O+ ion temperature partition coefficients β$\parallel$ and β$\perp$: The Effect of O+-O+ Coulomb self-collisions

Details O+ ion temperature partition coefficients β$\parallel$ and β$\perp$: The Effect of O+-O+ Coulomb self-collisions O+ ion temperature partition coefficients β$\parallel$ and β$\perp$: The Effect of O+-O+ Coulomb self-collisions

Jun 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005jgra..11006307b&link_type=abstract

Journal of Geophysical Research, Volume 110, Issue A6, CiteID A06307

Physics

Ionosphere: Auroral Ionosphere (2704), Ionosphere: Plasma Temperature And Density, Magnetospheric Physics: Numerical Modeling

Scientific paper

We have computed altitude profiles for O+ ion temperature partition coefficients β$\parallel$ and β$\perp$ in the auroral ionosphere for different values of electric field with the use of a Monte Carlo simulation. The Monte Carlo model includes the effect of E × B drift, O+-O (resonant charge exchange and polarization interaction) collisions, and O+-O+ Coulomb self-collisions. These effects were included self-consistently in the computations. At low altitudes, the O+ ion concentration is small compared with the O concentration; thus the role of O+-O+ Coulomb self-collisions in isotropizing the O+ ion velocity distribution is negligible and hence non-Maxwellian features (β$\perp$ > β$\parallel$) are obtained due to the effect of O+-O collisions. However, as altitude increases, the O+ concentration increases and consequently, the role of O+-O+ Coulomb collisions becomes significant in transferring energy from the field-perpendicular direction to the field-parallel direction. This explains the increase of β$\parallel$ and the decrease of β$\perp$ with altitude. Also, we have investigated the variation of β$\parallel$ and β$\perp$ with the electric field and found that as electric field increases, O+ ion temperature increases, β$\parallel$ decreases, and β$\perp$ increases due to the interplay between E × B drift and O+-O collisions. In other words, the effect of O+-O+ Coulomb collisions becomes less important because these Coulomb collisions are in turn dependent on O+ ion temperature. Therefore the combined effects of E × B drift, O+-O collisions, and O+-O+ Coulomb collisions determine the altitude profiles of β$\parallel$ and β$\perp$. Finally, a comparison has been made between the Monte Carlo calculations obtained in this paper and observations of the O+ ion temperature partition coefficient β$\parallel$. The comparison showed a remarkably close agreement in the corresponding results for the altitude variation of β$\parallel$. This close agreement provides further evidence that the Monte Carlo model described in this paper is a powerful tool for studying O+ ion behavior in the auroral ionosphere, especially when O+-O+ Coulomb self-collisions are included. As a result of the comparison, we were able to predict the real values of the convection electric field in the auroral ionosphere for three ion heating events.

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