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.
Bibliographical noteFunding Information:
This work was carried out under a programme of, and funded by the European Space Agency, in the frame of the ESA Living Planet Fellowship “Swarm Investigation of the Energetics of Magnetosphere-Ionosphere Coupling (SIEMIC).” The view expressed in this publication can in no way be taken to reflect the official opinion of the European Space Agency. The work was also supported in part by the Canadian Space Agency (CSA) Class Grant “What role do Alfvén waves play in energy transfer in the dynamical magnetosphere-ionosphere system?” IRM is supported by a Discovery Grant from Canadian Natural Sciences and Engineering Research Council (NSERC). RLL is supported by NSF grant AGS-1840891. JKB is supported in part with funding from the Canadian Space Agency. The authors thank the AMPERE team and the AMPERE Science Center for providing the Iridium-derived data products. The ESA Swarm data can be obtained from the ESA server at swarm-diss.eo.esa.int. AMPERE data can be downloaded from http://ampere.jhuapl.edu. Geomagnetic conditions and L1 information can be found at https://omniweb.gsfc.nasa.gov.
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- Alfvén waves
- Birkeland currents
- magnetosphere-ionosphere interaction
- wave reflection