Magnetometer and radar observations of magnetohydrodynamic cavity modes in the Earth's magnetosphere

Abstract

Night and early morning data from the Johns Hopkins University, Applied Physics Laboratory HF radar at Goose Bay and from the magnetometers in the Canadian CANOPUS array often show structured spectra with distinct spectral peaks at 1.3, 1.9, and 2.6 mHz. These frequencies are very stable and vary by less than ±5% for the 6 days of data we have analysed. The radar measurements of the F-region drift velocities indicate that these spectral peaks are often associated with field line resonances of the shear Alfvén wave, and that the resonances are seen at lower latitudes as the frequency increases. The magnetometer data indicate that the magnetohydrodynamic (MHD) waves have westward phase velocities. We shall show that these observations are compatible with the formation of MHD cavity modes in the early morning and nightside magnetosphere, between the magnetopause, at approximately 14.5 RE, and turning points in the dipolar magnetosphere, outside the plasmasphere. Likely sources of energy are compressional pulses from the solar wind or Kelvin–Helmholtz instabilities in the low-latitude boundary layer.