From AMS Glossary
A portion of the F-region, centered on the magnetic dip equator, in which electron densities are anomalously low with peaks at 15°–20° latitude on both sides.
The equatorial trough appears during daytime and is absent at night. It is attributed to diffusion of ionospheric plasma down the magnetic field lines from the magnetic equator as the F-region rises in response to an eastward electric field generated by dynamo action in the lower ionosphere. One or more troughs in electron density are also found at high latitudes and are magnetically linked to the plasmapause located at a radial distance of several earth radii in the outer magnetosphere.