Wavelet-based ULF wave diagnosis of substorm expansion phase onset

摘要

[1] Using a discrete wavelet transform with a Meyer wavelet basis, we present a newquantitative algorithm for determining the onset time of Pil and Pi2 ULF waves in thenightside ionosphere with ~20- to 40-s resolution at substorm expansion phase onset. Wevalidate the algorithm by comparing both the ULF wave onset time and location to theoptical onset determined by the Imager for Magnetopause-to-Aurora Global Exploration(IMAGE)—Far Ultraviolet Imager (FUV) instrument. In each of the six events analyzed,five substorm onsets and one pseudobreakup, the ULF onset is observed prior to theglobal optical onset observed by IMAGE at a station closely conjugate to the optical onset.The observed ULF onset times expand both latitudinally and longitudinally away from anepicenter of ULF wave power in the ionosphere. We further discuss the utility of thealgorithm for diagnosing pseudobreakups and the relationship of the ULF onset epicenterto the meridians of elements of the substorm current wedge. The importance of thetechnique for establishing the causal sequence of events at substorm onset, especially insupport of the multisatellite Time History of Events and Macroscale Interactions DuringSubstorms (THEMIS) mission, is also described.