Double layers above the aurora

Two different kinds of double layers have been found in association with auroral precipitation. One of these is the so-called 'electrostatic shock,' which is oriented at an oblique angle to the magnetic field in such a way that the perpendicular electric field is much larger than the parallel electric field. This type of double layer is often found at the edges of regions of upflowing ion beams, and the direction of the electric field in the shock points towards the ion beam. The other kind of double layer is the small amplitude double layer found in regions of upward flowing ion beams, often in association with electrostatic cyclotron waves. The parallel and perpendicular electric fields in these structures are comparable in magnitude. Some important questions concerning double layers and their relation to parallel electric fields in the aurora are discussed. (author)