Abstract
Observations of the change in Earth’s magnetic field—the secular variation—provide information about the motion of liquid metal within the core that is responsible for the magnetic field’s generation. High-resolution observations from the European Space Agency’s Swarm satellite mission show intense field change at high latitude, localized in a distinctive circular daisy-chain configuration centred on the north geographic pole. Here we show that this feature can be explained by a localized, non-axisymmetric, westward jet of 420 km width on the tangent cylinder, the cylinder of fluid within the core that is aligned with the rotation axis and tangent to the solid inner core. We find that the jet has increased in magnitude by a factor of three over the period 2000–2016 to about 40 km yr−1, and is now much stronger than typical large-scale flows inferred for the core. We suggest that the current accelerating phase may be part of a longer-term fluctuation of the jet causing both eastward and westward movement of magnetic features over historical periods, and may contribute to recent changes in torsional-wave activity and the rotation direction of the inner core. Satellite observations have detected localized magnetic field changes at high latitudes. Simulations suggest these changes can be explained by a westward jet in the liquid core, which has been accelerating over the past 15 years.
Highlights
Between 2004 and 2016 these SV patches, required by the observational data, have notably increased in magnitude
The latitudinal position and extent of the northern polar SV patches suggest an intimate link to dynamics close to the tangent cylinder, whose intersection with the CMB is at latitude ±69◦
We investigate whether the localised SV on the CMB close to the tangent cylinder can be accounted for by such a jet
Summary
Between 2004 and 2016 these SV patches, required by the observational data, have notably increased in magnitude. Where SVobs is the radial component of secular variation from the observational model, and SVsyn is the synthetic SV as determined from the interaction of the flow with the background field.
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