Variations in the chorus source location deduced from fluctuations of the ambient magnetic field: Comparison of Cluster data and the backward wave oscillator model

Abstract

We study the motion of the source region of magnetospheric chorus emissions using multipoint measurements of VLF wave emissions and geomagnetic field onboard the Cluster spacecraft. The geomagnetic field data are matched to a parameterized model of the local magnetic field, and the spatiotemporal dynamics of the magnetic field are obtained on this basis. The wave data from the Wide Band Data instrument are used to obtain the power spectral density and number of chorus elements. Comparison of these data shows that the chorus remains related to the magnetic field minimum, while the position of this minimum can vary rather strongly during periods of enhanced geomagnetic activity. These results support the backward wave oscillator (BWO) model of chorus emissions, which attributes chorus generation to an absolute instability of whistler mode waves in the presence of a step-like velocity distribution of energetic electrons. Such an instability takes place in a small vicinity of the local "magnetic equator" of a magnetic field line. Quantitative agreement between the data and the model is demonstrated by variation of the statistical chorus characteristics with changing of the deduced BWO parameters.