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Abstract

Four‐point magnetic field measurements in space allow estimates of the electric current density through the curlometer technique, which estimates electric current density from Ampère's law, and is relevant to the magnetosphere and surrounding regions, which contain high conductivity plasma. Knowledge of spacecraft separations, magnetic field measurement accuracy, and the form of the current structures sampled (e.g., relative scale size) limits the accuracy of the method. Despite these conditions of application, in many regions of the magnetosphere it has been shown to be robust and reliable. A number of studies have applied the method successfully such as: the ring current; the magnetotail current sheet; the magnetopause currents, and field aligned currents, as well as to other current structures (e.g., flux tubes). Where time stationarity and other special assumptions can be made, or where the spacecraft configuration is highly irregular or less than four spacecraft are available, the method can still be applied to obtain partial components of the current. We discuss the application of the curlometer technique to the four‐point observations from the Cluster mission in terms of its adaptability and performance (including the lessons learned) and illustrate its use with recent data from the MMS mission, which covers a much smaller spatial regime.