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Abstract:
Theoretically, there is a multitude of waves in the liquid outer core of the Earth with periods ranging from days to hundreds of years. Their direct observation is not possible, and their determination relies on observations on and above the Earth's surface. Periodic changes in the magnetic field, the Earth's rotational parameters, and possibly the Earth's gravitational field can be related to the velocity and magnetic field variations of the waves in the core. By modeling magnetohydrodynamic waves, we can investigate their sensitivity to possible physical conditions. Waves with periods near the characteristic magnetic time scale (which is expected to be a few years for the Earth) are expected to have a strong dependence on a steady background magnetic field in the interior. One type of such waves, the torsional Alfvén waves, have been used to infer the cylindrical-radial component of the magnetic field deep in the core. However, not all observations can be explained by torsional Alfvén waves, and recent work suggests that Magneto-Coriolis waves, previously thought to have periods of decades to centuries, may also contribute to magnetic field changes at interannual periods. This is possible if the velocity of the wave is nearly invariant along the axis of rotation (quasi-geostrophic) and the longitudinal length scale is much larger than the cylindrical radial length scale. By modeling these waves with the dynamical parameters expected for Earth, inaccessible to non-linear simulations, we can attempt to relate changes in the geomagnetic field directly to fluid motions in the core.
