Diagnosing the Role of Alfven 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 Alfven 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 Alfven 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 Alfven wave interpretation, validated using the Lysak (1991, https://doi.org/10.1029/90JA02154) ionospheric Alfven resonator model constrained by inferred local Swarm plasma mass density. Local large amplitude E and B fields indicate the importance of Alfvenic 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 Alfven wave paradigm: quasi-static large-scale FACs representing the omega -> 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 Alfvenic ionospheric feedback. Plain Language Summary The study demonstrates the importance of electromagnetic wave phenomena in global magnetosphere-ionosphere coupling dynamics during conditions associated with southward interplanetary magnetic fields. Such conditions are associated with strong energy input into geospace and with driving large flows in the ionosphere. The importance of wave phenomena for transporting energy from the magnetosphere to the ionosphere is not well understood, and this study assesses the potential role of waves during conditions of strong flows, which are typically encountered in near-Earth geospace at these times. The study demonstrates the importance of wave phenomena during these conditions and, in particular, evaluates the energetic significance of small-scale electromagnetic perturbations.