The effect of negative ions on collision‐dominated Thomson scattering

Abstract

An expression for the collision-dominated Thomson scatter power spectrum is derived from the fluid or continuum equations. This expression includes the possibility of multiple ion species of arbitrary charge. The total power and power spectral shape are investigated for scattering from a plasma composed of negative ions, positive ions, and electrons. The results indicate that total scattered power is modified by negative ion presence and that a limited amount of information about the negative ion content of the ionospheric D region may be found from Thomson scatter experiments. In particular, negative ions are found to increase the width of the power spectrum as would increased ion temperature or decreased ion neutral collision frequency. Spectra over a wide range of parameter values display the property of 'scaling': however, this effect may be sorted out such that negative ion presence is deduced from anomalously small ion-neutral collision frequencies. radio signal (Evans, 1969) and may be treated theoretically by use of the continuum or fluid equa- tions as has been done by Tanenbaum (1968), Seasholtz and Tanenbaum (1969), and Seasholtz (1971a, b). We extend their results to include the possibility of multiple ion species and multiply charged ions. The parameters of the theoretical spectra then include number density, charge, temperature, and collision frequency for each plasma component; overall charge neutrality is of course enforced. We investigate in particular the effects of negative ions, heavy (with respect to the majority ion) ions, and electron heating over what is probably a reasonable parameter range for the ionosphere D region. Our results indicate that while the information contained in a single experimental spectrum is very limited, there is the possibility of considerable informa- tion about the scattering medium when several experimental conditions are enforced. We also find the important result that total scattered power is modified by the presence of negative ions. This fact renders more difficult the evaluation, in terms of' electron number density, of Thomson scatter power measure- ments from the lower D region. The results of this investigation could also be obtained from various other approaches, such as the approach of Dougherty and Farley (1963), who use a relaxation type collision model, or of Hagfors and Brockelman (1971), who employ a random walk plus ballistic path type treatment of collisions. These various methods all result in spectra which are quite similar, as has been noted by Tanenbaum (1968) and Hagfors and Power (1972). Heretofore the fluid equation approach has received rela- tively little attention, perhaps because it is applicable only to the collision-dominated regime. However, its simplicity in fi- nal form and in numerical evaluation renders the continuum theory Thomson scatter spectral expression quite useful and illuminating for our task in this paper. These results are especially appropriate to the Arecibo 430- M Hz radar, which is capable of performing the measurements