Exospheric distributions of minor ions in the solar wind

Abstract

We investigate the acceleration of heavy solar wind ions on the basis of an exospheric Lorentzian model and show that the heavy ions can flow faster than the protons when their temperatures in the corona are more than proportional to their mass. The Lorentzian kinetic exospheric model [Pierrard and Lemaire, 1996], initially developed only for electrons and protons of the solar wind in a monotonie potential energy, is here generalized for the case of a nonmonotonic potential energy and applied to the heavy solar wind ions. We study how the mass, the charge, and the coronal temperature of the heavy ions influence their motion. The present work indicates that the temperature of the ions in the corona' characterizing the dispersion of their velocities is a crucial factor determining their bulk velocity at large radial distances. Pierrard and Lamy [2003] showed that the velocity filtration effect can produce very high values of ion temperatures in the solar corona, considering that ion velocity distribution functions are enhanced with suprathermal tails in the low corona. We show that for sufficiently high ion temperatures at the exobase, our exospheric model can account for large bulk speeds of the heavy solar wind ions and for their relative abundances at 1 AU. Copyright 2004 by the American Geophysical Union.