Antisymmetric standing wave structure associated with the compressional Pc 5 pulsation of November 14, 1979

Abstract

Model calculations are performed to explain ion and electron flux pulsations observed at geostationary orbit in association with the compressional Pc 5 magnetic pulsation of November 14, 1979. The amplitude of the flux perturbation is calculated based on the observed value of the wave frequency, the azimuthal wave number, and the average distribution f(W, α) of the particles, for assumed forms of a standing wave structure along the ambient magnetic field. A theory developed by Southwood and Kivelson is followed to calculate the changes in energy and the location of the guiding center, which are experienced by particles in a given wave, and the consequential modulation of particle fluxes. It is found that if the compressional magnetic field component b∥ of the wave has a symmetric structure about the equator, net acceleration of the particles contributes significantly to the total amplitude of flux pulsations; whereas if b∥ has an antisymmetric structure, the total energy is conserved with the exchange of perpendicular and parallel energy leading to the modulation of the pitch angle distribution as the dominant cause of the flux pulsation. For the latter case, local magnetic field compression and the pitch angle dependence of the average particle distribution determine the amplitude of the flux pulsation. We find that antisymmetric models of b∥ reproduce the observations better than symmetric models. This result implies that there exists a class of compressional Pc 5 waves that do not have a fundamental standing wave structure.