. For a sequence of particularly strong pulses in the December period, the ‘instantaneous’ wave normal directions were calculated and found to have angles of 120-140° to the magnetic field of the earth pointing 0-40° eastward. These pulses have amplitudes of the order of 10 µV/m and refractive indices above 30. At times the amplitude envelope of the signals is unmodulated, but often the amplitude and the wave normal direction are strongly modulated on a time scale of 0.2 s. We attribute these features to ray path mixing. The signals are often prolonged up to 1 s, which is interpreted as an overlap of the primary pulse with the echo of the pulse reflected from the topside ionosphere in the southern hemisphere. While the signals do not trigger strong emissions, we do observe ∼ 0.1-s long bursts at the tail of some direct pulses.