R. Bodmer et P. Bochsler, Influence of Coulomb collisions on isotopic and elemental fractionation inthe solar wind acceleration process, J GEO R-S P, 105(A1), 2000, pp. 47-60
In view of new observational evidence from isotope spectrometers on WIND, S
OHO (Solar Heliospheric Observatory), and ACE (Advanced Composition Explore
r), we explore the efficiency of isotope fractionation processes in the inn
er corona. We reinvestigate the role of Coulomb collisions in the accelerat
ion of minor ions using a multifluid model. To model the main gas, we study
stationary solutions for the continuity and momentum equations of electron
s, protons, and alpha particles. As a closure of the system of equations, w
e prescribe expansion geometry and temperature profiles based on observatio
ns. The behavior of minor ions, which are treated as test particles, depend
s 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 e
ffects 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 i
n 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 al
pha particles. Isotopic fractionation of helium of the order of 30% is poss
ible, while the isotope effect on heavier elements amounts at most to a few
percent per mass unit.