Low-frequency waves in a high-beta collisionless plasma: polarization, compressibility and helicity

The linear theory of waves near and below the ion cyclotron frequency in an isothermal electron-ion Vlasov plasma which is isotropic, homogeneous and magnetized is considered. Numerical solutions of the full dispersion equation for the magnetosonic whistler and Alfven/ion cyclotron modes at βsub(i)=1.0 are presented, and the polarizations, compressibilities, helicities, ion Alfven ratios and ion cross-helicities are exhibited and compared. At sufficiently large βsub(i) and theta, the angle of propagation with respect to the magnetic field, the real part of the polarization of the Alfven/ion cyclotron wave changes sign, so that, for such parameters, this mode is no longer left-hand polarized. The Alfven/ion cyclotron mode becomes more compressive as the wavenumber increases, whereas the magnetosonic/whistler becomes more compressive with increasing theta. At oblique propagation, the helicity of both modes approaches zero in the long-wavelength limit; in contrast, the ion cross-helicity is of order unity for the Alfven/ion cyclotron wave and decreases as theta increases for the magnetosonic/whistler mode. (author)