“Caterpillar” ULF Waves in the Dusk Side Sub‐Auroral Region: Conjugate Observations With SuperDARN, Arase and Ground Magnetometers

On 23 November 2022, during exceptionally quiet geomagnetic conditions, a distinct ultra‐low frequency (ULF) wave in the Pc5 band (∼2.4 mHz) was observed in the duskside sub‐auroral region by two Canadian SuperDARN (Super Dual Auroral Radar Network) radars. The wave exhibited a periodic Doppler velocity signature resembling a “caterpillar,” characterized by anti‐sunward propagation and an azimuthal wave number of ∼12. Ground‐based magnetometer data revealed latitudinal variations in wave amplitude and phase consistent with field line resonance, peaking at ∼66° magnetic latitude. The Arase satellite, whose ionospheric footprint traversed the region of the caterpillar ULF wave, detected toroidal oscillations in electric and magnetic fields at magnetically conjugate locations in the inner magnetosphere, with high coherence and phase consistency with ground‐based observations. Arase observed the electric field variation preceding the magnetic field variation by ∼45°, which is unusual for a pure standing mode and suggests a departure from a simple standing‐wave interpretation. Instead, the observations can be interpreted as mixed standing and propagating characteristics along the field line. Simultaneously, the clear anti‐sunward azimuthal phase propagation and sustained wave activity suggest a driving mechanism by a large‐scale surface perturbation at the magnetospheric flank. These properties are consistent with excitation by the Kelvin‐Helmholtz instability. A minor enhancement in solar wind dynamic pressure was observed, potentially contributing to the wave onset. This study highlights SuperDARN's capability to detect ULF waves during quiet intervals and demonstrates the utility of multi‐instrumental conjugate observations in revealing the spatiotemporal variations of ULF waves and their generation mechanisms.