Theoretical models for unstable IAWs and nonlinear structures in the upper ionosphere

Physical mechanisms are discussed for the excitation of ion-acoustic waves (IAWs) by field-aligned shear flow of ions and parallel current produced by electrons in the upper ionospheric oxygen hydrogen plasma within auroral latitudes. Theoretical models are presented for the formation of solitary structures by nonlinear IAWs. It is pointed out that the small concentration of hydrogen ions in the oxygen plasma should not be ignored, because it plays important role in the linear instability of IAWs and in determining the size of the nonlinear electrostatic structures. The growth rates of IAWs and size of nonlinear structures vary with altitude, because both depend upon the density ratio of oxygen-to-hydrogen ions along with other parameters. Current-driven electrostatic ion-acoustic waves are studied using kinetic theory which shows that parallel current produces these waves if the concentration of protons is very small about 4% or lesser in the presence of field-aligned shear flow of both kind of ions. Fluid theory is used to look for shear flow-driven instabilities and formation of nonlinear structures ignoring ion temperature effects in this plasma where Freja observations indicate $$T_{i}\approx $$ (0.3–0.1)$$T_{e}$$. Effects of nonthermal electrons and density gradient on the instabilities and size of the structures are also pointed out.