Electrostatic waves and nonlinear structures in Titan’s ionosphere

Abstract

The temporal and spatial scales associated with linear and nonlinear ion acoustic waves (IAWs) in Titan’s ionosphere have been estimated in the presence of an extremely small ambient magnetic field of the order of \(B_0 \simeq 2 \times 10^{-4}\) G. Titan’s ionosphere has many kinds of positive and negative ions but for analytical calculations, the dominant ions HCNH⁺ and CN⁻ are taken into account along with free electrons keeping in view the observations of the Cassini spacecraft. The frequencies of IAW turn out to be of the order of (0.001) rad/s, and parallel wavelengths of the order of several kilometers, which are very different from Earth’s ionosphere. Shear flow-driven purely growing instability is also investigated. It is found that the IAWs become unstable in the presence of field-aligned shear flow of positive and negative ions. It is pointed out that the nonlinear IAWs can give rise to large solitary electrostatic structures with widths of the order of several hundred kilometers. The values of plasma beta corresponding to positive and negative ions are \(\beta _{a}\simeq (0.046)\ll 1\) and \(\beta _{b}\simeq (0.006)\ll 1\), which are suitable for the propagation of electrostatic waves.