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In this paper we present two substorm events with coordinatedground-based and in-situ THEMIS observations, and focus our interest on thewave activities in Pi1 and Pi2 bands from minutes before the substormexpansion phase (EP) onset to minutes after the local current disruption(CD). We find that Pi2 band (40–100 s) wave appears 1–2 min before thesubstorm onset and last over the entire EP interval, while higher-frequencywave within Pi1 band (10–30 s) emerges within few tens of seconds after theEP onset, intensifies during the local CD, and fades afterwards. Thepre-onset Pi2 waves are attributed to a ballooning mode which acts as theseed perturbation to the substorm EP onset process. The azimuthal wavenumberestimated from the Doppler shift nature of the ballooning mode is consistentwith the longitudinal "wavelength" inferred from the onset auroralstructures. The Pi1 waves appearing within few tens of seconds after the EPonset are interpreted as supportive of a two-fluid instability mode of thincurrent sheet investigated in an accompanying paper (Liu and Liang, 2009).During the local CD, broadband wave activities from Pi2 band to well abovethe ion gyrofrequency are observed, suggesting the coexistence of variousplasma instabilities featuring different frequency ranges

In-situ observation of ULF wave activities associated with substorm expansion phase onset and current disruption