Upstream gyrating ion events: Cluster observations and simulations.

Localized events of low-frequency quasi-monochromatic waves in the 30s range observed by Cluster in the upstream region of Earth are analyzed. They are associated with a gyro-motion of the two ion populations consisting of the incoming solar wind protons and the back-streaming ions from the shock. A coordinate system is chosen in which one axis is parallel to the ambient magnetic field B0 and the other one is in the vsw×B0 direction. The variation of the plasma parameters is compared with the result of two-fluid Hall-MHD simulations using different beam densities and velocities. Keeping a fixed (relative) beam density (e.g. α=0.005), non-stationary ‘shock-like’ structures are generated if the beam velocity exceeds a certain threshold of about ten times the Alfven velocity. Below the threshold, the localized events represent stationary, nonlinear waves (oscillitons) in a beam-plasma system in which the Reynold’s stresses of the plasma and beam ions are balanced by the magnetic field stress. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]