The Dynamic Behavior of Plasmas Observed near Geosynchronous Orbit

The plasmas observed in the geosynchronous region of the earth's magnetosphere are complex and dynamic. Hot plasmas coexist with cold plasmas. These different populations interact, but each in general occupies its own niche in phase space depending on its source, and each experiences its own distinct scattering, energization, and loss processes. Although the plasma behavior is so variable that it is difficult to characterize, two predominant features are the sudden appearance of hot plasma, called injections, and the presence of marked magnetic field-aligned structure. Injections occur at least daily, more frequently during magnetic activity, with variable spectra, and frequently with field-aligned structure. The injection phenomenon is the geosynchronous manifestation of the global acceleration process that produces the aurora. Field-aligned fluxes can be as much as two orders of magnitude larger than perpendicular fluxes, but lower field-aligned fluxes are also common as a result of precipitation losses. Ions and electrons often display different field-aligned structure which changes with energy, indicating field-aligned acceleration processes. Other frequently observed phenomena are wave activity in both particles and fields, ion clusters bouncing between hemispheres, separate patches of low-energy plasma, and various kinds of plasma heating. Hot plasma can charge spacecraft to kilovolt potentials in eclipse and to hundreds of volts in sunlight. This kind of complex and dynamic plasma behavior cannot be unique to the earth's magnetosphere. Such behavior must predominate in other astrophysical regimes. Theoretical treatments which assume thermodynamic equilibrium and quasi-static behavior are probably far removed from reality.