Abstract
Abstract. Whistler-mode chorus is a structured wave emission observed in the Earth's magnetosphere in a frequency range from a few hundreds of Hz to several kHz. We investigate wave packets of chorus using high-resolution measurements recorded by the WBD instrument on board the four Cluster spacecraft. A night-side chorus event observed during geomagnetically disturbed conditions is analyzed. We identify lower and upper frequencies for a large number of individual chorus wave packets inside the chorus source region. We investigate how these observations are related to the central position of the chorus source which has been previously estimated from the Poynting flux measurements. We observe typical frequency bandwidths of chorus of approximately 10% of the local electron cyclotron frequency. Observed time scales are around 0.1 s for the individual wave packets. Our results indicate a lower occurrence probability for lower frequencies in the vicinity of the central position of the source compared to measurements recorded closer to the outer boundaries of the source. This is in agreement with recent research based on the backward wave oscillator theory.
Highlights
Whistler-mode chorus is generated by a nonlinear mechanism involving wave-particle interactions with energetic electrons (Helliwell, 1967; Tsurutani and Smith, 1974; Nunn et al, 1997; Trakhtengerts, 1999)
Trakhtengerts et al (2007) showed that the spectrum of chorus wave packets lacks lower frequencies close to the center of the source region and explained this effect on the basis of the backward wave oscillator model (Trakhtengerts, 1995, 1999; Trakhtengerts and Rycroft, 2000; Trakhtengerts et al, 2004). This model links the frequency of chorus wave packets to the parallel component of the velocity of a steplike deformation of the electron distribution function. This velocity decreases as the resonant electrons move along the magnetic field lines through the chorus source region
The results indicate a tendency of slightly lower bandwidths for chorus wave packets observed close to the central position of the source
Summary
Trakhtengerts et al (2007) showed that the spectrum of chorus wave packets lacks lower frequencies close to the center of the source region and explained this effect on the basis of the backward wave oscillator model (Trakhtengerts, 1995, 1999; Trakhtengerts and Rycroft, 2000; Trakhtengerts et al, 2004). This model links the frequency of chorus wave packets to the parallel component of the velocity of a steplike deformation of the electron distribution function. We analyze properties of chorus wave packets as a function of the position inside the generation region relative to the direction of the Poynting flux
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