Application of 20-momen approximation for ion velocity distribution in calculations of incoherent scattering spectra

According to Grad´s theories, the ion velocity distribution function is expanded to a 20-moment approximation based on the Maxwellian distribution. The effects of the stress tensor and the heat flow vector on the ion velocity distributions are discussed. When the electric field is weak, it is reasonable to neglect the heat flow vector for 20-moment approximation in the directions parallel to E times B and the stress tensor in the directions parallel to E , respectively. According to Sheffield´s theories, the incoherent scattering radar spectra are calculated by using the 20-moment approximation. Effects of parameter E on spectra shape are studied in details using the 20-moment approximation. Numerical results show that it is reasonable to use the 20-moment approximation to describe the Non-Maxwellian plasma characterized by the temperature anisotropy. When the electric field becomes weak, the 20-moment approximation may be reduced, this can simplify the calculation of the incoherent scatter spectra very much.