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

I. P. Pakhotin; I. R. Mann; D. J. Knudsen; R. L. Lysak; J. K. Burchill

doi:10.1029/2019JA027277

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

I. P. Pakhotin, I. R. Mann, D. J. Knudsen, R. L. Lysak, J. K. Burchill

Research output: Contribution to journal › Article › peer-review

12 Scopus citations

Abstract

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.

Original language	English (US)
Article number	e2019JA027277
Journal	Journal of Geophysical Research: Space Physics
Volume	125
Issue number	1
DOIs	https://doi.org/10.1029/2019JA027277
State	Published - Jan 1 2020
Externally published	Yes

Bibliographical note

Funding 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.

Publisher Copyright:
©2019. American Geophysical Union. All Rights Reserved.

Keywords

Alfvén waves
Birkeland currents
ionosphere
magnetosphere
magnetosphere-ionosphere interaction
wave reflection

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title = "Diagnosing the Role of Alfv{\'e}n Waves in Global Field-Aligned Current System Dynamics During Southward IMF: Swarm Observations",

abstract = "Field-aligned currents (FACs) are a primary signature of magnetosphere-ionosphere coupling (MIC). However, establishing FACs requires the propagation of Alfv{\'e}n 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 Alfv{\'e}n 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 Alfv{\'e}n wave interpretation, validated using the Lysak (1991, https://doi.org/10.1029/90JA02154) ionospheric Alfv{\'e}n resonator model constrained by inferred local Swarm plasma mass density. Local large amplitude E and B fields indicate the importance of Alfv{\'e}nic 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 Alfv{\'e}n 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 Alfv{\'e}nic ionospheric feedback.",

keywords = "Alfv{\'e}n waves, Birkeland currents, ionosphere, magnetosphere, magnetosphere-ionosphere interaction, wave reflection",

author = "Pakhotin, {I. P.} and Mann, {I. R.} and Knudsen, {D. J.} and Lysak, {R. L.} and Burchill, {J. K.}",

note = "Funding 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 Alfv{\'e}n 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. Publisher Copyright: {\textcopyright}2019. American Geophysical Union. All Rights Reserved.",

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AU - Knudsen, D. J.

AU - Lysak, R. L.

AU - Burchill, J. K.

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N2 - 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.

AB - 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.

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

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Bibliographical note

Keywords

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