On the curlometer measurement of field-aligned and perpendicular currents in low Earth orbit: Swarm observations and whole geospace simulations

TL;DR

This study evaluates the curlometer technique for measuring field-aligned currents in low Earth orbit using Swarm data and simulations, highlighting its limitations and the importance of measurement quality.

Contribution

It demonstrates the challenges of applying the curlometer method at small scales and emphasizes the need for high-quality four-point measurements for accurate FAC analysis.

Findings

FACs are highly uncorrelated at scales below 100 km.

Time-shifted FAC estimates can diverge from true values at meso-scales.

Poor tetrahedral configurations can produce spurious currents due to numerical instability.

Abstract

Measuring field-aligned currents (FACs) using magnetic field observations provides a powerful means to probe the multi-scale interactions between the magnetosphere, ionosphere and thermosphere. In this study, we apply the curlometer technique to Swarm spacecraft observations and to simulations of the coupled magnetosphere-ionosphere system. We begin by correlating current density curlometer estimates derived from Swarm tetrahedra with varying spatial scales and barycentre locations. This confirms an apparent departure from stationarity for FACs at spatio-temporal scales below 100 km where measurements appear highly uncorrelated. We then analyse simulated magnetic perturbations, where true four-point measurements are available. This shows how, even at meso-scales of hundreds of kilometres, time-shifted FAC estimates can diverge significantly from this ground truth. In both observational and simulated data we find poor tetrahedral configurations can produce spurious perpendicular currents due to numerical instability in the inversion process. This can be mitigated using appropriate quality metrics and high-quality FAC reconstructions still achieved with a tetrahedral face well-aligned to the local magnetic field. These results highlight the dynamic nature of FACs at large as well as small scales, and underscore the substantial advantages of true four-point observations for their accurate analysis.