Investigating the southern daytime midlatitude trough's relation with the daytime Weddell Sea Anomaly during equinoxes

Abstract

A detailed study with statistical investigations was conducted on the morning topside ionosphere for the 1996/1997 equinoctial period utilizing Defense Meteorological Satellite Program data and coupled thermosphere‐ionosphere plasmasphere wind simulations. Highly structured plasma densities, including a daytime Weddell Sea Anomaly (WSA; about 0900 LT) over the southeastern Pacific and localized high‐latitude plasma enhancements (about 0800 LT) in the Australian sector, were tracked. Poleward and equatorward winds, respectively, were strongest there. High‐resolution regional surface maps and field‐aligned profiles tracked their thermal structures, O+ and H+ composition, vertical drifts (VZ, m/s) and horizontal plasma flows {FY; [i+/(cm2s)]}. A daytime heavy‐ion (Ni‐O+) stagnation trough [(54.5 ± 4.5)°S; geomagnetic] appeared equatorward of both the WSA [60.0 ± 4.0)°S (center), 185–310°E; geographic] and these enhancements [>(62.0 ± 3.5)°S; 30–180°E]. Meanwhile, the plasmapause developed poleward of the WSA and equatorward of these enhancements. Plasma convection became significantly increased by enhanced electron temperatures nearby upflows underlying localized plasma enhancements. Upflow velocities, modeled as about 694.5 m/s, shows close similarities to the actual direct measurements of about 700 m/s. Both slow and enhanced plasma convections promoted stagnation events and trough formation. However, regular trough features became significantly altered by the development of a downward dipping poleward wall in the WSA region and by the absence of regular trough walls in the region of localized plasma enhancements. Statistically, the trough and localized plasma enhancement locations varied linearly with magnetic activity and showed an equatorward movement with increasing activity. A rate of (0.026 ± 0.002)°S/nT for the trough and (0.067 ± 0.003)°S/nT for the localized plasma enhancements were modeled. This higher (approximately two times) rate of movement indicates the localized plasma enhancements' more responsiveness.