Beam-excited whistler waves at oblique propagation with relation to STEREO radiation belt observations

Abstract. Isotropic electron beams are considered to explain the excitation of whistler waves which have been observed by the STEREO satellite in the Earth's radiation belt. Aside from their large amplitudes (~240 mV/m), another main signature is the strongly inclined propagation direction relative to the ambient magnetic field. Electron temperature anisotropy with T_e⊥>T_e||, which preferentially generates parallel propagating whistler waves, can be excluded as a free energy source. The instability arises due to the interaction of the Doppler-shifted cyclotron mode ω=−Ω_e+kV_bcosθ with the whistler mode in the wave number range of kc/ω_e≤1 (θ is the propagation angle with respect to the background magnetic field direction, ω_e is the electron plasma frequency and Ω_e the electron cyclotron frequency). Fluid and kinetic dispersion analysis have been used to calculate the growth rate of the beam-excited whistlers including the most important parameter dependencies. One is the beam velocity (V_b) which, for instability, has to be larger than about 2V_Ae, where V_Ae is the electron Alfvén speed. With increasing V_Ae the propagation angle (θ) of the fastest growing whistler waves shifts from θ~20° for V_b=2V_Ae to θ~80° for V_b=5V_Ae. The growth rate is reduced by finite electron temperatures and disappears if the electron plasma beta (β_e) exceeds β_e~0.2. In addition, Gendrin modes (kc/ω_e≈1) are analyzed to determine the conditions under which stationary nonlinear waves (whistler oscillitons) can exist. The corresponding spatial wave profiles are calculated using the full nonlinear fluid approach. The results are compared with the STEREO satellite observations.