Diagnosing the Role of Alfvén Waves in Global Field-Aligned Current System Dynamics During Southward IMF: Swarm Observations

Field-aligned currents (FACs) are a primary signature of magnetosphere-ionosphere coupling (MIC). However, establishing FACs requires the propagation of Alfvén waves. Large-scale quasi-static FACs are well-organized into large-scale Region 1 (R1) and Region 2 (R2) systems during intervals of southward interplanetary magnetic field (IMF); however, the scale-dependent spatiotemporal variability and related electrodynamics are less well understood. Using the electric and magnetic field data from Swarms A and C, we examine the role of Alfvén waves in MIC at a range of scales during two auroral crossings during southward IMF on May 16, 2016. Interspacecraft techniques reveal large amplitude small-scale (10s km) non-stationary magnetic fields inconsistent with a quasi-static formulation. Cross-phase techniques reveal a frequency-dependent E/B ratio and E-B phase difference consistent with an Alfvén wave interpretation, validated using the Lysak (1991, https://doi.org/10.1029/90JA02154) ionospheric Alfvén resonator model constrained by inferred local Swarm plasma mass density. Local large amplitude E and B fields indicate the importance of Alfvénic energy transport at small scales. Evidence for Poynting flux concentration at the boundary between large-scale upward and downward FACs is also presented. Our results suggest that cross-scale FAC characteristics can be explained by a single Alfvén wave paradigm: quasi-static large-scale FACs representing the ω → 0 limit of a broader continuum of spatial scales associated with MIC. Future work should assess in more detail the energetic significance of small scales and the potential localization of large amplitude small-scale disturbances at large scale FAC boundaries and assess related scale-dependent MIC including Alfvénic ionospheric feedback.