Vectorial Association of Linearly Oriented Residua: Application to Swarm High-latitude Magnetic Field Measurements

Abstract We introduce the methodological framework VALOR for the analysis of field-aligned current (FAC) sheets based on dual-spacecraft magnetic field observations. VALOR generalizes existing approaches in two respects. First, it takes advantage of the primary observables at both spacecraft, namely, the full magnetic field vectors, instead of derived scalar quantities like single- or dual-spacecraft FAC densities. Second, rather than being restricted to linear correlation measures, VALOR allows to define a custom-made statistical measure of association between the signals at both spacecraft, that is chosen to be particularly sensitive to small-scale fluctuations for this first demonstration. Additionally, VALOR offers to consider the linear orientation of the magnetic field residual vectors, i.e., the sheets’ polarization, in order to infer their orientation and to accentuate the association measure. This paper illustrates the method on the basis of an exemplary auroral oval crossing on March 1, 2019 by the lower pair of ESA’s multi-spacecraft mission Swarm. Explicit comparisons to competing methods are performed to give credence to the results of the case study while the general capability of VALOR is demonstrated on the basis of a statistical study comprising around 9700 auroral oval crossings from 2014-2020. One suitable area of application is the analysis of meso- to small-scale (tens of km to below one km) auroral arc systems in combination with optical data from modern all-sky imagers. Conceptually, VALOR is neither bound to the auroral zone nor to Swarm and can be applied by any multi-spacecraft geospace mission with suitable instrumentation and orbit configuration.