Stochastic diffusion of interplanetary dust grains orbiting under Poynting-Robertson forces

The effects of the fluctuating interplanetary magnetic field (IMF) on charged dust grains orbiting around the sun under radiation pressure and Poynting-Robertson drag forces are examined for general ecliptic and inclined orbits. The Fokker-Planck equation describing the evolution of the charged grain distribution function is reduced to a diffusion-type equation by adopting the constants of the impulse-free motion as variable parameters. The Fokker-Planck coefficients are expressed in terms of the power spectrum of the fluctuating IMF, choosing a relatively simple model of fluctuations superposed on the Parker spiral mean field. For the model, the covariance matrix elements reduce to integrals over the slowly-varying orbital parameters, giving simple algebraic solutions for circular orbits at arbitrary inclinations.