On whistler-mode trapping in the vicinity of the earth's magnetopause

AMPTE-UKS observations are used to show that the intensification of whistler-mode waves observed near the earthward boundary of the Earth's magnetopause can be related to wave trapping in this region. In particular, the local increase in intensity of whistler-mode waves at frequencies, f, below about half the local electron gyrofrequency, fH/2, during the magnetopause encounter on 9 October, 1984 was accompanied by a corresponding increase in the component of the whistler-mode refractive index in the direction parallel to the magnetic field, N1. A similar amplitude increase of waves at f≥fH/2 was observed on 2 October, 1984 and was accompanied by a decrease of N1. In both cases, consideration of geometrical optics predicts wave trapping in the region where the intense waves were actually observed. Similar relations between intensification of waves and maxima in N1 were observed during three other crossings, although this was not true for two more crossings studied