Characteristics of electromagnetic Rayleigh-Taylor modes in nighttime equatorial plasma

摘要

The unstable Rayleigh-Taylor plasma modes that are believed to be responsible for the density irregularities in the nighttime F region equatorial ionosphere have been studied before by assuming that the modes are electrostatic. In this paper, these plasma modes are studied without such assumption in order to determine the strength and the characteristics of the magnetic field fluctuations and to determine the physical condition under which the electrostatic assumption is justified. It is found that the relevant magnetic field fluctuations ( Bρ), which arise from the fluctuating parallel (to B 0) current density, are associated with the shear Alfven waves. The parameter that determines the amplitude of Bρ is α(s) ≡ D m (s)k 2/γ, where D m k 2 represents the rate at which magnetic field fluctuations with perpendicular wavelength λφ (≡ 2π/k φ) diffuse away due to parallel resistivity and γ is the rate at which the fluctuations grow. Typically, α 1 in the equatorial ionosphere, which means that the excited magnetic field fluctuations diffuse at a rate much faster than their growth rate and, consequently, their amplitudes remain very small (compared to B 0). Thus the Rayleigh-Taylor modes in the equatorial ionosphere are predominantly electrostatic in nature, and the electrostatic assumption of the previous analyses is quite justified. If the maximum amplitude of the electric field fluctuations ( ) is taken to be 1 mV/m, then the maximum amplitude of ρ is found to be less than 0.2 nT when λφ is 500 m, and it increases to about 2.4 nT when λ is 20 km. Longer wavelength modes have somewhat larger magnetic field fluctuations, but their growth rates are smaller. While the maximum amplitude of occurs at the magnetic equator, that of Bρ occurs at a distance away from it.