The Structure of Field‐Aligned Current Systems as Inferred From the Multiscale Minimum Variance Analysis.

Auroral field‐aligned currents (FACs) have an intrinsic complexity caused by the superposition of contributions from a broad spectrum of scales and diversity of locations. The complex FAC systems are investigated by using the multiscale minimum variance analysis. This technique provides a scale based decomposition of the FAC systems by identifying the constituting FAC elements as well as their structure. At the basis, the analysis exploits the scale dependence of the eigenvalues of the magnetic field variance matrix. The scale decomposition along the transversal (latitudinal) direction results from the scale derivative of the maximum eigenvalue. The complementary information from the scale derivative of the minimum eigenvalue helps to infer the full structure of each FAC element by providing estimates of the FAC length (longitudinal) scale. The scale derivative of minimum and maximum eigenvalues are used to identify FAC signatures associated to different types of aurora (e.g., highly extended, finite arcs, gradient regions) as well as to characterize the influence of the crossing location with respect to the FAC structures (e.g., near edge crossings). The multiscale analysis is applied to simulated FACs and to spacecraft observations made by the Swarm mission. The use with real world data illustrates the power of this analysis, whose full benefits for magnetosphere‐ionosphere coupling investigations are yet to be explored. Plain Language Summary: The analysis in this work applies to magnetic field observations from satellites crossing above the aurora. The complexity of auroral displays relates to complex currents that flow along the Earth main magnetic field. These currents are observed as a perturbation magnetic field. The added contributions from currents of different spatial scales, locations and life‐times lead to complex signatures of the field‐aligned currents. The analysis here traces this complexity by a decomposition of the measured magnetic field perturbation into discrete current elements. The aim is to decompose the magnetic field observations into the constituent discrete elements and to characterize their respective structures. The analysis is applied to Swarm observations whereas the ground optical observations provide a qualitative cross‐check to the analysis. Key Points: Single‐spacecraft decomposition of complex field‐aligned current (FAC) systems associated to finite length (east‐west) auroral structuresEstimation of the geometric structure of finite FACs by the transversal and longitudinal scalesIdentification of FAC signatures associated to finite FACs like the crossings near the eastward/westward edges of longitudinally extended FACs [ABSTRACT FROM AUTHOR]