The ionosphere serves as a closure path for electrical currents flowing from one part of the magnetosphere to another. How this happens is shown in detail by recent work of R. A. Greenwald at the Max Planck Institute, using the Scandinavian twin auroral radar experiment (Stare). Built in northern Scandinavia as part of the ground-based program of the International Magnetospheric Study, Stare is producing measurements that are a significant advance in the study of the ionospheric current systems in the auroral zones.
Explaining that electric fields which drive currents build up in the ionosphere in response to influences such as changes in solar activity, Greenwald says that the nature of these currents changes with the time of day. On the evening side of the earth these currents flow generally eastward, driven by a northward electric field; on the morning side they flow westward under the influence of a southward electric field. These auroral electrojets can extend over hundreds of kilometers north-south and flow for thousands of kilometers east-west. Where they flow, the electrons moving through the ions create plasma instabilities that can be observed by the radars.