Theoretical analysis of Poynting flux and polarization for ELF‐VLF electromagnetic waves in the Earth's magnetosphere

Abstract

We discuss properties of extremely low‐frequency (ELF) and very low‐frequency (VLF) electromagnetic waves in the Earth's magnetosphere. General expressions for wave magnetic field polarization and for the angle θP between the direction of the Poynting flux vector and the ambient magnetic field are derived for low‐amplitude (linear) waves taking into account first‐order finite gyroradius effects. The wave magnetic field is always in a plane perpendicular to the direction of the wave propagation, and the polarization depends on wave frequency (or wavelength), dispersion, and plasma parameters. In a warm plasma, the Poynting flux is not aligned with the group velocity and generally deviates from the wave propagation direction, except for parallel propagation. Numerical estimates for θP and wave polarization are made for plasmaspheric hiss (at L=2, 4, and 6) for typical plasma parameters and for the case of elevated electron temperature (5 keV). The plasmaspheric hiss wave magnetic field is right‐hand circularly polarized, except for highly oblique hiss waves which are elliptically polarized. We note a possible transition to the magnetosonic wave regime at short wavelengths. The maximum Poynting flux angle is ∼20°. Estimates for daytime outer zone chorus (L=6) show two regions of oblique Poynting flux corresponding to a low‐frequency band (ω<ωce/2) and to a high‐frequency band (ωce/2<ω<ωce) where ω and ωce are the wave frequency and the electron cyclotron frequency, respectively. The wave magnetic field polarization varies from circular to elliptical at shorter wavelengths. In a hot plasma, chorus is elliptically polarized. The maximum θP in the low‐frequency band is ∼20°. θP can reach ∼60° in the high‐frequency band. Our results are consistent with the Poynting flux statistics of Polar measurements.