ABSTRACT
This chapter provides a brief history of observations of ultra-low-frequency (ULF) magnetic oscillations or waves in the solar wind and geomagnetic field. It provides and outline of how they came to be understood as Magnetohydrodynamic (MHD) waves. There is a considerable degree of MHD wave activity in the solar wind. Much of this is MHD turbulence but there are a number of observations of coherent waves. With a large number of observations and some theoretical tools becoming available, the stage was now set for quantitative physical understanding. The substantial advances that have been made in the understanding of the nature of ULF pulsations that have come about as the result of major advances in experimental technique proceeding hand-in-hand with theoretical progress. Sophisticated spacecraft measurements, linked with observations by large arrays of ground-based instruments, have allowed the temporal and spatial behaviour of many types of phenomenon to be unravelled.
