A Self-Consistent Marginally Stable State for Parallel Proton-Cyclotron Waves

Physics – Plasma Physics

Scientific paper

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[7827] Space Plasma Physics / Kinetic And Mhd Theory, [7829] Space Plasma Physics / Kinetic Waves And Instabilities, [7867] Space Plasma Physics / Wave/Particle Interactions

Scientific paper

Ion cyclotron waves propagating through a plasma nearly parallel to the large-scale magnetic field act to fundamentally govern the structure of the ion distribution. In the solar wind, proton distributions are commonly observed in shapes interpreted as quasilinear plateaus caused by an essentially complete resonant cyclotron scattering of the protons by the cyclotron waves. The instability of these waves in sufficiently anisotropic plasmas may also play a part in regulating the evolution of the solar wind ion distributions inside 1 AU. However, the form of the marginally stable state which specifies the plateaus and instability thresholds has never been self-consistently determined. Typically, an assumption is made about the proton distribution - that it is bi-Maxwellian or some other specific shape - and the dispersion relation appropriate to that plasma is calculated, yielding wave frequencies and growth/damping rates. These assumed distributions are not marginally stable, since the growth/damping rates are not zero for all wavenumbers. Finite growth/damping rates mean that the assumed distributions will evolve on quasilinear time scales in response to the generation or absorption of the waves, so they cannot be maintained in a steady state. We derive a self-consistent marginally stable state, consisting of a proton distribution which transmits dispersive proton cyclotron waves without growth or damping. The dispersion relation of these waves is obtained as a function the plasma beta of the protons. The anisotropy of the marginally stable distributions rigorously defines the threshold for the proton anisotropy instability.

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