Diagnosing the Role of Alfven Waves in Magnetosphere-Ionosphere Coupling: Swarm Observations of Large Amplitude Nonstationary Magnetic Perturbations During an Interval of Northward IMF

作者：Pakhotin I P^*; Mann I R; Lysak R L; Knudsen D J; Gjerloev J W; Rae I J; Forsyth C; Murphy K R; Miles D M; Ozeke L G; Balasis G

来源：JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, 2018, 123(1): 326-340.

DOI：10.1002/2017JA024713

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 Alfven 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 Alfven 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 Alfven resonator model including Pedersen conductance effects. Indeed, within this Alfven wave incidence, reflection, and interference paradigm, even quasi-static field-aligned currents might be reasonably interpreted as very low frequency (omega -> 0) Alfven waves. Overall, our results not only indicate the importance of Alfven 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.
Plain Language Summary The study shows evidence that electromagnetic waves in the ionosphere and currents flowing into the ionosphere can be described by the same physical model. This is important for estimating the total energy going into the ionosphere and potentially allows deriving important high-resolution information about the ionosphere by studying data recorded when spacecraft fly over the auroral region.

• 出版日期2018-1