Oscillitons and gyrating ions in a beam-plasma system

The resonant excitation of right-hand polarized waves by an ion beam propagating parallel to the ambient magnetic field is a well-known effect of linear dispersion theory. In this paper, stationary nonlinear structures in such a beam-plasma system are studied on the basis of Hall-MHD description. Soliton-like structures with superimposed spatial oscillations have been found which arise from the momentum coupling between the main plasma population and the beam ions via the LF electromagnetic field. They have similar signatures as the recently described oscillitons of linearly stable two-ion plasmas [Sauer et al., 2001] where effective momentum coupling may happen in the vicinity of their cross-over frequencies. Our message is that gyrophase-bunched ions seen upstream of the Earth's bow shock in association with ULF wave packets may find an explanation by oscillitons in a beam-plasma system.