Comparison of transport equations based on Maxwellian and bi-Maxwellian distributions for anisotropic plasmas

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Because of the relevance to space physics and aeronomy, the authors studied the extent to which transport equations based on both Maxwellian and bi-Maxwellian series expansions can describe plasma flows characterised by non-Maxwellian velocity distributions, with emphasis given to modelling the anisotropic character of the distribution function. For the Maxwellian expansion the authors considered Grad's 5-, 13-, and 20-moment approximations, while for the bi-Maxwellian expansion they considered the 6- and 16-moment approximations. To determine the adequacy of a given series expansion, they selected a simple plasma flow problem which possessed an analytic solution so that the velocity distribution functions obtained from the different series expansions could be compared with the exact solution. The comparisons indicate that the Maxwellian-based 20-moment approximation is a reasonable approximation for temperature anisotropies up to T/sub ////Tperpendicular to approximately 2-3, while the bi-Maxwellian-based 16-moment approximation can describe temperature anisotropies as large as T/sub ////Tperpendicular to approximately 20.