An Improved CHAOS-Based Correction Model for Storm-Time External Magnetic Fields

Summary Accurate separation of the Earth’s internal and external geomagnetic fields requires robust modeling, especially during the geomagnetic storms. Because the comprehensive CHAOS model is primarily designed to represent the Earth’s magnetic field under geomagnetically quiet periods, residuals of satellite observations relative to the CHAOS model during storms exhibit systematic dependencies on magnetic local time (MLT) and the disturbance levels, highlighting limitations in its representation of external field morphology. To address this limitation, we developed an empirical correction for the CHAOS model that specifically targets the magnetic signatures of the residual ring current at low and mid-latitudes. Using the high-quality scalar magnetic data from Swarm A/B and the Macau Science Satellite-1 (MSS-1) at nighttime hours, between November 2023 and September 2025, we modeled the systematic perturbations remaining after CHAOS background removal. Our separable parameterization explicitly resolves MLT structure while incorporating dependencies on geomagnetic disturbance levels. Event analyses demonstrate that our correction model can successfully capture the pronounced dawn-dusk MLT asymmetries of the external field during storms main and recovery phases. Statistically, the empirical model robustly captures residual ring current signals across all evaluated nighttime sectors, yielding the greatest improvements at dusk. Ultimately, the proposed correction effectively mitigates the activity-dependent biases in the CHAOS model’s external field, yielding a more reliable representation for the accurate interpretation of storm-time magnetic residuals.