ALBERT

Description: High resolution multi-spacecraft Swarm data are used to examine magnetosphere-ionosphere coupling (MIC) during a period of northward IMF on 31 st 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 seconds 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.