Oblique Whistler Waves Research Articles

Correction to “Pitch angle scattering of energetic particles by oblique whistler waves” by U.S. Inan and T.F. Bell

Correction to “Pitch angle scattering of energetic particles by oblique whistler waves” by U.S. Inan and T.F. Bell

Reduced magnetofluid dynamics in the lower-hybrid frequency range

The electromagnetic two-fluid dynamics for plasmas in a strong applied magnetic field is investigated using a model for phenomena having frequencies in the range Ωi ≪ ω ≪ Ωe and long parallel wavelengths ε≡L⊥/L∥ ≪ 1. The model retains the essential nonlinear physics of the magnetosonic, lower-hybrid, and oblique whistler waves. At least two unstable modes can exist for equilibria with magnetic shear; a local magnetic interchange whistler mode and a collisionless tearing mode. These modes are studied using a quadratic energy relation for linear stability and by three-dimensional time-dependent numerical computations.

Near‐equatorial pitch angle diffusion of energetic electrons by oblique whistler waves

The pitch angle scattering of trapped, energetic electrons by obliquely propagating whistler waves in the equatorial regions of the plasmasphere is investigated. Storm‐injected electrons moving along field lines near the equator interact with electromagnetic waves whose frequencies are Doppler‐shifted to some harmonic of the cyclotron frequency. The wave normals are distributed almost parallel to the geomagnetic field. Waves grow from the combined contributions of a large reservoir of energetic electrons that are driven into the loss cone by the highest‐harmonic interactions permitted to them. Relativistic, quasi‐linear theory is applied to obtain self‐consistent equations describing the temporal evolution of waves and particles over time scales which are longer than the particle bounce time and group time delay of the waves. The equilibrium solutions and their stability are studied, considering the reflection of the waves by the ionosphere and the coupling of multiple harmonic resonances. The contributions of nonlocal wave sources are also included in the theory. Numerical computations based on our theoretical analysis for regions inside the plasmasphere (L ≤ 2) and near the plasmapause (L ∼ 4.5) and for the first three harmonic resonances are presented.

Pitch angle scattering of energetic particles by oblique whistler waves

First order cyclotron or Landau resonant pitch angle scattering of electrons by oblique whistler waves propagating at large angles to the ambient field are found to be at least as large as that due to parallel propagating waves. Commonly observed precipitation of >40 keV electrons in association with ducted whistlers may thus be accompanied by substantial fluxes of lower energy (10 eV–40 keV) electrons precipitated by the nonducted components.

Obliquely propagating whistler instability in an inhomogeneous plasma penetrated by a helical electron beam

The excitation of the whistler instability due to normal gyroresonant interaction is studied in a warm inhomogeneous plasma in the presence of a helical electron beam. The results show that it is possible to excite oblique whistler waves propagating at large angles with respect to the ambient magnetic field. It is found that the inhomogeneity increases the frequency and growth rate of the instability.

EXPERIMENTS ON WHISTLER WAVE FILAMENTATION AND VLF HISS IN A LABORATORY PLASMA

With the development of a large magnetized plasma source it has become possible to investigate space plasma physics problems in the laboratory. First, the nonlinear effects associated with the excitation of a large amplitude whistler wave have been explored. It is found that the radiation pressure of the wave and thermal effects give rise to a field-aligned density depression in which the wave becomes completely trapped. Hyperfine filaments with diameters small compared with the parallel wavelength are observed. Second, the stability of oblique whistler waves in the presence of an electron beam has been studied. A broadband whistler instability is observed and identified as a Cherenkov interaction between beam electrons and whistlers propagating near the resonance cone. These observations confirm the present model for the generation of VLF hiss in the aurora.

Observation of beam-generated VLF hiss in a large laboratory plasma

The paper describes a lab experiment on a beam-plasma instability under conditions for which the theory of VLF hiss is applicable. An energetic electron beam is injected into a cold, dense, almost collisionless magnetoplasma. Beam and plasma dimensions are so large that oblique whistler wave propagation and growth are readily observable. The observations of frequency and wave number spectra, growth rate, and instability level are consistent with theory and satellite observation in regions of electron precipitation.