Electrodynamic coupling between the magnetosphere and ionosphere is accomplished by means of the passage of Alfvén waves between these regions. These waves can be accompanied by parallel electric fields when the perpendicular scale size becomes small. There are two main regimes of this acceleration. At lower altitudes where the plasma is cold, electron inertial effects become important and can lead to the bulk acceleration of the cold plasma. At higher altitudes, the primary particle acceleration mechanisms are electron trapping and Landau damping, which preferentially interact with electrons with velocities near the Alfvén wave phase velocity. These mechanisms are favored in regions where there are sharp plasma gradients, such as at the plasma sheet boundary layer or on the edges of the auroral density cavity, since phase mixing is an efficient mechanism for reducing the perpendicular wavelength.
These waves may be generated in a number of ways. One common mechanism is generation due to shear flows, such as the bursty bulk flows often observed in the plasma sheet. This mechanism can explain the connection between these flows and auroral streamers. In addition, shear Alfvén waves can be produced by linear mode conversion of fast mode waves. Such mode conversion can take place where there are strong perpendicular gradients in the Alfvén speed. In such a situation, phase mixing can reduce the perpendicular wavelength of the Alfvén waves, which favors the development of parallel electric fields. Theory and modeling of these processes will be presented.