MONOCHROMATIC ULF WAVE EXCITATION IN THE DIPOLE MAGNETOSPHERE

Hydromagnetic waves are studied in the dipole magnetosphere in the case when an almost monochromatic energy source is assumed to be located at the magnetopause. The wave spectrum is investigated through a fast Fourier transform and the wave structures are shown on the meridional plane. We have found that relatively large amplitude field line resonances (FLR) are produced only where the source frequency is matched to a harmonic of the Alfven shear modes. On the other hand, the compressional wave spectra become rather broad banded. We suggest that boundary effects may be responsible for the broadening of these spectra. It is also suggested that the narrow bandwidth of the source plays an important role in determining the nature of the FLR excitations. These results may help to explain why compressional and transverse ULF wave spectra are often found to be broad banded even in the presence of a monochromatic source.