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 from Swarm A and C, approximately 10 s apart along track, and the correspondence between the observed electric and magnetic fields at 16 samples per second, provide significant evidence for an important role for Alfvén waves in magnetosphere‐ionosphere coupling even during northward IMF conditions. Spectral comparison between the wave E‐ and B‐fields reveals a frequency‐dependent phase difference and amplitude ratio consistent with interference between incident and reflected Alfvén waves. At low frequencies, the E/B ratio is in phase with an amplitude determined by the Pedersen conductance. At higher frequencies, the amplitude and phase change as a function of frequency in good agreement with an ionospheric Alfvén resonator model including Pedersen conductance effects. Indeed, within this Alfvén wave incidence, reflection, and interference paradigm, even quasi‐static field‐aligned currents might be reasonably interpreted as very low frequency (ω → 0) Alfvén waves. Overall, our results not only indicate the importance of Alfvén waves for magnetosphere‐ionosphere coupling but also demonstrate a method for using Swarm data for the innovative experimental diagnosis of Pedersen conductance from low‐Earth orbit satellite measurements.

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

“…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.…”

“…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.…”

“…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.…”

“…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.…”

“…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.…”