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Commentary on accessing 3‐D currents in space: Experiences from Cluster

The curlometer was introduced to estimate the electric current density from four‐point measurements in space; anticipating the realization of the four spacecraft Cluster mission which began full science operations in February 2001. The method uses Ampère's law to estimate current from the magnetic field measurements, suitable for the high‐conductivity plasma of the magnetosphere and surrounding regions. The accuracy of the method is limited by the spatial separation knowledge, accuracy of the magnetic field measurement, and the relative scale size of the current structures sampled but nevertheless has proven to be robust and reliable in many regions of the magnetosphere. The method has been applied successfully and has been a key element, in studies of the magnetopause currents; the magnetotail current sheet; and the ring current, as well as allowing other current structures such as flux tubes and field aligned currents to be determined. The method is also applicable to situations where less than four spacecraft are closely grouped or where special assumptions (particularly stationarity) can be made. In view of the new four‐point observations of the MMS mission taking place now, which cover a dramatically different spatial regime, we comment on the performance, adaptability, and lessons learnt from the curlometer technique. We emphasize the adaptability of the method, in particular, to the new sampling regime offered by the MMS mission; thereby offering a tool to address open questions on small‐scale current structures.