Tropical Ionization Trough in the Ionosphere Seen by Swarm-A Satellite

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\textdegree magnetic latitudes in the winter hemisphere during the solstices. The trough is pronounced around 330\textdegreeE longitude in the Southern Hemisphere (negative magnetic declination) during the June solstices and around 210\textdegreeE 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.