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.
Bibliographical noteFunding Information:
This work was supported in part by ESA contract 4000114090 (Swarm Investigation of the Role of High-Frequency (0.1–5 Hz) ULF Waves in Magnetosphere-Ionosphere Coupling) Swarm + Support to Science Element. I. R. Mann is supported by a discovery grant from Canadian NSERC. This work was also partially supported by the Canadian Space Agency Geospace Observatory (GO) Canada Science and Applications program. C. Forsyth is supported by a Natural Environment Research Council Independent Research Fellowship (NE/N014480/1). I. J. Rae is supported in part by a Science and Technology Facilities Council Grant (STFC) ST/N000722/1 and NERC grants NE/L007495/1 and NE/P017150/1. D. M. Miles was supported by an NSERC PGSD graduate scholarship and by funding from the Canadian Space Agency. R. L. Lysak is supported by NSF grant AGS- 1558134. The authors thank Johnathan Burchill for assistance with the 16 s−1 TII data. The authors thank the AMPERE team and the AMPERE Science Center for providing the Iridium-derived data products. The authors would like to separately thank Haje Korth for assistance with the AMPERE visualization toolkit. The ESA Swarm data can be obtained from the ESA server at swarm-diss.eo.esa.int. AMPERE data can be downloaded from http://ampere. jhuapl.edu. Geomagnetic conditions and L1 information used to make Figure 1a can be found on https:// omniweb.gsfc.nasa.gov. The CARISMA ground magnetometer data used to make Figure 1c can be obtained from http://www.carisma.ca.
- Alfvén waves
- Birkeland currents
- magnetosphere-ionosphere interaction
- wave reflection