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Abstract

Field‐aligned currents play a major role in magnetized plasmas. They are the main agents for coupling the dynamics between magnetosphere and ionosphere. Perturbations of magnetic field distribution, plasma pressure or flow on one end are communicated by field‐aligned currents along the whole flux tube. Furthermore, they transport energy, practically lossless, from dynamo regions in the magnetosphere into the upper atmosphere. The knowledge of field‐aligned current distribution is therefore of great interest for understanding the magnetosphere‐ionosphere system. In this chapter we describe the expressions that illustrate the relationship between plasma motion in the magnetosphere and electric currents flowing along the background magnetic field. These currents can only be sensed above the ionospheric E‐layer, e.g. by low‐Earth orbiting satellites. However, to obtain reliable current density estimates is quite challenging. In a climatological sense, field‐aligned currents appear along two concentric rings in the high‐latitude ionosphere with opposite flow directions. Details of these patterns are controlled by the orientation of the interplanetary magnetic field. The transverse scale sizes determine important characteristics of field‐aligned currents.