BETWEEN ELECTRIC FIELDS. “In the vicinity of this boundary, the plasma density has a sharp gradient; this is known as the plasmapause. Particles inside the trapping boundary remain on closed drift paths around the Earth while those outside the trapping boundary drift under the dawn-to-dusk electric field to the dayside boundary and are lost in the outer magnetosphere and eventually to the solar wind.”
https://link.springer.com/article/10.12942/lrsp-2007-1