This figure illustrates how the density of the plasmasphere varies with geomagnetic activity. The plot at the top shows changes in the Dst index over a ten-day period. As indicated by the drops in the Dst, two geomagnetic storms occur during this period, a large one at time 2 and a smaller one at time 5. The effects of these storms on the plasmasphere are reflected in the lower plot, which shows plasmaspheric electron density versus position in L. Curve 1 gives the density profile of the pre-disturbance plasmasphere. With the onset of the geomagnetic storm at time 2, increased convection erodes the outer plasmasphere, creating a steep drop (known as the "plasmapause") in density at L = 3 (curve 2). With the quieting of the magnetosphere (times 3 and 4), density in the eroded region (the "plasma trough") increases (curves 3 and 4) as upflowing ionospheric plasma refills the plasmasphere. Recovery is interrupted by the occurrence of the second storm, however, and a new plasmapause forms at L ~ 4.2 (curve 5).

Source: Carpenter, D. L., and C. G. Park, On what ionospheric workers should know about the plasmapause-plasmasphere, Rev. Geophys. Space Phys., 11, 133, 1973.