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The generation mechanism for magnetosheath lion roars

Abstract

‘Lion roars’ are intense electromagnetic whistler-mode waves which occur throughout the magnetosheath1. The emissions have a mean frequency near a few tenths of the electron gyrofrequency, a typical amplitude of 0.1 nT and occur in bursts with an average duration of a few seconds2. When played through a loudspeaker the emissions sound like a lion roaring, hence the name. We report here that analysis of simultaneous wave and plasma data obtained on the ISEE-1 satellite indicates that the waves can originate by cyclotron resonant instability with anisotropic magnetosheath electrons whenever the magnetic energy per particle, B2/8πN, falls to values comparable with the electron thermal energy. The variation in this critical energy for instability seems to be associated with the hydromagnetic mirror instability which in turn is excited by the observed pressure anisotropy in the magnetosheath ion population.

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Thorne, R., Tsurutani, B. The generation mechanism for magnetosheath lion roars. Nature 293, 384–386 (1981). https://doi.org/10.1038/293384a0

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