Identifying intervals of temporally invariant field‐aligned currents from Swarm: Assessing the validity of single‐spacecraft methods

Forsyth, C.; Rae, I. J.; Mann, I. R.; Pakhotin, Ivan

doi:10.1002/2016ja023708

Cited by 24 publications

(52 citation statements)

References 19 publications

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“…This aspect, in line with previous observations (see, e.g., Forsyth et al, 2017;, adversely influences the outcome of the dual-s/c estimation. In the latter, the fine structure of FAC density is revealed.…”

Section: Dual-s/c Methods On Configurations That Use Simultaneous Meassupporting

confidence: 90%

“…The latter condition is usually checked by performing a cross-correlation analysis (see, e.g., in Forsyth et al, 2017 a more involved technique has been used that combines cross correlation and linear fitting). The latter condition is usually checked by performing a cross-correlation analysis (see, e.g., in Forsyth et al, 2017 a more involved technique has been used that combines cross correlation and linear fitting).…”

Section: Applicability Of the Three-s/c Methodsmentioning

confidence: 99%

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Multipoint Field‐Aligned Current Estimates With Swarm

Blăgău

Vogt

2019

JGR Space Physics

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Field‐aligned currents (FACs) are a key component of the magnetosphere‐ionosphere system, providing the transfer of energy and momentum between the distant collision‐free magnetospheric plasma and the collisional ionosphere. The European Space Agency's Swarm mission offers a unique opportunity to explore the FACs low‐altitude end by a broad range of single‐spacecraft and multispacecraft techniques. The present technical report demonstrates the application of dual‐ and three‐satellite FAC estimation methods based on the least squares approach, with the goal to enhance the use of Swarm data in exploring the high‐latitude current structures, beyond the official FAC product presently available to the end user. The dual‐satellite method presents some clear advantages since it provides stabler solutions, can be applied on a more general spacecraft configuration, and offers a robust error estimation scheme. Consequently, the method provides significantly more data near the singularity where the satellites' orbits intersect, allows FAC estimation with configurations that involves the upper Swarm satellite, or to fine tune the constellation geometry to the problem at hand. Similarly, the three‐satellite method, meant to be applied when Swarm forms a close configuration, brings additional valuable information, associated to a different (larger) scale. Moreover, it can estimate the FAC density with high time resolution when instantaneous measurements are used. The performance of the methods are thoroughly analyzed both on Swarm events and on simulated data, and the results are compared with other available methods. Particular emphasis is put on how different FAC estimation methods complement each other and provide consistent results.

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Section: Dual-s/c Methods On Configurations That Use Simultaneous Meassupporting

confidence: 90%

Section: Applicability Of the Three-s/c Methodsmentioning

confidence: 99%

Multipoint Field‐Aligned Current Estimates With Swarm

Blăgău

Vogt

2019

JGR Space Physics

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“…These authors explained such features as occurring as a natural result of R1 and R2 current closure in the auroral zone due to the fact that Pedersen current and its associated electric field reach a maximum at the R1-R2 boundary; they labelled these features Birkeland current boundary flows (BCBFs). Forsyth et al, 2017). The disturbances shown in Figures 11 and 12 are clearly electromagnetic and polarized in the transverse plane, similar to the other time periods considered in this paper.…”

Section: 1029/2019ja027277supporting

confidence: 75%

“…Similarly, such nonstationarity will pollute the two-spacecraft ESA Swarm FAC product as well. The Swarm A single-(blue line at 1-Hz resolution) and dual-spacecraft (red line, 20-s low-pass filtered, 1-Hz resolution; Ritter et al, 2013) FAC estimates in panel c imply significant small-scale electrodynamics, producing fine structure in the single-spacecraft FAC estimate but which most likely violate the quasi-static approximation (e.g., Forsyth et al, 2017). The importance of small-scale electrodynamics is further analysed and emphasized in Figure 4, which demonstrates the impact of applying a 20-s filter as compared to studies using 16-sps data.…”

Section: Consistent With the Global Ampere Fac Map Shown Inmentioning

confidence: 99%

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

et al. 2020

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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. Plain Language SummaryThe 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.

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“…Here, we incorporate additional analysis steps consisting of the application of the CHAOS-6 model (prior to the wavelet analysis) and the utilization of Swarm Level 2 products (i.e., the FAC and IBI products) to impose constraints on the Swarm data for the wave event selection. Forsyth et al (2017) suggested that many of the magnetic field perturbations on small scales may contain a significant fraction of perturbations which are the result of wave activity in the vicinity of the spacecraft or current sheets inclined to the motion of the spacecraft. As a result, they noted that extreme care must be taken in interpreting the temporal variation of the magnetic field observed by a moving spacecraft as relating to the spatial structures which can be used to infer field-aligned currents.…”

Section: Data Processing and Analysis Techniquementioning

confidence: 99%

An initial ULF wave index derived from 2 years of Swarm observations

et al. 2018

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Abstract. The ongoing Swarm satellite mission provides an opportunity for better knowledge of the near-Earth electromagnetic environment. Herein, we use a new methodological approach for the detection and classification of ultra lowfrequency (ULF) wave events observed by Swarm based on an existing time-frequency analysis (TFA) tool and utilizing a state-of-the-art high-resolution magnetic field model and Swarm Level 2 products (i.e., field-aligned currents -FACs -and the Ionospheric Bubble Index -IBI). We present maps of the dependence of ULF wave power with magnetic latitude and magnetic local time (MLT) as well as geographic latitude and longitude from the three satellites at their different locations in low-Earth orbit (LEO) for a period spanning 2 years after the constellation's final configuration. We show that the inclusion of the Swarm single-spacecraft FAC product in our analysis eliminates all the wave activity at high altitudes, which is physically unrealistic. Moreover, we derive a Swarm orbit-by-orbit Pc3 wave (20-100 MHz) index for the topside ionosphere and compare its values with the corresponding variations of solar wind variables and geomagnetic activity indices. This is the first attempt, to our knowledge, to derive a ULF wave index from LEO satellite data. The technique can be potentially used to define a new Level 2 product from the mission, the Swarm ULF wave index, which would be suitable for space weather applications.

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Identifying intervals of temporally invariant field‐aligned currents from Swarm: Assessing the validity of single‐spacecraft methods

Cited by 24 publications

References 19 publications

Multipoint Field‐Aligned Current Estimates With Swarm

Multipoint Field‐Aligned Current Estimates With Swarm

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

An initial ULF wave index derived from 2 years of Swarm observations

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