Estimation of the Chorus Group Velocity from THEMIS Wave Observations
Résumé
Chorus waves can play an important role for the energy budget of Earth's radiation belts. Some nonlinear analytical models describing chorus generation and energy transfer between waves and energetic electrons need the wave group velocity as a model parameter. The group velocity is the propagation velocity of the amplitude envelope of a wave packet. Theoretically, its absolute value is derived from a derivative of frequency (om) with respect to wave number (k), i.e. v_g = dₒm/dₖ. It is difficult, if not impossible, to infer this quantity directly from electromagnetic wave observations in space. We propose to take a "detour" over the Poynting velocity (vₚ), which is related to the Poynting vector (S) and the average wave energy density (W). Both, S and W can be inferred from a combination of electric and magnetic signals measured by triaxial antenna systems. We demonstrate the concept and show first results from an application to chorus observations made by the THEMIS spacecraft.