Modeling ULF waves in a compressed dipole magnetic field

This paper presents the results of a linear model for global scale magneto-hydrodynamic (MHD) waves in a compressed dipole model magnetosphere. We examine scenarios where a localized monochromatic source along the magnetopause boundary launches MHD fast mode ultralow frequency (ULF) waves into the magnetosphere, where they couple to shear Alfven waves. Sharply peaked field line resonance (FLR) structures are found to form at discrete locations within the magnetosphere in response to the fast mode driver. The extent in local time and relative amplitudes of FLR structures are found to depend strongly on the source location along the magnetopause boundary, indicating how the addition of day/night asymmetry affects the penetration of MHD fast waves within the magnetosphere. This also suggests that observed FLR structures within the magnetosphere may be used to deconvolve the spatial characteristics of the ULF wave source at the magnetopause, giving insight to the excitation mechanism responsible for observed ULF waves. As an example, we consider narrow band ULF activity observed on 25 November 2001 during a high solar wind speed interval following a geomagnetic storm and qualitatively reproduce the spatial and temporal characteristics of observations made by the Prince George SuperDARN radar by constraining the ULF wave source characteristics.