ALBERT

Description: 〈span〉〈div〉SUMMARY〈/div〉We present a new model of time-dependent flow at low latitudes in the Earth’s core between 2000 and 2018 derived from magnetic field measurements made on board the 〈span〉Swarm〈/span〉 and CHAMP satellites and at ground magnetic observatories. The model, called 〈span〉CoreFlo-LL.1〈/span〉, consists of a steady background flow without imposed symmetry plus a time-dependent flow that is dominated by geostrophic and quasi-geostrophic components but also allows weak departures from equatorial symmetry. Core flow mode amplitudes are determined at 4-month intervals by a robust least-squares fit to ground and satellite secular variation data. The 〈span〉l〈/span〉〈sub〉1〈/sub〉 norm of the square root of geostrophic and inertial mode enstrophies, and the 〈span〉l〈/span〉〈sub〉2〈/sub〉 norm of the flow acceleration, are minimized during the inversion procedure. We find that the equatorial region beneath the core–mantle boundary is a place of vigorous, localized, fluid motions; time-dependent flow focused at low latitudes close to the core surface is able to reproduce rapid field variations observed at non-polar latitudes at and above Earth’s surface. Magnetic field acceleration pulses are produced by alternating bursts of non-zonal azimuthal flow acceleration in this region. Such bursts are prominent in the longitudinal sectors from 80–130°E and 60–100°W throughout the period studied, but are also evident under the equatorial Pacific from 130°E to 150°W after 2012. We find a distinctive interannual alternation in the sign of the non-zonal azimuthal flow acceleration at some locations involving a rapid crossover between flow acceleration convergence and divergence. Such acceleration sign changes can occur within a year or less and, when the structures involved are of large spatial extent, they can give rise to geomagnetic jerks at the Earth’s surface. For example, in 2014, we find a change in the sign of the non-zonal azimuthal flow acceleration under the equatorial Pacific as a region of flow acceleration divergence near 130°E changes to a region of flow acceleration convergence. This occurs at a maximum in the amplitude of the time-varying azimuthal flow under the equatorial Pacific and corresponds to a geomagnetic jerk at the Earth’s surface.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉We present a new model of time-dependent flow at low latitudes in the Earth’s core between 2000 and 2018, derived from magnetic field measurements made on board the 〈span〉Swarm〈/span〉 and CHAMP satellites and at ground magnetic observatories. The model, called 〈span〉CoreFlo-LL.1〈/span〉, consists of a steady background flow without imposed symmetry plus a time-dependent flow that is dominated by geostrophic and quasi-geostrophic components but also allows weak departures from equatorial symmetry. Core flow mode amplitudes are determined at 4-month intervals by a robust least squares fit to ground and satellite secular variation data. The 〈span〉l〈/span〉〈sub〉1〈/sub〉 norm of the square root of geostrophic and inertial mode enstrophies, and the 〈span〉l〈/span〉〈sub〉2〈/sub〉 norm of the flow acceleration, are minimised during the inversion procedure. We find that the equatorial region beneath the core-mantle boundary is a place of vigorous, localised, fluid motions; time-dependent flow focused at low latitudes close to the core surface is able to reproduce rapid field variations observed at non-polar latitudes at and above Earth’s surface. Magnetic field acceleration pulses are produced by alternating bursts of non-zonal azimuthal flow acceleration in this region. Such bursts are prominent in the longitudinal sectors from 80-130°E and 60-100°W throughout the period studied, but are also evident under the equatorial Pacific from 130°E to 150°W after 2012. We find a distinctive interannual alternation in the sign of the non-zonal azimuthal flow acceleration at some locations, involving a rapid cross-over between flow acceleration convergence and divergence. Such acceleration sign changes can occur within a year or less, and when the structures involved are of large spatial scale they can give rise to geomagnetic jerks at the Earth’s surface. For example, in 2014, we find a change in the sign of the non-zonal azimuthal flow acceleration under the equatorial Pacific, as a region of flow acceleration divergence near 130°E changes to a region of flow acceleration convergence. This occurs at a maximum in the amplitude of the time-varying azimuthal flow under the equatorial Pacific and corresponds to a geomagnetic jerk at the Earth’s surface.〈/span〉