How do core surface flow models vary when inverted from IGRF-14 candidate field models?

Area of Science:

• Geophysics and Earth Sciences
• Geomagnetism and Earth's Magnetic Field
• Core Dynamics and Geodynamo Theory

Background:

• The International Geomagnetic Reference Field (IGRF) models the Earth's large-scale magnetic field, crucial for navigation, space weather, and resource exploration.
• The 14th generation (IGRF-14) provides definitive models for 2020.0, predictions for 2025.0, and secular variation (SV) for 2025.0-2030.0.
• Secular variation (SV) is linked to the flow at the Earth's outer core, making IGRF candidate models valuable for studying core dynamics.

Purpose of the Study:

• To investigate predicted core flow variability using the ensemble of IGRF-14 candidate models.
• To analyze how differences in IGRF candidate models affect inferred core flow.
• To assess the impact of spherical harmonic truncation degree on SV predictions and core flow.

Main Methods:

• Utilized the pygeodyn Python package with the AR-1 'dense' methodology and a 71% geodynamo prior.
• Kept inversion parameters fixed to isolate the effect of IGRF candidate model variations on core flow.
• Compared SV candidate models truncated at spherical harmonic degree 13 versus degree 8.

Main Results:

• While all IGRF candidates produced similar flow spectra, greater deviations from the median model led to larger flow differences, primarily at small spatial scales.
• Flow speed differences remained below 25% of the mean model's maximum flow speed, with highest uncertainty in the Pacific and polar regions.
• Increasing SV truncation degree to 13 altered flow spectral energy and increased maximum flow speed by up to 31%, while maintaining high correlation in flow maps.

Conclusions:

• Deviations in IGRF candidate models significantly influence inferred core flow, highlighting the importance of model consensus.
• Core flow uncertainty is greatest in regions with weaker magnetic field constraints.
• The study supports increasing the spherical harmonic truncation degree to 13 for the SV component of future IGRF models to improve accuracy.