Field-line resonances in a time-varying magnetosphere

Field line resonances, which represent shear Alfven standing modes of magnetohydrodynamic (MHD) waves are studied in space for the case in which magnetosphere is being perturbed in time by solarwind variations. A linearizedMHD wave model has been developed with time-varying Alfven speeds. When the magnetosphere becomes quiet, field line resonances consistently occur over many harmonics. A time-varying Alfven speed is found to cause a significant broadening of each spectral peak, which strongly depends on the frequency. While fundamental mode or lower harmonics become relatively stable with less broadening, higher harmonics appear with strong dispersion. Our results are consistent with the observed feature that fundamental or lower harmonic modes are dominant and higher harmonics are less frequently excited in space where the system is often perturbed in time.