Alfvén waves in the vicinity of Uranus: The role of energetic proton thermal anisotropy

Spacecraft observations in the vicinity of planetary bow shocks indicate the existence of proton beams and of magnetohydrodynamic (MHD) waves. We analyze the dispersion of such waves in the presence of energetic beams and show that the role of temperature anisotropy in the growth of low‐frequency MHD waves is significantly enhanced by weak relativistic effects. The present analysis of ion beam instability near Uranus shows that thermal anisotropy of energetic protons coupled with the relativistic dependence of their mass on energy leads to an increase of the resonant Alfvénic wave growth rate and to a dispersion curve which differs from that of the isotropic case. MHD‐like waves, excited by energetic proton beams near the planetary bow shock have (in the presence of thermal anisotropy) a dispersion relation similar to that of Alfvén waves at wavelengths of the order of 2 × 10 8 m and can propagate in the solar wind plasma away from Uranus.