Refraction Shock and Its Consequences for the Auroral Acceleration Processes

Satellite observations have now amply demonstrated that the accelerations of electrons and ions in the upward current region of the auroral plasma occur in two layers situated at low and high altitudes. Large localized upward electric fields in the layers accelerate electrons downward and ions upward. Thus, between the two layers accelerated electrons and ions exist simultaneously. Recently we identified that the fields in the lower layer belong to a rarefaction shock. The rarefaction shocks were first studied in connection with expansion of laser plasmas with two electron temperatures. In auroral context, the ionospheric plasma and the backscattered electrons constitute the needed two-electron-temperature plasma. The resulting rarefaction shock in the expanding ionospheric plasma takes up a part of the auroral parallel potential drop. The rest of the potential drop occurs at higher altitudes in a single double layer or multiple ones. Between the rarefaction shock and high-altitude double layer(s), electron and ion beams simultaneously exist. The interaction between the beams generates a variety of wave modes, which trap electrons and ions generating counterstreaming populations of charged particles. Nonlinear wave structures like electron and ion holes and even ion-acoustic shocks, resulting from ion-ion instabilities, become a common feature of the region between the two acceleration layers. Below the low-altitude acceleration layer (rarefaction shock) the cold-hot electron mixture supports the electron-acoustic mode and when destabilized by the downward electron beam, large-amplitude electron holes form. Besides the auroral plasma, when the time history of the ionosphere allows for the presence of warm electrons with a hot to cold temperature ratio of ~10, a rarefaction shock forms in the polar wind, accelerating ions upward.