A study is made of the process of generation and evolution of parallel electric fields on closed geomagnetic field lines. Charge separation that is due to the different character of the drift of energetic electrons and protons of inhomogeneous magnetospheric plasma in the geomagnetic field is considered to be the factor responsible for the generation of the electric field. In a magnetic flux tube that drifts together with the cold plasma, this separation of charges is manifested as the injection of energetic electrons or protons. A numerical self-consistent calculation of the evolution of the distribution functions of particles and of the parallel electric field is carried out for the injection of energetic electrons into the geomagnetic flux tube. An analytical investigation of the macroscopic and microscopic processes accompanying such injection is carried out. The calculations take into account cold ionospheric particles and energetic magnetospheric protons and electrons. It was found that an intense parallel electric field is produced in the form of two collisionless rarefaction shock waves that separate from the equatorial plane in the northward and southward directions. Below the front of each shock wave, there is mainly the ionospheric plasma, while behind the front there are the upward accelerated ionospheric ions and the hot magnetospheric plasma. Microscopic structure behind the shock front is formed by electrostatic oscillations of a large amplitude. Generation conditions for intense longitudinal fields are formulated: The difference in densities of injected energetic electrons and protons near the equatorial plane must exceed the initial density of cold particles, and the current-associated velocity of cold electrons must be larger than the ion sound velocity. It is shown that these conditions can be satisfied, owing to a drift mechanism of charge separation. A rarefaction shock wave is excited with substantially smaller field-aligned currents as compared with anomalous resistance or double layers. The observed manifestations of the rarefaction shock waves are auroral plasma cavities. ¿ American Geophysical Union 1994