The Earth's plasmasphere is a torus of cold (~1 eV), dense (tens to thousands of particles per cubic centimeter) plasma that occupies roughly the same region of the inner magnetosphere as the ring current and radiation belts (between L = 2 and L = 7) and that is populated by the outflow of ionospheric plasma along mid- and low-latitude magnetic field lines (i. e., those that map to magnetic latitudes of ~60 degrees and less). H+ is the principal plasmaspheric ion, with singly ionized helium accounting for ~20% of the plasmaspheric plasma.

Unlike the plasma of the central plasma sheet, which in general "convects" (i. e., flows) sunward toward the dayside magnetopause, the cold, dense plasma of the plasmasphere is trapped on magnetic field lines that rotate with the Earth and thus "co-rotates" with them (see figure). It is the competitive interplay between these two flow regimes -- one convecting sunward, the other co-rotating -- that, together with the outflow of plasma from the ionosphere, determines the size, shape, and dynamics of the plasmasphere, which vary strongly according to the level of magnetospheric activity.

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