Internally driven inertial waves in geodynamo simulations

13 Apr 2018

Inertial waves are oscillations in a rotating fluid, such as the Earth’s outer core, which result from the restoring action of the Coriolis force. In an earlier work, it was argued by Davidson that inertial waves launched near the equatorial regions could be important for the α2 dynamo mechanism, as they can maintain a helicity distribution which is negative (positive) in the north (south). Here we identify such internally-driven inertial waves, triggered by buoyant anomalies in the equatorial regions in a strongly-forced geo-dynamo simulation. Using the time-derivative of vertical velocity, ∂uz /∂t, as a diagnostic for travelling wave-fronts, we find that the horizontal movement in the buoyancy field near the equator is well-correlated with a corresponding movement of the fluid far from the equator. Moreover, the azimuthally-averaged spectrum of ∂uz /∂t lies in the inertial wave frequency range. We also test the dispersion properties of the waves by computing the spectral energy as a function of frequency, π, and the dispersion angle, θ. Our results suggest that the columnar flow in the rotation-dominated core, which is an important ingredient for the maintenance of a dipolar magnetic field, is maintained despite the chaotic evolution of the buoyancy field on a fast-time scale by internally-driven inertial waves.