Characterizing north-south ionospheric gradients during geomagnetic storms : a variogram enhancement for wide area ionospheric models

Abstract

The intensified solar maximum of Cycle 25 has heightened space weather risks to Global Navigation Satellite Systems (GNSS), where geomagnetic storms critically challenge Positioning, Navigation, and Timing (PNT) availability. Current wide-area ionospheric models may fail to resolve extreme latitudinal Total Electron Content (TEC) gradients generated by solar wind-magnetosphere coupling, particularly directional asymmetries emerging during geomagnetic storms in sunset and post-sunset sectors. This limitation forces conservative error estimates that degrade Satellite Based Augmentation System (SBAS) availability in mid-low latitude regions supporting critical infrastructure. We introduce an independent variogram solution on latitudinal TEC estimation (ISOLATE). ISOLATE advances beyond conventional planar fit and kriging models by decoupling meridional (north-south) and zonal (east-west) gradient components using physics-informed directional variograms. Key findings demonstrate meridional gradient amplification dominates spatial variability before the geomagnetic storms peak, and the gradient are captured through variogram sills and ranges. Zonal gradients primarily influence spatial correlations as residual nugget effects. Crucially, ISOLATE achieves effective meridional decoupling. By establishing quantitative links between observable gradients and geomagnetic storm phase modeling, ISOLATE could enable physically constrained corrections that enhance wide-area ionospheric model resilience.