Radial energy transport by magnetospheric ULF waves: Effects of magnetic curvature and plasma pressure
Abstract
The ``radial'' transport of energy by internal ULF waves, stimulated by dayside magnetospheric boundary oscillations, is analyzed in the framework of onefluid magnetohydrodynamics. (The term radial is used here to denote the direction orthogonal to geomagnetic flux surfaces.) The model for the inhomogeneous magnetospheric plasma and background magnetic field is axisymmetric and includes radial and parallel variations in the magnetic field, magnetic curvature, plama density, and low but finite plasma pressure. The radial mode structure of the coupled fast and intermediate MHD wave is determined by numerical solution of the inhomogeneous wave equation; the parallel mode structure is characterized by a WKB approximation. Ionospheric dissipation is modeled by allowing the parallel wave number to be complex. For boundary oscillations with frequencies in the range from 10 to 48 mHz, and using a dipole model for the background magnetic field, the combined effects of magnetic curvature and finite plasma pressure are shown to (1) enhance the amplitude of field line resonances by as much as a factor of 2 relative to values obtained in a cold plasma or boxmodel approximation for the dayside magnetosphere; (2) increase the energy flux delivered to a given resonance by a factor of 24 and (3) broaden the spectral width of the resonance by a factor of 23. The effects are attributed to the existence of an ``Alfvén buoyancy oscillation,'' which approaches the usual shear mode Alfvén wave at resonance, but unlike the shear Alfvén mode, it is dispersive at short perpendicular wavelengths. The form of dispersion is analogous to that of an internal atmospheric gravity wave, with the magnetic tension of the curved background field providing the restoring force and allowing radial propagation of the mode. For nominal dayside parameters, the propagation band of the Alfvén buoyancy wave occurs between the location of its (field line) resonance and that of the fast mode cutoff that exists at larger radial distances.
 Publication:

Journal of Geophysical Research
 Pub Date:
 May 1995
 DOI:
 10.1029/94JA02293
 Bibcode:
 1995JGR...100.7599K
 Keywords:

 Curvature;
 Earth Magnetosphere;
 Extremely Low Frequencies;
 Flux (Rate);
 Geomagnetism;
 Magnetic Field Configurations;
 Magnetohydrodynamic Flow;
 Magnetohydrodynamics;
 Plasma Pressure;
 Transport Theory;
 Buoyancy;
 Earth Ionosphere;
 Magnetohydrodynamic Waves;
 Plasma Density;
 Radial Flow;
 Wave Equations;
 WentzelKramerBrillouin Method;
 Magnetospheric Physics: MHD waves and instabilities;
 Magnetospheric Physics: Solar wind/magnetosphere interactions;
 Magnetospheric Physics: Magnetosphere/ionosphere interactions;
 Space Plasma Physics: Waves and instabilities