Core Surface Flow Changes Associated With the 2017 Pacific Geomagnetic Jerk. Issue 15 (5th August 2022)

• Record Type: Journal Article
• Authors: Whaler, K. A.; Hammer, M. D.; Finlay, C. C.; Olsen, N.
• Is Part Of: Geophysical research letters. Volume 49:Issue 15(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-05
• Subjects: Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2022GL098616
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: A geomagnetic jerk was seen in Swarm satellite data in 2017 over the Pacific region. We invert time series of spatial gradient secular variation data between 2014 and 2020, reduced to a grid of points at satellite altitude, for spatially‐ and temporally‐regularized core surface flow. Pacific region flow acceleration was almost constant before and after the jerk, with a sharp change, especially in the azimuthal component, at the jerk epoch, despite the temporal regularization. Azimuthal acceleration is oppositely signed either side of 160°W, where it effectively vanishes, and also reverses sign at the jerk epoch. Acceleration features drift westward at about 900 km year −1 . Unlike previous studies, the evidence presented here for low latitude waves does not depend on imposing flow equatorial symmetry, quasi‐ or tangential geostrophy, or band‐pass filtering, and has no reliance on stochastic models or numerical simulations. Plain Language Summary: The European Space Agency Swarm mission has been measuring the geomagnetic field since 2013. The geomagnetic field has a number of sources, from dynamo action in the Earth's core to magnetospheric currents. We use data that have best estimates of all sources except the core removed, and they are reduced to a grid of points at satellite altitude every 4 months. The configuration of the trio of Swarm satellites enables spatial gradients of the field to be estimated, providing better resolution. We estimate the flow at the core surface from the time rate‐of‐change of these spatial gradients, assuming magnetic diffusion is negligible. Both vector and spatial gradient time rate‐of‐change data show rapid changes over the Pacific region in 2017, a phenomenon known as a geomagnetic jerk. Sharp flow changes, especially in the east‐west component, are associated with the geomagnetic jerk. Flow acceleration is essentially steady before and after the jerk, changes rapidly at the jerk epoch, and is larger beneath the Pacific region than elsewhere on the core surface. Flow speeds are typically 20 km year −1, but we also see suggestions of rapidly (∼900 km year −1 ) westward‐drifting acceleration features, adding to evidence for low latitude wave propagation in the core. Key Points: Inversion for core surface flow of Swarm ‐derived secular variation spatial gradients resolves ∼50% more coefficients than vector components Models include cross‐equatorial flow beneath Indonesia, a notable departure from tangential geostrophy and equatorial symmetry Although temporal flow variability is constrained, rapid acceleration changes are required to fit data in the Pacific region around 2017