Poynting vector as a diagnostic of hydromagnetic wave structure

Abstract

The Poynting vector of resonant shear Alfvén waves which are generated by surface waves in a cold, inhomogeneous plasma is investigated in a simple model of the magnetosphere with ionospheres of finite conductivity. The results are applied to solar‐wind‐driven magnetospheric pulsations with periods between 100 and 600 s. In the equatorial plane the Poynting vector has only perpendicular components (i.e., azimuthal and radial components) if the conductivities of both ionospheres and the mass distributions of both sides of the equatorial plane are equal and if the energy source is symmetric around the equator. The magnitude of the radial component, which is pointing inward everywhere, depends on the ionospheric resistance. If the latter is considerably less than the wave resistance of the shear wave (as typical in the dayside magnetosphere), the radial component of the Poynting vector is smaller than the azimuthal component near the magnetopause and in the resonance region. The azimuthal component is parallel to the phase velocity vector of the surface wave between the magnetopause and the resonant field line and is antiparallel on the earthward side of the resonance region for typical magnetospheric mass density distributions and magnetic field gradients. The Poynting vector component parallel to the background magnetic field in the resonance region near the ionospheres is smaller than the perpendicular components in the equatorial plane for typical ionospheric conductivities, indicating that most of the energy is converted by the surface waves into the geomagnetic tail and into the nose of the magnetosphere. A realistic range for the energy flux is 1015 to 1016 erg/s and 10−4 to 10−3 erg/(s cm²) for the Poynting vector of typical solar‐wind‐driven surface waves on the magnetopause and the low‐latitude boundary layer of the dawn and dusk flanks. As the ratio between the wave electric and magnetic fields near geostationary orbit generally should increase with increasing frequency for fundamental modes, it should be possible to measure Poynting fluxes there for long‐period pulsations with periods between 400 and 600 s.