Longitudinal variations of plasmapause radius and the propagation of VLF noise within small (ΔL ∼ 0.5) extensions of the plasmasphere

Simultaneous broadband whistler recordings made during the International Magnetospheric Study (IMS) at the two Antarctic stations Halley and Siple have been used to study longitudinal variations in the radius of the plasmapause observed during local afternoon. In both of the two periods studied thus far, whistler‐derived equatorial electron density profiles imply an increase in plasmapause radius between the longitudes of Siple and Halley (Δϕ ∼ 30°) of ΔL ∼ 0.5. Intense VLF noise (∼2.5 kHz) was observed at Halley but not at Siple, and by echo analysis its propagation path was identified with that of a whistler component travelling close to the plasmapause within the region of larger radius. This leads to the conclusion that the noise was generated by a gyroresonance instability when energetic electrons (typically 10 keV), drifting eastwards in the plasmatrough, encountered enhanced plasma density in the small extension of the plasmasphere.