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Models for the second time‐derivative of the geomagnetic field reveal prominent activity at high latitudes. Alternating patches of positive and negative geomagnetic acceleration propagate to the west at speeds that exceed nominal fluid velocities in the core. We show that waves are a viable interpretation of these observations. Magnetic Rossby waves produce a high‐latitude response with suitable phase velocities. However, the spatial complexity of the prediction is not compatible with the observations. Our preferred interpretation involves zonal MAC waves. These waves can account for the observed geomagnetic field when a stratified layer exists at the top of the core. The required layer has a thickness in excess of 100 km and a buoyancy frequency comparable to the rotation frequency. We anticipate a gradual reduction in the phase velocity over time, leading to a future change in the propagation direction.

 

Satellite observations reveal short pulses in the second time derivative of the geomagnetic field. We seek to interpret these signals using complex empirical orthogonal functions (CEOFs). This methodology decomposes the signal into traveling waves, permitting estimates for the period, angular wave number, and phase velocity. We recover CEOFs from the CHAOS‐6 model, focusing on three geographic regions with strong secular acceleration. Two regions are confined to the equator, while the third is located under Alaska. We find evidence for both eastward and westward traveling waves with periods between 7 and 20 years. There is also evidence for weaker standing waves with complex spatial patterns. Two of the three regions have waves that are compatible with predictions for waves in a stratified fluid. Our results yield estimates for the structure of fluid stratification at the top of the core.

 

Ekman layers develop at the boundaries of the Earth’s fluid core in response to precession. Instabilities in these layers lead to turbulence when a local Reynolds number, Re, based on the thickness of the Ekman layer, exceeds a critical value. The transition to turbulence is often assessed using experiments for steady Ekman layers, where the interior geostrophic flow is independent of time. Precessionally driven flow varies on diurnal timescales, so the transition to turbulence may occur at a different value of Re.We use 3-D numerical calculations in a local Cartesian geometry to assess the transition to turbulence in precessional flow. Calculations retain the horizontal component of the rotation vector and account for the influence of fluid stratification. The transition to turbulence in a neutrally stratified fluid occurs near Re = 500, which is higher than the value Re = 150 usually cited for steady Ekman layers. However, it is comparable to the nominal value for precessional flow in the Earth. Complications due to fluid stratification or a magnetic field can suppress the transition to turbulence, reducing the likelihood of turbulent Ekman layers in the Earth’s core. 

Recent satellite missions have detected short pulses of magnetic secular acceleration in the equatorial region of Earth’s core (Chulliat et al., 2010; Finlay et al., 2016). The new data provide an opportunity to detect dynamics in the Earth’s core on short timescales. To interpret these signals, we require a technique to separate distinct wave motions. The standard method, called Empirical Orthogonal Function (EOF), applies only to standing waves. An extension to deal with traveling waves (known as complex - EOF) relies on a Hilbert transform of the dataset before applying the EOF methodology (Horel, 1984; Susalito, 1994). This technique allows us to extract the period (T), the angular wave number (m) and the phase velocity (v), based solely on information in the CHAOS-6 model. We focused on two equatorial regions; one centered on Southeast Asia and the other on the Caribbean. The first two complex - EOFs in both regions account for over 90% of the signal. We find two eastward traveling waves in the Southeast Asia region (Tmode1=16.2 years, Tmode2=9.1 years, vmode1 = 3.5 ± 0.7 degrees/year, vmode2 = 7.1± 1.8 degrees/year and mmode1=mmode2=6). In the Caribbean region, the first mode represents a westward traveling wave (Tmode1 =6.7 years, vmode1 = -7.0 ±0.4 degrees/year and mmode1 = 6). The second mode appears to be a standing wave with a complicated spatial pattern. Extending our analysis beyond ±20º latitude causes a gradual loss of coherence, suggesting that the waves are confined to the equator, consistent with predictions for equatorially trapped MAC waves. In fact, both of the eastward waves in Southeast Asia are compatible with a thin layer of strongly stratified fluid in the outer 28 km of the Earth’s core. Confirmation of this result will require forward models to predict the magnetic secular acceleration expected from equatorially trapped MAC waves. As future work, we propose to use these forward models to reconstruct the CHAOS-6 model in the two equatorial windows.