Whistlers and VLF noises propagating just outside the plasmapause

Abstract

Ground‐based recordings of broad‐band whistler mode signals propagating along the outer ‘surface’ of the plasmasphere, just beyond the region of steep plasmapause density gradients, exhibit a number of features that have not been observed in other field line regions. The features include extension of signal frequencies into the range ≃0.5–0.8 fHeq, where fHeq is the equatorial electron gyrofrequency of the path, and echoing or repeated propagation over the path at frequencies above 0.5 fHeq. The effects are frequently exhibited by tne ‘knee trace,’ the whistler component propagating just beyond the plasma‐pause in the low‐density or plasma trough region.The unusual features include VLF noise bands and bursts that are frequently triggered or ‘activated’ by knee whistler traces or echoes of knee traces. The noise bands and bursts tend to occur within the frequency range 0.4–0.8 fHeq and near the frequency of an amplitude peak in knee whistler trace activity. The wave activity is documented in VLF data from Eights, Antarctica (L≃4), for 1963 and 1965 and from Siple, Antarctica (L≃4.2), for 1973 through 1975. Certain features of the whistler and noise activity and of equatorial electron densities deduced from whistlers support earlier findings that the plasmapause equatorial radius may on occasion be irregular, varying by up to 1 RE within the longitudinal viewing range (≃30°) of a whistler receiver. Propagation effects such as guidance along the outer plasmapause surface and coupling of wave energy into and out of the ionosphere are not yet well understood. The VLF noise effects are of a kind recently found to be associated with detectable bursts of electron precipitation into the nighttime lower ionosphere. Because of the special propagation and warm plasma effects associated with the plasmapause, that region appears to offer advantages for experiments on magnetospheric wave injection both from the ground and from satellites.