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 provideimportant information concerning the chorus generation mechanism. They confirm such propertiesof the wave source as their strong localization near the equatorial cross section of a magnetic fluxtube, an almost parallel average wave-vector direction with respect to the geomagnetic field, and anenergy flux direction pointing outward from the generation region. Inside this region, Clusterdiscovered strong temporal and spatial variations in the amplitude with correlation scale lengths ofthe order of 100 km across the magnetic flux. The wave electric field reached 30 mV/m, and themaximum growth and damping rates are of the order of a few hundreds of s21. These and otherproperties of the detected chorus emissions are discussed here in relation with the backward waveoscillator mechanism. According to this mechanism, a succession of whistler wave packets isgenerated in a small near-equatorial region with temporal and spatial characteristics close to theCluster data. Amplitudes and frequency spectra, as well as dynamical features of the Poynting fluxof chorus are estimated and compared with the Cluster measurements.