Impulse-excited hydromagnetic cavity and field-line resonances in the magnetosphere

Abstract We present detailed numerical results from a model which determines the time development of hydromagnetic waves within a hemi-cylindrical magnetospheric cavity subject to a short-duration compressional stimulus at the magnetopause. The model allows a realistic radial variation of Alfven speed, arbitrary axial asymmetry, and the inclusion of ionospheric Joule dissipation. The results show the development of a set of compressional cavity resonances, and their coupling to a corresponding set of field-line resonances at positions where the cavity eigenperiods match the uncoupled field-line eigenperiods. Transient solutions having variable period with radial distance are important in establishing the field-line resonances, and can be locally dominant in certain regions of the outer magnetosphere. The variation with axial wavenumber m of the cavity eigenperiods is obtained, and it is shown that the magnetopause impulse drives the field-line resonances most efficiently for m ∼ 3. This suggests that small azimuthal wavenumbers are important in the terrestrial magnetosphere under analogous conditions. A variety of waveform and polarisation diagrams is given to aid in observational identification of such coupled resonances.