The Third Plasma Density Peak at Poleward of EIA Crest: Swarm and ICON Observations

Abstract

AbstractIn this study, we focus on the presence of the ionospheric third plasma density peak associated with the equatorial ionization anomaly (EIA) based on observations from the ESA's Swarm constellation. By statistically analyzing the third peaks observed by the Swarm A and B at two different altitudes, we found that such structure appears mainly around ±20° magnetic latitude (Mlat), namely the poleward of the EIA crests. In the meanwhile, the third peak shows prominent season and local time dependences, that are mainly observed in the summer hemisphere during solstice seasons and with the highest occurrence in the afternoon hours. However, no clear solar flux and magnetic activity dependences are found. By further analyzing the simultaneous neutral wind measurements from the ionospheric connection explorer satellites, we found that the summer‐to‐winter hemispheric meridional wind is responsible for causing the third peak as well as its seasonal dependence (higher occurrence in the summer hemisphere). In addition, the third‐peak structure shows prominent longitudinal dependence. In regions where the magnetic declination directs eastward, it has a larger occurrence in the southern hemisphere, while in regions where the magnetic declination reverses, it has a larger occurrence in the northern hemisphere. Such a longitudinal dependence suggests that the zonal wind, which has a magnetic field‐aligned component at favorable magnetic declination regions, contributes also to the formation of the third‐peak structure.