Variations in the ultra-low frequency (ULF) range, with periods from 0.1 to 600 seconds, were noted in the Earth's magnetic field long before they were understood to be indicators of plasma processes in the Earth's space environment. They are now also recognized as a significant means of dissipating solar wind and/or magnetospheric energy and as a primary means of coupling magnetospheric and ionospheric processes. ULF waves also constitute the source field for electromagnetic induction studies of the structure and motions of conductive bodies in the Earth. Knowledge of the spatial and temporal variation of these waves thus can aid in our understanding of properties of both Earth's external plasma environment and its underlying oceans, crust, and mantle.
This report focuses on waves and transients that perturb the Earth's magnetic field. It emphasizes areas in which exceptional progress has been made, or in which new ideas or observations have raised significant questions. Space does not allow coverage of ULF wave studies at other planets, or even of many important facets of ULF wave studies in the Earth's space environment. Interested readers are referred to the more complete reviews of much of this field contained in several papers found in the recent AGU Monograph Solar Wind Sources of Magnetospheric Ultra-Low-Frequecy Waves [ Engebretson et al., 1994a], including especially the introductory tutorial by Hughes [1994], and to an earlier ULF review article by Takahashi [1991]. A summary of related issues in all areas of geomagnetic studies (but with no references) can be found in the report titled The National Geomagnetic Initiative [1993], recently issued by the U. S. National Research Council.