The CHAOS-7 geomagnetic field model and observed changes in the South Atlantic Anomaly

Christopher C Finlay,Alexander Grayver,Lars Tøffner-Clausen,Alexey Kuvshinov,Magnus D Hammer,Nils Olsen,Clemens Kloss

doi:10.1186/s40623-020-01252-9

Christopher C Finlay, Alexander Grayver + Show 5 more

Open Access

https://doi.org/10.1186/s40623-020-01252-9

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Abstract

We present the CHAOS-7 model of the time-dependent near-Earth geomagnetic field between 1999 and 2020 based on magnetic field observations collected by the low-Earth orbit satellites Swarm, CryoSat-2, CHAMP, SAC-C and Ørsted, and on annual differences of monthly means of ground observatory measurements. The CHAOS-7 model consists of a time-dependent internal field up to spherical harmonic degree 20, a static internal field which merges to the LCS-1 lithospheric field model above degree 25, a model of the magnetospheric field and its induced counterpart, estimates of Euler angles describing the alignment of satellite vector magnetometers, and magnetometer calibration parameters for CryoSat-2. Only data from dark regions satisfying strict geomagnetic quiet-time criteria (including conditions on IMF B_z and B_y at all latitudes) were used in the field estimation. Model parameters were estimated using an iteratively reweighted regularized least-squares procedure; regularization of the time-dependent internal field was relaxed at high spherical harmonic degree compared with previous versions of the CHAOS model. We use CHAOS-7 to investigate recent changes in the geomagnetic field, studying the evolution of the South Atlantic weak field anomaly and rapid field changes in the Pacific region since 2014. At Earth’s surface a secondary minimum of the South Atlantic Anomaly is now evident to the south west of Africa. Green’s functions relating the core–mantle boundary radial field to the surface intensity show this feature is connected with the movement and evolution of a reversed flux feature under South Africa. The continuing growth in size and weakening of the main anomaly is linked to the westward motion and gathering of reversed flux under South America. In the Pacific region at Earth’s surface between 2015 and 2018 a sign change has occurred in the second time derivative (acceleration) of the radial component of the field. This acceleration change took the form of a localized, east–west oriented, dipole. It was clearly recorded on ground, for example at the magnetic observatory at Honolulu, and was seen in Swarm observations over an extended region in the central and western Pacific. Downward continuing to the core–mantle boundary, we find this event originated in field acceleration changes at low latitudes beneath the central and western Pacific in 2017.

Highlights

The Earth’s magnetic field is a fundamental part of our planetary environment and an integral component of many modern navigational systems, providing a naturalFinlay et al Earth, Planets and Space (2020) 72:156 the Earth’s magnetic field for the wider scientific community including space physicists, high-energy particle physicists, exploration geologists, engineers and biologists
An obvious question that arises is whether this type of localized acceleration change at low latitudes has been seen before, or is it something unusual? It is well known that similar events have been seen at low latitudes under southern America and in the Atlantic sector in CHAllenging Minisatellite Payload (CHAMP) data that cover the 2000s (Chulliat and Maus 2014; Finlay et al 2015), but what about going further back? To explore this question, we present in Fig. 19 a selection of impressive 60 year long observatory records from mid-to-low latitudes spanning 1960–2020, showing annual differences of revised monthly means of the radial field component
We have presented the CHAOS-7 geomagnetic field model, the basis for DTU’s International Geomagnetic Reference Field (IGRF)-13 candidate field models, and used it to investigate geomagnetic field evolution, focusing on changes occurring over the past 6 years when excellent data coverage has been available from the Swarm trio of satellites

Summary

Introduction

The Earth’s magnetic field is a fundamental part of our planetary environment and an integral component of many modern navigational systems, providing a naturalFinlay et al Earth, Planets and Space (2020) 72:156 the Earth’s magnetic field for the wider scientific community including space physicists, high-energy particle physicists, exploration geologists, engineers and biologists. Histograms of the residuals for the newly reported ground observatory secular variation data and the newly collected Swarm scalar data are presented in Fig. 9 and misfit statistics (unweighted mean and rms residuals) are given, similar unweighted statistics for data used in the construction of CHAOS-7 are presented for reference.

Results

Conclusion