CHAOS-8 geomagnetic field model

Summary

The CHAOS-8 model describes the time-dependent near-Earth geomagnetic field between 1997 and 2025. It is based on magnetic field observations collected by the low-Earth orbit satellites Swarm, MSS-1, CSES, CryoSat-2, CHAMP, SAC-C, and Ørsted, and on annual differences of monthly means of ground observatory measurements.

The model consists of a time-dependent internal field up to spherical harmonic degree 20, a static internal field that merges to the LCS-1 lithospheric field model above degree 25, a model of the magnetospheric field and its induced counterpart, a climatological model of the ionospheric E-layer field, estimates of Euler angles describing the alignment of satellite vector magnetometers, and magnetometer calibration parameters for CryoSat-2. Only data satisfying strict geomagnetic quiet-time criteria (including conditions on IMF B_z and B_y at all latitudes) were used for the model estimation. Data from dark regions were used to estimate the internal and magnetospheric field models, while data from all sunlight conditions constrained the ionospheric field model.

Model parameters were estimated using an iteratively reweighted regularized least-squares procedure. The temporal regularization of the internal field has been improved compared with the previous version, CHAOS-7. Here, it is based on temporal covariances extracted from geodynamo simulations rather than on minimizing the square of the radial field acceleration integrated over the core surface, which tends to artificially suppress field variations at high degrees.

Acknowledgments

This work has been supported by the European Space Agency (ESA) in the framework of EO Science for Society through contract 4000127193/19/NL/IA (SWARM + 4D Deep Earth: Core) and as part of Swarm Data Innovation and Science Cluster (Swarm DISC) activities, funded by the ESA contract no. 4000109587/13/I-NB.

CHAOS-8.3 Update