Dispersion surface and electromagnetic responses of the magnetosonic/whistler mode in the high-β collisionless plasmas

Abstract

Near-parallel and highly oblique magnetosonic/whistler (M/W) mode waves are frequently observed in solar wind and interplanetary shocks. These observed waves are usually analyzed by the plasma kinetic and fluid theories. This study compares the dispersion surface and electromagnetic responses of the M/W mode wave under both kinetic and fluid theories in the plasmas with βp = βe ≃ 1 in detail. In comparison with the kinetic theory, it proposes that the hot fluid theory is suitable to obtain the dispersion relation, the ratio of the parallel to perpendicular magnetic fluctuation Bz/B⊥, and the ratio of the electric to magnetic fluctuation E/B. Except the near-parallel low-frequency M/W mode, the hot fluid theory would be used to calculate the ratio between two magnetic fluctuations perpendicular to the wave vector B1/B2, the magnetic helicity σ, and the ratio of the parallel to perpendicular electric fluctuation Ez/E⊥ of the M/W mode waves. The cold fluid theory is good at describing the magnetic responses including Bz/B⊥, B1/B2, and σ, but it underestimates ω, E/B, and Ez/E⊥ for the obliquely propagating M/W mode wave. Furthermore, both hot and cold fluid theories can obtain the ratio of the longitudinal to transverse electric fluctuation relative to the wave vector EL/EP of the obliquely propagating low-frequency M/W mode. These results provide an indicator about how to choose the adequate theory to analyze the observations of the M/W mode waves in the solar wind and interplanetary shocks. Besides, from the dispersion surface in kinetic theory, it shows that the relative weak damping arises for the low-frequency branch at the angle around 0°, 60°, and 90° and for the high-frequency branch with ω ∼ [ωcp, 300ωcp] at the normal angle smaller than 60°, which indicates the possible angle and frequency of the freely propagating M/W mode waves.