The Transverse Dimensions of Whistler-Mode Chorus Emissions in the Source Region

Physics

Scientific paper

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2772 Plasma Waves And Instabilities, 6984 Waves In Plasma, 7871 Waves And Instabilities

Scientific paper

High-resolution electric field waveforms from the Wideband Plasma Wave Instrument (WBD) on the four Cluster spacecraft are used to study the spatial scale of whistler-mode chorus emissions. From a variety of measurements it is now known that chorus is generated very close to the magnetic equator, typically within 3 to 5 degrees of the magnetic equator. On April 18, 2002, a nearly continuous series of chorus observations were obtained as the four Cluster spacecraft passed south to north through the source region at about 4.4 Earth radii. The separation distances on this pass were unusually small, less than about 250 km along the magnetic field, and less than about 75 km transverse to the magnetic field. The six transverse baseline distances were also well distributed, from a minimum of 7 km to a maximum of 72 km. By computing the linear correlation coefficient of the chorus intensities across these six transverse baselines, the transverse size of the chorus wave packets can be estimated. A statistical analysis shows that the average transverse half-width of the chorus wave packets, based on a Gaussian fit, is about 35 km. This transverse scale size is quite small, implying that the transverse size of the source is also comparably small, only a few wavelengths. The small size of the chorus source region implies that the waves cannot be emitted in a narrow beam, but rather must be emitted over a substantial range of wave vector directions. The relatively broad beam width has important implications for the subsequent temporal evolution of the wave packet, and may explain why small but easily observable frequency shifts are sometimes observed fin the chorus elements detected by the various spacecraft.

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