THEMIS observations of electron cyclotron harmonic emissions, ULF waves, and pulsating auroras

We present multiprobe, multi‐instrument observations of the electron cyclotron harmonic (ECH) emissions and ultralow‐frequency (ULF) waves from Time History of Events and Macroscale Interactions during Substorms (THEMIS) and explore their potential linkage to the concurrent ground‐observed pulsating auroras (PsA) on 4 January 2009. The ECH emissions were observed as discrete packets modulated by the ULF flapping motion of the neutral sheet around the probes. Combining different data sets of the ECH observations, we infer that the ECH emission intensities were strongly fluctuating and contained multi‐time scale fine structures. The distribution of PsA patches featured longitudinal “wavelength” in concert with the in situ ULF wave characteristics inferred from a cross‐phase analysis. The overall activeness of the PsA correlated with the in situ‐measured energetic electron fluxes and ECH wave intensities. We suggest that ECH waves played the key role in the pitch angle diffusion of the plasma sheet electrons that led to the PsA, while the ULF waves structured the plasma sheet and imposed a macroscopic effect over the spatial distribution of the PsA.