资源环境科技发展态势分析平台

A new type of ionization trough, the tropical ionization trough, is identified in the tropical F region from the Swarm-A satellite observations in December 2013 to January 2017. The trough is formed around midnight near 25 degrees magnetic latitudes in the winter hemisphere during the solstices. The trough is pronounced around 330 degrees E longitude in the Southern Hemisphere (negative magnetic declination) during the June solstices and around 210 degrees E longitude in the Northern Hemisphere (positive magnetic declination) during the December solstices. Weak signatures of the trough also appear at those locations during the equinoxes. We interpret the formation of the trough in the winter hemisphere during the solstices by the convergence of meridional winds in the winter tropics. The longitudinal variation of the trough intensity is explained by the magnetic declination control of the zonal wind effect and the longitudinal variation of the meridional wind velocity.

Plain Language Summary The plasma density in the ionosphere is sensitive to the height of the ionosphere because the loss rate of ions by the reaction with molecular gases depends on the height. Our study reports a new ionospheric feature, the tropical ionization trough, which is closely related to the modulation of the ionospheric height. Ionization trough develops around midnight in the tropics, and its intensity varies with season and longitude. The formation of the trough at the winter tropics is explained by the lowering of the ionospheric height by summer-to-winter winds. The longitudinal variation of trough intensity shows the dependence on the declination of geomagnetic field lines; the field-aligned plasma flow driven by zonal winds varies with the declination and intensifies the trough in the region where the field-aligned flows driven by summer-to-winter and zonal winds are parallel. Our results demonstrate the significant role of neutral winds and Earth's magnetic field configuration on the ionospheric morphology and provide the global picture of the nighttime ionospheric morphology by connecting the phenomena in low and middle latitudes.