Influence of Coulomb collisions on isotopic and elemental fractionation in the solar wind acceleration process

In view of new observational evidence from isotope spectrometers on WIND, SOHO (Solar Heliospheric Observatory), and ACE (Advanced Composition Explorer), we explore the efficiency of isotope fractionation processes in the inner corona. We reinvestigate the role of Coulomb collisions in the acceleration of minor ions using a multifluid model. To model the main gas, we study stationary solutions for the continuity and momentum equations of electrons, protons, and alpha particles. As a closure of the system of equations, we prescribe expansion geometry and temperature profiles based on observations. The behavior of minor ions, which are treated as test particles, depends in a complicated manner on their mass and on their charge, structured by the interplay of acceleration, gravity, pressure gradient, electromagnetic fields, Coulomb drag, and thermal diffusion. We compare the fractionation effects in different solar wind regimes: In our model high-speed solar wind emanating from polar coronal holes, Coulomb friction practically equalizes the velocities of all species, and no substantial fractionation takes place. In the case of a rapidly expanding magnetic field geometry, for example, in the vicinity of a coronal streamer, the proton flux and thus the Coulomb friction on minor ions is reduced, leading to depletion of heavy species in the solar wind. The model also predicts a substantial depletion of alpha particles relative to protons in the heliospheric current sheet, consistent with observations. In such a situation, heavy elements are depleted in the solar wind relative to protons as well, but the effect is strongest for alpha particles. Isotopic fractionation of helium of the order of 30% is possible, while the isotope effect on heavier elements amounts at most to a few percent per mass unit.