Observational evidence for marginal stability of solar wind ion beams

Solar wind proton double streams and proton-alpha differential streaming may provide sufficient free energy to drive field-aligned magnetosonic waves unstable. We carry out a statistical analysis of this electromagnetic beam instability based on measured Helios ion distributions. Observational evidence is indeed found for unstable distributions, whereby the largest growth rates are obtained in the collisionless high-speed wind. Alpha particles by themselves are not able to excite these waves but mostly tend to stabilize an existing proton double stream configuration by enhancing the cyclotron damping of the main proton population.