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

Abstract

AbstractField‐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.