A Statistical Study of Seasonal and Interhemispheric Asymmetries in Low-Latitude Pi2 Pulsations Across All Local Times

This study statistically investigates the seasonal and interhemispheric asymmetry of low-latitude Pi2 pulsations by analyzing 6026 events from 2013--2017 geomagnetic data at the conjugate stations Kakioka (KAK) and Alice Springs (ASP). The east-west (D) component amplitude is significantly enhanced near the solar terminator, with a strong dawn-dusk asymmetry; the enhancement is most pronounced at dawn (around 06:00--09:00 LT) during local summer, whereas at dusk it extends into the nightside. In contrast, while the nightside north-south (H) component amplitude lacks clear seasonality, the dayside H component is systematically larger in the summer hemisphere across both morning and afternoon sectors. This is opposite to the D component in the afternoon, which tends to be smaller in summer. On the nightside, the D-component amplitude is persistently larger in the summer hemisphere. Dayside features in both the D and H components are largely explained by a model combining the ionospheric current system with remote effects of nightside field-aligned currents (FACs). We suggest that the non-seasonality of the nightside H component is due to its dominance by compressional waves, as the low-conductivity nightside ionosphere is effectively transparent to this wave mode. These compressional waves are coupled with Alfvén waves that drive oscillating FACs, which in turn results in the observed seasonal variation of the D component and dayside H component.