Modeling of Magnetic Local Time Asymmetry in Storm‐Time Low‐Latitude Geomagnetic Field Disturbances Due To Partial Ring Current

Abstract The intensity of storm‐time disturbance in the ground magnetic field varies significantly at different longitudes due to the magnetic local time (MLT) dependent contributions from different magnetospheric and ionospheric currents. Local geomagnetic field disturbances at low‐to‐mid latitudes often deviate considerably from the global depression represented by symmetric geomagnetic storm indices (such as Dst/SymH/SMR). In this study, we quantitatively investigated the geomagnetic horizontal field depressions (ΔH) at different local time sectors, compared to the longitudinally averaged SuperMAG Ring current (SMR), at eleven low‐latitude stations during a large number (665) of geomagnetic storms that occurred from 1996 to 2024. The relative disturbances (i.e., ΔH‐SMR) exhibit significant asymmetry with respect to MLT, which further varies with storm evolution, intensity, and phase. The MLT asymmetry of ΔH grows rapidly in the early main phase and then grows gradually with storm intensity. Further, the MLT sector of weakest/strongest ΔH depression shifts from post‐dawn/post‐dusk to pre‐dawn/pre‐dusk periods as storm intensity increases. Finally, an empirical model is derived that can quantitatively represent the MLT variations in the low‐latitude ΔH disturbances during geomagnetic storms. This model is very useful in estimating the low‐latitude geomagnetic field disturbances at different longitudes/MLT sectors from the global SMR index and can have significant applications in space weather studies.