Very-low-frequency saucers observed on DEMETER

摘要

Observations of very-low-frequency saucers by the electric field instrument (Instrument Champ Electrique, ICE) aboard the DEMETER spacecraft have added new evidence about the nature of the highly localized source of this radiation. DEMETER orbited sun-synchronously at altitudes around 660 km, significantly below those at which earlier spacecraft detected saucers. Also, DEMETER data establish the existence of saucer sources in the dayside ionosphere. Frequency-time slopes of saucers in DEMETER spectrograms have been analyzed with two-dimensional ray tracing. To produce such slopes requires long vertical separations between the source and the spacecraft, in some cases much greater than the height of the spacecraft above ground. It is concluded that the sources lie above the satellite and radiate downward. Bidirectional radiation patterns and the broadband quasielectrostatic whistler mode energy spectrum are consistent with the published results of simulations of nonlinear two-stream instabilities.