On the origin of donut-shaped electron distributions within magnetic cavities

Abstract:

Abstract: Magnetic cavities, also known as magnetic holes, are ubiquitous in space plasmas characterized by localized regions with depressed magnetic strength and enhanced plasma pressure. Most of the observed magnetic cavities are also associated with strongly anisotropic particle distributions, with higher fluxes in the direction perpendicular to the magnetic field than in the parallel direction. Recent observations of kinetic-scale magnetic cavities, however, have identified another types of electron distributions in the pitch angle spectrum, the so-called donut-shaped distributions, although their formation processes remain unclear. Here we present a simple model of magnetic cavity contraction, in which electrons are traced backward in time to the initial, equilibrium-state cavity. The resulting electron distributions, determined from the Liouville’s theorem, are consistent with the observations in the presence of donut-shaped pitch angle structures. The model also enables a quantitative evaluation on the roles of betatron cooling, Fermi acceleration and radial transport in the formation of donut-shaped electron distributions, and therefore, sheds new light into the evolution and particle dynamics of kinetic-scale magnetic cavities.