PLASMAPAUSE. “The location of the outer boundary of the plasmasphere, the plasmapause, is controlled by the relative intensities of the solar wind-imposed electric field and the co-rotation electric field. Although roughly circular in shape, the plasmapause often shows an elongation in the duskside, following the general electric field pattern.”
https://link.springer.com/article/10.12942/lrsp-2007-1