Dependence of the Sweep Rate of Whistler-mode Chorus Emissions on the Plasma Density

Whistler-mode chorus consists of intense electromagnetic wave packets generated by a nonlinear mechanism involving wave-particle interactions. Chorus wave packets are discrete frequency-time structures in a frequency range from a few hundreds of Hz to several kHz changing their frequency on a time scale from a few tenths of seconds to a few seconds. The source region of chorus emissions is localised close to the geomagnetic equatorial plane. Our investigation is based on multipoint measurements of the wideband (WBD) plasma wave instruments on board the four Cluster spacecraft. We investigate the sweep rate of the chorus frequency as a function of the cold plasma density in the equatorial plane and then we compare it with theoretical and simulation results. A theoretical model, numerical simulations and also our preliminary results from the measurements predict an increasing sweep rate with decreasing cold plasma density. Introduction Whistler-mode chorus emissions are among the most intense naturally occurring electromagnetic waves in the Earth’s magnetosphere. The whistler-mode is named after lightning generated ’whistlers’ which propagate in this plasma mode of propagation [Helliwell, 1965]. Chorus emissions consist of discrete elements (wave packets). Chorus wave packets are observed more frequently as rising tones (risers) with increasing sweep rate (typically few kHz/s), but falling tones (fallers) are also observed. The duration of a chorus element varies between a few milliseconds and a few seconds. The generation process is still not well known, although properties of chorus emissions have been studied for more than fifty years (Storey, [1953]). Intense chorus emissions are known to originate from the injection of substorm electrons [Tsurutani and Smith, 1974] close to the geomagnetic equator outside the plasmasphere, through the cyclotron resonance loss cone instability [Andronov and Trakhtengerts, 1964; Kennel and Petschek, 1966]. The source of chorus emissions is now known to be located within a few degrees of the magnetic equator and the estimate of the characteristic scale of the source region parallel WDS'07 Proceedings of Contributed Papers, Part II, 71–80, 2007. ISBN 978-80-7378-024-1 © MATFYZPRESS