PerspectivePlanetary Science

The Origin of Plasmaspheric Hiss

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Science  08 May 2009:
Vol. 324, Issue 5928, pp. 729-730
DOI: 10.1126/science.1172878

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The role of electromagnetic waves in shaping the space environment around our planet has been studied since the early 1960s (1, 2). Initial analysis of these waves at audible frequencies consisted of playing the recorded data through a loudspeaker. The historical terminology in this field thus resembles an experimental musical score where we can encounter whistlers, noise, hiss, and chorus. On page 775 of this issue, Bortnik et al. (3) invite us to this world of “space sound.” On the basis of measurements by NASA's THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft mission, the authors describe two types of natural electromagnetic waves: chorus and plasmaspheric hiss. They show that plasmaspheric hiss can be interpreted as arising from transformed chorus waves, thus providing important clues as to its origin.