Recent satellite measurements of the morphology and dynamics of the plasmasphere

The very low energy thermal plasma (∼1 eV or several thousand degrees Kelvin) is often overlooked in reviews of magnetospheric plasma populations, usually as a result of energy considerations in which thermal particles are believed unimportant. In reality this cold plasma, which surrounds the earth in a region known as the plasmasphere, has an important direct influence on the characteristics of the ionospheric F region and on the location of regions of plasma turbulence in the magnetosphere, and it is an excellent indirect measure of the intensity of magnetospheric convection. The characteristic morphology and dynamics of the plasmasphere vary with local time and with geomagnetic conditions. On the nightside the plasmapause position changes predictably with changing magnetic activity. Once established at a specific L ‐shell value, the steep density gradient on the nightside corotates into the dayside, where filling from the ionosphere takes place. In the duskside bulge region the characteristic density profile inside the plasmapause displays a smooth decrease proportional to 1/ R 4 , where R is radial distance. Plasmasphere morphology and dynamics can be understood in terms of a time‐varying convection electric‐field model of the magnetosphere that includes the bulge region as part of the main circulation pattern of the plasmasphere.