OBSERVATIONS OF WEAK DOUBLE-LAYERS ON AURORAL FIELD LINES

The wave instrument on the Swedish Viking satellite performed measurements of plasma density and potential fluctuations with high temporal and spatial resolution. In the auroral regions, double-layer structures were observed which have scale lengths of about 100 m and where the plasma density is reduced by up to 50%. The structures are negatively charged and have net potential drops of up to a few volts directed upwards in the same direction as the electric currents along the geomagnetic field lines; i.e., the structures are dissipative elements of the magnetospheric electric circuit. In some regions double-layer structures are so ubiquitous that it is possible that series of such double layers may form a large-scale equipotential pattern, with a substantial potential drop along the field lines, thus contributing to the energization of the electrons that produce the aurora and upward flowing ions. Support for this view is found from simultaneous observation of the accelerated ions and electrons, and from measurements of large-scale potential variations along the satellite orbit.