Rare ground‐based observations of Siple VLF transmitter signals outside the plasmapause

Abstract

Signals from the Siple, Antarctica (L ∼4.3), VLF transmitter observed at the conjugate ground station Roberval, Canada, have previously been found to propagate either within the outer plasmasphere or within the region of steep plasmapause density gradients, but on paths with equatorial electron density that is within a factor of 2 of nearby plasmaspheric levels. We report here two cases in which propagation occurred just outside the plasmapause and at plasma trough density levels. In one case a series of 1‐s pulses and frequency ramps was observed; many of the pulses triggered risers with slopes of ∼10 kHz/s, much steeper than those usually observed within the plasmasphere. In the other case, no evidence of triggered emissions was seen on the Siple pulses, but instead efficient echoing occurred and noise band precursors to large wave bursts were initiated. The wave bursts were of a type that has been previously identified as driving transient bursts of electron precipitation into the ionosphere. Both cases occurred under magnetic conditions more disturbed than those typical of strong Siple signal propagation in the outer plasmasphere.