Abstract
We investigate slow magnetic Rossby waves in convection-driven dynamos in rotating spherical shells. Quasi-geostrophic waves riding on a mean zonal flow may account for some of the geomagnetic westward drifts and have the potential to allow the toroidal field strength within the planetary fluid core to be estimated. We extend the work of Hori et al. (2015) to include a wider range of models, and perform a detailed analysis of the results. We find that a predicted dispersion relation matches well with the longitudinal drifts observed in our strong-field dynamos. We discuss the validity of our linear theory, since we also find that the nonlinear Lorentz terms influence the observed waveforms. These wave motions are excited by convective instability, which determines the preferred azimuthal wavenumbers. Studies of linear rotating magnetoconvection have suggested that slow magnetic Rossby modes emerge in the magnetostrophic regime, in which the Lorentz and Coriolis forces are in balance in the vorticity equation. We confirm this to be predominant balance for the slow waves we have detected in nonlinear dynamo systems. We also show that a completely different wave regime emerges if the magnetic field is not present. Finally we report the corresponding radial magnetic field variations observed at the surface of the shell in our simulations and discuss the detectability of these waves in the geomagnetic secular variation.
Highlights
Observations of waves can provide us with information on many aspects of geophysical and astrophysical flows
Revisiting a hypothesis of Hide (1966), Hori et al (2015) demonstrated in dynamo simulations that these longitudinal drifts could be produced by the propagation of slow magnetic Rossby (MR) waves riding on mean flow advection
The longitudinal drifts observed in the radial velocity match very well with the predicted wave speeds. (ii) Of particular interest are the dynamics of these waves: whether the identification could represent a predominant magnetostrophic balance, and to what extent assumptions required for the wave theory could be appropriate. (iii) In the light of the analysis of the internal dynamics, we examine whether these wave motions could be detected in data of the magnetic field that is inferred at the top of the core
Summary
Observations of waves can provide us with information on many aspects of geophysical and astrophysical flows. The excitation of torsional oscillations (TOs) has become evident and is a plausible candidate for 6 year variations that are observed in core flow models and length-of-day (LOD) fluctuations (Gillet et al, 2010) This finding is used to infer the radial profile of the poloidal magnetic field within the core and to suggest a z-mean rms strength of approximately 3 mT. Revisiting a hypothesis of Hide (1966), Hori et al (2015) (hereafter referred to as HJT15) demonstrated in dynamo simulations that these longitudinal drifts could be produced by the propagation of slow magnetic Rossby (MR) waves riding on mean flow advection The advantage of their approach is that it did not specify the configuration of the background magnetic field, but computed it from a dynamo model.
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