Pi2 pulsations observed with the Polar satellite and ground stations: Coupling of trapped and propagating fast mode waves to a midlatitude field line resonance

Simultaneous measurements from the Polar satellite and several ground stations of two substorm-related Pi2 pulsation events (separated by ∼6 min) provide evidence for radially trapped and propagating fast mode waves and a coupled field line resonance (FLR). The Pi2 pulsations were observed at five ground stations located between 2130 and 2330 magnetic local time (MLT) ranging from L=1.83 to 3.75, which showed nearly identical waveforms in the H component with a frequency of ∼20 mHz. Five additional ground stations located between L=4.48 and ∼15 (on similar meridians) recorded weaker less-correlated signals. The pulsations were also detected both simultaneously and with a time delay of ∼38 s at two low-latitude stations (L=1.17 and 1.23) on the dayside at ∼1030 and ∼0612 MLT, respectively, indicating the global extend of the pulsations. The nightside ground data showed an amplitude maximum and a phase reversal in the H component between L=3.4 and 3.75. During the oscillations the Polar satellite moved (on the same meridian as the ground stations in the nightside) from 14° to 10° magnetic latitude and from L=4.1 to 3.7. Electric and magnetic field measurements also showed two Pi2 pulsation events (∼20 mHz) in both the compressional (B_z and E_y) and transverse (B_y and E_x) mode components with waveforms almost identical to the ground signals. Whereas the first Pi2 had a standing wave structure in the compressional mode, the second Pi2 was a propagating wave. Both Pi2s had standing wave signatures in the transverse mode. The amplitude of the compressional magnetic field component (B_z) was ∼40% of that of the azimuthal component (B_y). Although the two Pi2 events showed equal amplitudes in the H component of ground data, Polar recorded much larger oscillations in the azimuthal magnetic field component (B_y) of the first Pi2 event; the fast mode amplitude (B_z) was nearly unchanged for both Pi2s. This suggests that Polar was at or near a localized FLR excited by the oscillations of the fast mode waves. During the in situ FLR observation, Polar's footpoint was closest to the ground stations which recorded the amplitude maximum and the phase reversal in the H component.