Interpretation of Cluster data on chorus emissions using the backward wave oscillator model

Abstract

The measurements of chorus emissions by four closely separated Cluster spacecraft provide important information concerning the chorus generation mechanism. They confirm such properties of the wave source as their strong localization near the equatorial cross section of a magnetic flux tube, an almost parallel average wave-vector direction with respect to the geomagnetic field, and an energy flux direction pointing outward from the generation region. Inside this region, Cluster discovered strong temporal and spatial variations in the amplitude with correlation scale lengths of the order of 100 km across the magnetic flux. The wave electric field reached 30 mV/m, and the maximum growth and damping rates are of the order of a few hundreds of s−1. These and other properties of the detected chorus emissions are discussed here in relation with the backward wave oscillator mechanism. According to this mechanism, a succession of whistler wave packets is generated in a small near-equatorial region with temporal and spatial characteristics close to the Cluster data. Amplitudes and frequency spectra, as well as dynamical features of the Poynting flux of chorus are estimated and compared with the Cluster measurements.

Dynamics of the Magnetospheric Cyclotron ELF/VLF Maser in the Backward-Wave-Oscillator Regime. I. Basic Equations and Results in the Case of a Uniform Magnetic Field

“…A linear theory of the magnetospheric BWO considered in [7,10] allows one to find the threshold flux of energetic electrons and the instability growth rate for a given excess over the threshold. Qualitative consideration of the nonlinear stage of the instability [8] and comparison with statistical properties of chorus emissions detected onboard the satellite Magion 5 [3] and with the Cluster spacecraft data, obtained in the generation region [11], showed that the BWO model can explain both the typical amplitude and frequency drift of chorus elements.…”

“…As for the specific features of the chorus-spectrum formation, a closed self-consistent scenario of this process has been proposed fairly recently [7][8][9][10][11]. This model is based on the following experimentally established properties of chorus emissions:…”

New results of investigations of whistler-mode chorus emissions

“…Properties of the observed chorus emissions are compared by Trakhtengerts et al (2004) with the theoretical properties of the BWO mechanism. According to this theory, a succession of whistler wave packets is generated in a nearequatorial source region with temporal and spatial characteristics close to those observed by the four CLUSTER spacecraft.…”

Frequencies of wave packets of whistler-mode chorus inside its source region: a case study

“…Chorus typically has a discrete structure of separate wave packets (elements) in a frequency range from a few hundreds of Hz to several kHz (see reviews by Omura et al, 1991;Sazhin and Hayakawa, 1992, and references therein). 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(Trakhtengerts, , 1999Trakhtengerts 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.…”