Evidence for Rapid Low Latitude Waves at the Surface of the Core from Flow Models Derived from Geomagnetic Observatory and Satellite Observations

Geomagnetic secular variation (SV) data and models have long been used to infer the flow at the core surface, normally assuming only advective flow is responsible for generating the SV. Here we use recent satellite and ground magnetic observatory observations of SV to infer core surface flows over a ~25-year time interval, from 1999, just before the availability of Ørsted satellite data, to 2023, just after the latest date from which Swarm-based SV estimates can reliably be derived and the last data for which definitive geomagnetic observatory monthly means were available. Our flow models are constrained to have energy concentrated in large scales and minimise acceleration, but have no other assumptions. They are predominantly equatorially symmetric, toroidal and tangentially geostrophic, in agreement with previous studies. The temporal constraint means that the flow evolves rather slowly, though with an accelerating jet beneath the Bering Strait, and a change in flow direction beneath the equatorial Pacific in around 2010. The azimuthal component of the flow acceleration at low latitudes is consistent with wave-like structure. These waves propagate at around 1700 km/yr with periods of about 7 years. The energy is focused in wavenumbers -2 and -5, with some in wavenumber +2, where negative wavenumbers indicate westward propagation. Pulses of azimuthal acceleration change polarity at times when geomagnetic jerks occur.