Comprehensive investigation of hemispheric and longitudinal asymmetries in midlatitude TEC

The midlatitude ionosphere exhibits persistent hemispheric and longitudinal asymmetries that vary with solar activity, geomagnetic configuration, and thermospheric dynamics. In this study, we systematically characterize these asymmetries by analyzing the base-level Total Electron Content (TEC) coefficient derived from a locally planar trend model. Using 2-hourly data from 2000 to 2024 across 48 non-overlapping midlatitude regions, we construct hourly-monthly median coefficient maps and evaluate regional TEC behavior under low, moderate, and high solar activity (SA) levels. A metric-based framework employing Root Mean Square Difference (RMSD), Dissimilarity Measure (DM), and Symmetric Kullback-Leibler Distance (SKLD) is used to quantify north–south and east–west asymmetries across time and space. Our results reveal consistent TEC enhancements in the Northern Hemisphere, dynamic longitudinal patterns modulated by geographic-geomagnetic equator deviation, and seasonal signatures including the Midlatitude Summer Nighttime Anomaly (MSNA). This study presents the first large-scale climatological asymmetry atlas derived from planar trend coefficients and offers a compact, model-independent reference for ionospheric variability under varying SA conditions.