MAGNETIC ROSSBY WAVES IN A STABLY STRATIFIED LAYER NEAR-THE-SURFACE OF THE EARTHS OUTER CORE

The stratification profile of the Earth's magnetofluid outer core is u nknown, but there have been suggestions that its upper part may be sta bly stratified. Braginsky (1984) suggested that the magnetic analog of Rossby (planetary) waves in this stable layer (the 'H' layer) may be responsible for a portion of the short-period secular variation. In th is study, we adopt a thin shell model to examine the dynamics of the H layer. The stable stratification justifies the thin-layer approximati ons, which greatly simplify the analysis. The governing equations are then the Laplace's tidal equations modified by the Lorentz force terms , and the magnetic induction equation. We linearize the Lorentz force in the Laplace's tidal equations and the advection term in the magneti c induction equation, assuming a zeroth order dipole field as represen tative of the magnetic field near the insulating core-mantle boundary. An analytical beta-plane solution shows that a magnetic field can rel ease the equatorial trapping that non-magnetic Rossby waves exhibit. A numerical solution to the full spherical equations confirms that a su fficiently strong magnetic field can break the equatorial waveguide. B oth solutions are highly dissipative, which is a consequence of our ne cessary neglect of the induction term in comparison with the advection and diffusion terms in the magnetic induction equation in the thin-la yer limit. However, were one to relax the thin-layer approximations an d allow a radial dependence of the solutions, one would find magnetic Rossby waves less damped (through the inclusion of the induction term) . For the magnetic field strength appropriate for the H layer, the rea l parts of the eigenfrequencies do not change appreciably from their n on-magnetic values. We estimate a phase velocity of the lowest modes t hat is rather rapid compared with the core fluid speed typically presu med from the secular variation.