Time-dependent low-latitude core flow and geomagnetic field acceleration pulses

Abstract

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 Swarm and CHAMP satellites and at ground magnetic observatories. The model, called CoreFlo-LL.1, 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 l 1 norm of the square root of geostrophic and inertial mode enstrophies, and the l 2 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.

Equatorially trapped waves in Earth’s core

“…The n = 0 wave predicts flow across the equator, whereas the n = 1 wave yields symmetric flow on either side of the equator. The latter wave (n = 1) appears to be more consistent with recent estimates of flow in the equatorial region (Kloss & Finlay 2019).…”

“…As an example, we consider the n = 1 wave in a 20 km layer; the wave period at m = 8 is 4.6 yr. Adopting a flow velocity of 5 km yr −1 gives an acceleration of 6.8 km yr −2 , which is comparable to estimates from Kloss & Finlay (2019). However, the estimates for horizontal divergence differ by roughly order of magnitude.…”

“…However, the estimates for horizontal divergence differ by roughly order of magnitude. The eMAC wave gives a horizontal divergence of 0.6 × 10 −3 yr −1 , whereas Kloss & Finlay (2019) recover a value of roughly 10 −2 yr −1 . The difference is partially related to the way that the velocity field is represented by Kloss & Finlay (2019).…”

“…When velocities for the wave and the background flow are similar, we might expect a superposition of an eastward propagating eMAC wave and a westward equatorial flow (e.g. Kloss & Finlay 2019) to produce a standing wave in observations of geomagnetic acceleration. However, the period of this oscillation would be much longer than the short-period fluctuations detected in the observations (Chulliat & Maus 2014).…”

“…Other disturbances are reported below the equatorial region of South America, notably in 2014. Some of these features propagate to the east , while others propagate to the west (Finlay & Jackson 2003;Kloss & Finlay 2019). In some instances the velocity of the disturbance exceeds the largest fluid velocities in the core.…”