During periods of low activity, when co-rotation dominates the near-Earth plasma flow, the plasmasphere becomes "saturated" with upflowing ionospheric plasma and extends to L = 6 or beyond, its density decreasing steadily with increasing distance from the Earth. When the magnetosphere is disturbed by a magnetic storm, enhanced convection "erodes" the outer plasmasphere, capturing plasma in the afternoon-dusk sector and transporting it outward and sunward toward the magnetopause. The plasma entrained in the outward convective flow forms a feature known as the "duskside bulge." Traditionally, the bulge was thought to be a part of the main plasmasphere and was attributed to the slowing of the plasmaspheric flow at local dusk, where the co- rotating flow is counteracted by the oppositely directed convection flow. However, recent studies suggest that the bulge and the main plasmasphere should be considered distinct regions. As the magnetosphere returns to a quiet state, the bulge plasmas continue to be influenced by convection, while co-rotation dominates the main body of the plasmasphere. (See figure for illustration showing how the plasmasphere evolves.)

What happens to the plasma that is swept away from the plasmasphere?

The fate of the material eroded from the plasmasphere is not fully known. Some of the eroded plasma convected to the magnetopause may escape into the solar wind, while some is thought to be transported into the magnetotail and eventually into the plasma sheet. Recent calculations indicate that the contribution of this material to the plasma sheet may be substantial. With the quieting of the magnetosphere and weakening of convection, as much as 10-30% of the eroded plasma appears to become trapped between the plasmasphere and the magnetopause and to remain for several days in the afternoon-dusk sector of the outer magnetosphere (beyond L = 6). Finally, some of the eroded plasmaspheric material, in the form of extended plasma streamers or tails attached to the main body of the plasmasphere, may begin to rotate with the Earth as convection weakens and may eventually wrap around the plasmasphere, contributing to its post-disturbance recovery.