An Observation Linking the Origin of Plasmaspheric Hiss to Discrete Chorus Emissions

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Science  08 May 2009:
Vol. 324, Issue 5928, pp. 775-778
DOI: 10.1126/science.1171273

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Chorus Hissing

Plasmaspheric hiss, a type of unstructured broadband, low-frequency radio emission, has long been known to exist in Earth's plasmasphere, but its origin has been uncertain. The source of hiss could be a different type of radio wave, called chorus, which originates outside the plasmasphere during geomagnetic storms. Both types of radio wave influence the behavior of energetic electrons in the near-Earth space environment, with implications for spacecraft and astronaut safety, but a correlation between the two has been difficult to establish experimentally. Recently, two of the five satellites of the THEMIS constellation were fortuitously able to record 4 minutes of electromagnetic wave data at high resolution during geomagnetically active conditions, detecting both chorus and hiss. An analysis of the data by Bortnik et al. (p. 775; see the Perspective by Santolik and Chum) revealed that the two sets of waves were well correlated, with hiss lagging behind chorus as expected, implying that one indeed evolved into the other.


A long-standing problem in the field of space physics has been the origin of plasmaspheric hiss, a naturally occurring electromagnetic wave in the high-density plasmasphere (roughly within 20,000 kilometers of Earth) that is known to remove the high-energy Van Allen Belt electrons that pose a threat to satellites and astronauts. A recent theory tied the origin of plasmaspheric hiss to a seemingly different wave in the outer magnetosphere, but this theory was difficult to test because of a challenging set of observational requirements. Here we report on the experimental verification of the theory, made with a five-satellite NASA mission. This confirmation will allow modeling of plasmaspheric hiss and its effects on the high-energy radiation environment.

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