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Electrodynamics of a substorm‐related field line resonance observed by the Polar satellite in comparison with ground Pi2 pulsations

[1] Observations from the Polar satellite during a substorm on 29 August 1996 are used to characterize electrodynamic properties of a nightside field line resonance (FLR). A negative H bay and Pi2 pulsations were simultaneously observed at midlatitude SAMNET ground stations (L = 2.6 − 4.5, ∼0000 MLT). During the pulsations, Polar was moving outbound on a 0330 MLT meridian and crossing L shells from 4 to 5.3. While in the central plasma sheet and outside the plasmasphere, Polar recorded toroidal standing waves for ∼20 min starting simultaneously with the ground pulsations. The magnetic-field-aligned Poynting flux associated with the FLR was ≤4 μW m−2. Whereas on the ground at L ≤ 4, the Pi2 frequency remained nearly constant and was similar for all stations, the toroidal frequency of the in situ FLR (L ≥ 4) decreased as the satellite crossed field lines with increasing L value. Although this L dependence of frequency of in situ FLR has been inferred in previous studies using multiple satellites at different local times, the 29 August 1996 event directly confirms this frequency dependence for substorm-related FLR over a large L range on the same meridian. Furthermore, ground (L ≤ 4) and satellite (L ≥ 4) observations allow for the possibility that the oscillations were manifestations of two different magnetospheric oscillation modes, resulting from the same initial magnetospheric disturbance.