Abstract
AbstractHigh‐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.
Highlights
Field-aligned currents (FACs) flowing between the magnetosphere and the ionosphere provide the basis for the transfer of energy and momentum between the two systems (e.g., Cowley, 2000; Foster et al, 1983; Lu et al, 1998)
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
An analysis of the magnetic field data for a FAC system crossed by Swarm A and C during northward IMF conditions revealed evidence for ~100 nT large-amplitude nonstationary magnetic structures which evolve on 10 s timescales
Summary
Field-aligned currents (FACs) flowing between the magnetosphere and the ionosphere provide the basis for the transfer of energy and momentum between the two systems (e.g., Cowley, 2000; Foster et al, 1983; Lu et al, 1998). Under the assumption that the FACs consist of long current sheets orientated perpendicular to the spacecraft orbit, FACs can be calculated from the observed transverse magnetic field variation via Ampère’s law, neglecting the displacement current (e.g., Ritter et al, 2013, and references therein). This single-spacecraft approach must assume that all the transverse magnetic oscillations occur as the result of spacecraft motion through static and infinite current sheets. Any temporal variations in the magnetic field during the crossing of a FAC element will result in a violation of this
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