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# Geomagnetic jerks and rapid hydromagnetic waves focusing at Earth’s core surface

## Abstract

Geomagnetic jerks are abrupt changes in the second time derivative—the secular acceleration—of Earth’s magnetic field that punctuate ground observatory records. As their dynamical origin has not yet been established, they represent a major obstacle to the prediction of geomagnetic field behaviour for years to decades ahead. Recent jerks have been linked to short-lived, temporally alternating and equatorially localized pulses of secular acceleration observed in satellite data, associated with rapidly alternating flows at Earth’s core surface. Here we show that these signatures can be reproduced in numerical simulations of the geodynamo that realistically account for the interaction between slow core convection and rapid hydromagnetic waves. In these simulations, jerks are caused by the arrival of localized Alfvén wave packets radiated from sudden buoyancy releases inside the core. As they reach the core surface, the waves focus their energy towards the equatorial plane and along lines of strong magnetic flux, creating sharp interannual changes in core flow and producing geomagnetic jerks through the induced variations in magnetic field acceleration. The ability to numerically reproduce jerks offers a new way to probe the physical properties of Earth’s deep interior.

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## Data availability

The data that support the findings of this study are available from the corresponding author on request.

## Code availability

The numerical simulation code used to generate the results of this study is available from the corresponding author on request.

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## Acknowledgements

J.A. acknowledges support from the French Programme National de Planétologie of CNRS/INSU, and from the Fondation Simone et Cino Del Duca of Institut de France (2017 research grant). This work was granted access to the HPC resources of S-CAPAD, IPGP, France, and to the HPC resources of IDRIS, CINES and TGCC under allocations A0020402122 and A0040402122 from GENCI. The results presented in this work rely on data collected at magnetic observatories. The authors thank the national institutes that support them and INTERMAGNET for promoting high standards of magnetic observatory practice (www.intermagnet.org). This is IPGP contribution 4011.

## Author information

J.A. designed the project, designed and carried out the numerical experiments and wrote the manuscript. C.C.F. processed the geomagnetic data, constructed the CHAOS-6x5 geomagnetic field model and led its comparison with the simulation results. J.A. and C.C.F. processed the results and discussed the manuscript.

### Competing interests

The authors declare no competing interests.

Correspondence to Julien Aubert.

## Supplementary information

### Supplementary Information

Supplementary Figs, Tables and references

### Supplementary Video 1

Hammer projection of the core surface radial secular geomagnetic acceleration the CHAOS-6x5 geomagnetic field model filtered at spherical harmonic degree 9 from 1999 to 2018.

### Supplementary Video 2

Hammer projection of the core surface radial secular geomagnetic acceleration from the Midpath model, filtered at spherical harmonic degree 9, in the vicinity of the jerk event occurring at 0 yr.

### Supplementary Video 3

Hammer projection of the core surface azimuthal flow acceleration (blue is westwards) from the Midpath model, in the vicinity of the jerk event occurring at time 0 yr.

### Supplementary Video 4

Partial equatorial cut and meridional cut outside the tangent cylinder of the convective density anomaly (orange denotes lighter fluid) from the Midpath model in the vicinity of the jerk event occurring at time 0 yr.

### Supplementary Video 5

Partial equatorial cut and meridional cut outside the tangent cylinder of azimuthal flow acceleration (blue is westwards) from the Midpath model in the vicinity of the jerk event occurring at time 0 yr.

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