Update on scales and energetics of auroral field-aligned currents as observed by Swarm

Auroral field-aligned currents (FACs) are of key importance for the electromagnetic coupling and the energy transport in the magnetosphere-ionosphere system. We use Swarm multi-spacecraft magnetic and electric field measurements from a selection of auroral oval crossing events to advance our understanding of the spatial scales and the electromagnetic energy flux (Poynting flux) associated with sheets of auroral FACs. Our study comprises the derivation of a scale-dependent correlation function based on dual-satellite vectorial magnetic field perturbation time series, in order to identify and analyze planar current structures. Applying concepts from multi-point boundary crossing analysis to data from Swarm-A and Swarm-C, a correlation measure is constructed using the mean square deviation of the observed magnetic perturbations and an empirical pattern function. Peak correlations indicate the positions and the scales of auroral FAC sheets, which we contextualize with the magnetic local time, the geomagnetic latitude, and geomagnetic activity indices (e.g., AL). In a parallel strand of work, we estimate the associated Poynting flux from the combination of the magnetic field perturbations and those of the electric field as deduced from the observed cross-track ion drift velocity. We assess the quality of our Swarm-based estimate by a comparison to the Poynting flux given by the “Cosgrove-PF” empirical model, which is based on FAST data from 1996 to 2001 and available from NASA’s Community Coordinated Modeling Center. Connecting both strands of work, we check to what degree this data-model comparison depends on the current sheets’ spatial scale. Throughout the study, we adopt a framework for describing planar magnetic structures that facilitates error analysis and accommodates not only boundary analysis, but also single-spacecraft polarization techniques.