Fig. 4: ULF waves and FLR observed on the ground.

a Stations selected from the IMAGE magnetometer array with the background auroral image observed by DMSP F17 satellite at 13:27:45 UTC (see also Fig. 1b). The blue chain is aligned in almost the same magnetic longitudes of ~106 ± 4° with the magnetic latitude increasing from 54.5° at TAR to 75.1° at LYR. The red chain is aligned in almost the same magnetic latitudes of ~61.5 ± 0.5° with the magnetic longitude increasing from 93.3° at RVK to 106.1° at OUJ. The black/cyan arrow represents the footpoints of VAP-A/ERG satellite moving towards low/high latitudes between 13:40 UT and 15:00 UT with the diamond marking plasmapause crossing. b Amplitude (black) and phase difference (blue) of the N component of the blue chain plotted as a function of geomagnetic latitude. c Bandpass filtered (1.4 ± 0.5 mHz) N component (north) of the geomagnetic field perturbations. d Bandpass filtered (1.4 ± 0.5 mHz) Z component (vertical) of the geomagnetic field perturbations. Clear poleward propagation of the waves is highlighted by the dashed lines. e Bandpass filtered (1.4 ± 0.5 mHz) E component (east) of the geomagnetic field perturbations. Clear sunward/westward propagation of the waves is highlighted by the dashed lines.