Diagnosing the Role of Alfvén Waves in Magnetosphere‐Ionosphere Coupling: Swarm Observations of Large Amplitude Nonstationary Magnetic Perturbations During an Interval of Northward IMF
Abstract:High‐resolution multispacecraft Swarm data are used to examine magnetosphere‐ionosphere coupling during a period of northward interplanetary magnetic field (IMF) on 31 May 2014. The observations reveal a prevalence of unexpectedly large amplitude (>100 nT) and time‐varying magnetic perturbations during the polar passes, with especially large amplitude magnetic perturbations being associated with large‐scale downward field‐aligned currents. Differences between the magnetic field measurements sampled at 50 Hz fr… Show more
“…However, as discussed also in Pakhotin et al (2018;and references therein), in the quasi-static case, there should be no dependence of impedance on frequency until perpendicular scales of the orders of only a few kilometers in transverse spatial scale are reached (e.g., Forget et al, 1991;Lessard & Knudsen, 2001). However, for the case of Alfvén waves at higher altitudes and/or higher frequencies, the wave impedance is no longer held to a constant value by the Pedersen conductance and is determined by wave interference.…”
Section: Alfvén Wave Impedance Analysismentioning
confidence: 81%
“…The methodology employed here is based on histogram analysis defined by Grzesiak (2000) and utilized by Pakhotin et al (2018) for Swarm observations of auroral oval crossings under northward IMF conditions. Fourier transforms of orthogonal transverse E-and B-field time series (in mean field-aligned coordinates) are taken, and a coherency test is utilized to retain only those spectral points that are coherent between E and B.…”
Section: Alfvén Wave Impedance Analysismentioning
confidence: 99%
“…The areas corresponding to low coherency in Figure 5e have been omitted. This is explored in terms of occurrence statistics in histogram form in Figure 6 and using the technique presented by Grzesiak (2000), which was previously employed for Swarm analysis of electrodynamics during conditions of northward IMF by Pakhotin et al (2018). These relative phase differences are of critical importance for assessing the characteristics of the MIC associated with these fields and are of particular importance for diagnosing the presence of Alfvénic wave effects.…”
Section: Consistent With the Global Ampere Fac Map Shown Inmentioning
confidence: 99%
“…Methodologies that use E/B ratio and phase to differentiate between quasi-static and Alfvénic disturbances in FAC systems, developed by, for example, Kelley et al (1991), Ishii et al (1992), and Knudsen et al (1992), were used by Pakhotin et al (2018) in their analysis of large-scale FAC systems observed by Swarm during a period of northward IMF. It complements the northward IMF work presented in Pakhotin et al (2018) since it demonstrates an important role for Alfvén waves not only in the cusp regions associated with Region 0 current systems, but also more widely in large-scale R1 and R2 current systems. The analysis presented here is significant since it addresses the role of Alfvén waves in the primary mode of MIC during southward IMF, and the FAC systems, which develop in the auroral ovals.…”
Section: Introductionmentioning
confidence: 99%
“…This was done by Pakhotin et al (2018) using data from Swarm during a period of northward IMF and by Miles et al (2018) using Swarm data from a nighttime pass over a pair of discrete auroral arcs in conjunction with the e-POP satellite (Yau & James, 2015). In the study we present here, we apply the same methodology used by Pakhotin et al (2018), with data from Swarm satellites during an interval of southward IMF and where the global-scale FACs were characterized by a typical R1/R2 current system. In the study we present here, we apply the same methodology used by Pakhotin et al (2018), with data from Swarm satellites during an interval of southward IMF and where the global-scale FACs were characterized by a typical R1/R2 current system.…”
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.
“…However, as discussed also in Pakhotin et al (2018;and references therein), in the quasi-static case, there should be no dependence of impedance on frequency until perpendicular scales of the orders of only a few kilometers in transverse spatial scale are reached (e.g., Forget et al, 1991;Lessard & Knudsen, 2001). However, for the case of Alfvén waves at higher altitudes and/or higher frequencies, the wave impedance is no longer held to a constant value by the Pedersen conductance and is determined by wave interference.…”
Section: Alfvén Wave Impedance Analysismentioning
confidence: 81%
“…The methodology employed here is based on histogram analysis defined by Grzesiak (2000) and utilized by Pakhotin et al (2018) for Swarm observations of auroral oval crossings under northward IMF conditions. Fourier transforms of orthogonal transverse E-and B-field time series (in mean field-aligned coordinates) are taken, and a coherency test is utilized to retain only those spectral points that are coherent between E and B.…”
Section: Alfvén Wave Impedance Analysismentioning
confidence: 99%
“…The areas corresponding to low coherency in Figure 5e have been omitted. This is explored in terms of occurrence statistics in histogram form in Figure 6 and using the technique presented by Grzesiak (2000), which was previously employed for Swarm analysis of electrodynamics during conditions of northward IMF by Pakhotin et al (2018). These relative phase differences are of critical importance for assessing the characteristics of the MIC associated with these fields and are of particular importance for diagnosing the presence of Alfvénic wave effects.…”
Section: Consistent With the Global Ampere Fac Map Shown Inmentioning
confidence: 99%
“…Methodologies that use E/B ratio and phase to differentiate between quasi-static and Alfvénic disturbances in FAC systems, developed by, for example, Kelley et al (1991), Ishii et al (1992), and Knudsen et al (1992), were used by Pakhotin et al (2018) in their analysis of large-scale FAC systems observed by Swarm during a period of northward IMF. It complements the northward IMF work presented in Pakhotin et al (2018) since it demonstrates an important role for Alfvén waves not only in the cusp regions associated with Region 0 current systems, but also more widely in large-scale R1 and R2 current systems. The analysis presented here is significant since it addresses the role of Alfvén waves in the primary mode of MIC during southward IMF, and the FAC systems, which develop in the auroral ovals.…”
Section: Introductionmentioning
confidence: 99%
“…This was done by Pakhotin et al (2018) using data from Swarm during a period of northward IMF and by Miles et al (2018) using Swarm data from a nighttime pass over a pair of discrete auroral arcs in conjunction with the e-POP satellite (Yau & James, 2015). In the study we present here, we apply the same methodology used by Pakhotin et al (2018), with data from Swarm satellites during an interval of southward IMF and where the global-scale FACs were characterized by a typical R1/R2 current system. In the study we present here, we apply the same methodology used by Pakhotin et al (2018), with data from Swarm satellites during an interval of southward IMF and where the global-scale FACs were characterized by a typical R1/R2 current system.…”
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.
Field‐aligned currents (FACs), also known as Birkeland currents, are the agents by which energy and momentum are transferred to the ionosphere from the magnetosphere and solar wind. This coupling is enhanced at substorm onset through the formation of the substorm current wedge. Using FAC data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and substorm expansion phase onsets identified using the Substorm Onsets and Phases from Indices of the Electrojet technique, we examine the Northern Hemisphere FACs in all local time sectors with respect to substorm onset and subdivided by season. Our results show that while there is a strong seasonal dependence on the underlying FACs, the increase in FACs following substorm onset only varies by 10% with season, with substorms increasing the hemispheric FACs by 420 kA on average. Over an hour prior to substorm onset, the dayside currents in the postnoon quadrant increase linearly, whereas the nightside currents show a linear increase starting 20–30 min before onset. After onset, the nightside Region 1, Region 2, and nonlocally closed currents and the SuperMAG AL (SML) index follow the Weimer (1994, https://doi.org/10.1029/93JA02721) model with the same time constants in each season. These results contrast earlier contradictory studies that indicate that substorms are either longer in the summer or decay faster in the summer. Our results imply that, on average, substorm FACs do not change with season but that their relative impact on the coupled magnetosphere‐ionosphere system does due to the changes in the underlying currents.
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