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-8.4 Update

• Extension of the CHAOS-8 model series.
• Uses Swarm L1b 1Hz magnetic field data (SW_MAGx_LR_1B), operational data up to 06-Sep-2025, product versions 0701. In the case of Swarm-A, operational data version 0606 was used after 18-Aug-2025 instead of 0701.
• Uses MSS-1A vector data, product versions V06XX of the operational datasets, up to 19-Aug-2025.
• Uses CryoSat2 vector data from FGM1, product version 0304, up to 31-Jul-2025.
• Uses CSES scalar data, version 0301, up to 31-Jan-2025.
• Uses ground observatory SV data up to 15-Jan-2025, derived from the BGS hourly mean database AUX_OBS version 0145.
• Starting with CHAOS-8.4, the interplanetary magnetic field components and the solar wind speed used for the ionospheric field model in CHAOS-8 are smoothed and interpolated before coupling functions and clock angles are computed. Previously, the coupling functions and clock angles were smoothed and interpolated, which led to incorrect clock angle values for strongly southward IMF conditions. This change has only a small effect on the modelled quiet-time ionospheric field because of the applied data selection, which permits only northward IMF conditions.