Multipoint Field-Aligned Current Estimates With Swarm

Field-aligned currents (FACs) are a key component of the magnetosphere-ionosphere system, providing the transfer of energy and momentum between the distant collision-free magnetospheric plasma and the collisional ionosphere. The European Space Agency's Swarm mission offers a unique opportunity to explore the FACs low-altitude end by a broad range of single-spacecraft and multispacecraft techniques. The present technical report demonstrates the application of dual- and three-satellite FAC estimation methods based on the least squares approach, with the goal to enhance the use of Swarm data in exploring the high-latitude current structures, beyond the official FAC product presently available to the end user. The dual-satellite method presents some clear advantages since it provides stabler solutions, can be applied on a more general spacecraft configuration, and offers a robust error estimation scheme. Consequently, the method provides significantly more data near the singularity where the satellites' orbits intersect, allows FAC estimation with configurations that involves the upper Swarm satellite, or to fine tune the constellation geometry to the problem at hand. Similarly, the three-satellite method, meant to be applied when Swarm forms a close configuration, brings additional valuable information, associated to a different (larger) scale. Moreover, it can estimate the FAC density with high time resolution when instantaneous measurements are used. The performance of the methods are thoroughly analyzed both on Swarm events and on simulated data, and the results are compared with other available methods. Particular emphasis is put on how different FAC estimation methods complement each other and provide consistent results.