Self-Consistent Ion Cyclotron Anisotropy-Beta Relation for Solar Wind Protons

摘要

Proton distributions with perpendicular anisotropies above a beta-dependent threshold trigger the quasilinear proton cyclotron instability, causing the generation of ion cyclotron waves and the scattering of protons to lower anisotropy. However, observations of solar wind proton distributions at 1 AU imply that this instability is ineffective in limiting the proton anisotropy. Here, we resolve this puzzle. We first point out that the commonly used estimates of the ion cyclotron threshold are based on bi-Maxwellians which are never marginally stable. Second, we derive a selfconsistent marginally stable state of protons and parallel ion cyclotron waves which yields true threshold anisotropies which do in fact bound the solar wind data.