Multi-spacecraft observations of a fundamental toroidal ULF wave event

Field line resonance (FLR) is an important source of Ultra-low-frequency (ULF) waves observed in the inner magnetosphere. In this study, we present multi-spacecraft observations of a toroidal ULF wave, detected by the Arase, Van Allen Probe-A (VAP-A), and GOES-14. During the event, Arase and VAP-A were positioned on two magnetic field lines in close proximity, providing a rare opportunity to examine the latitudinal structure of FLR. The temporal profiles of these toroidal components exhibited distinct, separated wave packets, with each packet persisting for ~10 minutes and one-to-one correspondence in timing and amplitude across all three spacecraft. The observed waveforms resemble transient toroidal waves associated with impulsive disturbances in the near-Earth magnetotail. These toroidal waves are identified as fundamental waves based on phase differences and harmonic eigenfrequencies. The oscillations observed in the residual flux of protons, along with the bump-on-tail structure, suggest that the waves are likely generated by drift resonance.