Multi-ion space plasma research: Beam-driven oscillitons as origin of coherent waves and gyrating ions

The presence of several ion populations which are in relative drift is a common feature in space plasmas. Cometary tails and the magnetospheres of unmagnetized planets as Venus and Mars are complex multi-ion plasmas which result from the interaction of the solar wind with extended sources of heavy ions. A characteristic signature of such plasmas is pronounced plasma structuring in form of rays, bunches, condensations, etc. which are considered as manifestation of nonlinear waves arising from different types of instabilities. In case of an ion beam which propagates parallel to the ambient magnetic field, e.g., right-hand polarized waves may resonantly excited. 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 of all parameters have been found which arise from the momentum coupling between the main plasma population and the beam ions via the low-frequency electromagnetic field. They have similar signatures as the recently described oscillitons of linearly stable two-ion plasmas where effective momentum coupling may happen in the vicinity of the cross-over frequencies. Two applications are considered: One are the coherent waves seen upstream of the Earth's bow shock in association with gyrophase-bunched ions which are linked to shock-reflected ions and the other are the large-amplitude upstream waves at Mars near the proton cyclotron frequency which are obviously caused by relative streaming between solar wind protons and protons of exospheric origin.