This chapter reviews historical ground-based observations of ultra-low-frequency (ULF) waves tied to substorms, and highlights new research linking these ULF waves explicitly to substorm onset itself. There are several robust methods that can be used to determine the characteristics of a nonstationary time series such as the ULF magnetic field traces observed in the auroral zone during substorms. These include the pure state filter, the Hilbert-Huang transform, and wavelet analysis. The first indication of a substorm is a sudden brightening of one of the quiet arcs lying in the midnight sector of the oval. The chapter focuses on the properties of ULF waves that are seen in two-dimensional images of auroral intensity near substorm expansion phase onset. It also discusses a wider range of magnetotail instabilities that could be responsible for the azimuthally structured auroral forms at substorm onset
