High latitude observations of whistlers using three spaced goniometer receivers
One-hop whistlers were recorded simultaneously at three high latitude stations in northern Norway during February 1978 using VLF goniometer receivers. Triangulation of the azimuthal bearings of whistlers received during an auroral substorm from 22.00 to 22.37 UT on 12 February located their whistler exit-points about 100 km to the north-east of Tromso, corresponding to an L-value for all the determinations of 6.4 ± 0.2. The frequency-time profiles of the same whistlers were analysed using the curve-fitting procedure of Tarcsai (J. atmos. terr. Phys.37, 1447, 1975) to determine their L-value of ducted propagation. These were found to lie in the L-value range 2.8–4.0, assuming a diffusive equilibrium distribution along the field lines. The L-values determined for the whistlers' exit-points were thus found to be considerably greater than that corresponding to the field line along which they were ducted. This discrepancy explained by the whistlers first following a field-aligned ducted path through the plasmasphere and then, after being reflected by sporadic-E ionisation in the lower ionosphere, following a sub-protonospheric path (Carpenteret al., J. geophys. Res.69, 5009, 1964) to higher latitude. It is shown by curve-fitting to whistler frequency—time profiles obtained by ray-tracing that such a path combination yields whistler spectra consistent with those observed.