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

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.