RADIAL ENERGY-TRANSPORT BY MAGNETOSPHERIC ULF WAVES - EFFECTS OF MAGNETIC CURVATURE AND PLASMA PRESSURE

The ''radial'' transport of energy by internal ULF waves, stimulated b y dayside magnetospheric boundary oscillations, is analyzed in the fra mework of one-fluid magnetohydrodynamics. (The term radial is used her e to denote the direction orthogonal to geomagnetic flux surfaces.) Th e model for the inhomogeneous magnetospheric plasma and background mag netic field is axisymmetric and includes radial and parallel variation s in the magnetic field; magnetic curvature, plasma density, and low b ut finite plasma pressure. The radial mode structure of the coupled fa st and intermediate MHD waves is determined by numerical solution of t he inhomogeneous wave equation; the parallel mode structure is charact erized by a WKB approximation. Ionospheric dissipation is modeled by a llowing the parallel wave number to be complex. For boundary oscillati ons with frequencies in the range from 10 to 48 mHz, and using a dipol e model for the background magnetic field, the combined effects bf mag netic curvature and finite plasma pressure are shown to (1) enhance th e amplitude of field line resonances by as much as a factor of 2 relat ive to values obtained in a cold plasma or box-model approximation for the dayside magnetosphere; (2) increase the energy flux delivered to a given resonance by a factor of 2-4; and (3) broaden the spectral wid th of the resonance by a factor of 2-3. The effects are attributed to the existence of an ''Alfven buoyancy oscillation,'' which approaches the usual shear mode Alfven wave at resonance, but unlike the shear Al fven mode, it is dispersive at short perpendicular wavelengths. The fo rm of dispersion is analogous to that of an internal atmospheric gravi ty wave, with the magnetic tension of the curved background field prov iding the restoring force and allowing radial propagation of the mode. For nominal dayside parameters, the propagation band of the Alfven bu oyancy wave occurs between the location of its (field line) resonance and that of the fast mode cutoff that exists at larger radial distance s.