Generation Mechanisms for Magnetic-Field-Aligned Electric Fields in the Magnetosphere

抄録

Magnetic-field-aligned electric fields in the magnetosphere can be generated in several different ways.
Current driven wave instabilities can lead to either anomalous resistivity or electric double layers. Both can support potential drops of many kilovolts. In the former case this requires wave turbulence extended over large distances, because observed wave amplitudes allow only moderate field strength. In the latter case the voltage drop is concentrated to one or more (perhaps numerous) thin regions, sometimes referred to as electrostatic shocks, each of the order of several tens of Debye lengths thick. Once established, such layers may exist with or without substantial wave turbulence. It has also been proposed that wave-particle interaction may establish a collisionless version of the thermoelectric effect. In the hot magnetospheric plasma potential drops can be created as a consequence of the magnetic mirror force. In a magnetically trapped plasma this, in principle, requires no current, although some leakage current is expected. The mirror effect has its most dramatic impact in regions of upward Birkeland currents, where it limits the current density to moderate levels unless large voltages are applied (many times the voltage-equivalent of the thermal energy of the magnetospheric plasma). The distribution of this voltage depends critically on the distribution functions of the plasma particles involved, and it may well degenerate into electric double layers.
Observational data now available indicate that more than one of the mechanisms mentioned are operative in the magnetosphere but it is not yet possible to evaluate their relative importance.