The Titan wakefield effects due to solar wind streaming

Abstract

Motivated by the observations of significant ionospheric escape from Titan by the Cassini spacecraft, and Voyager 1 and 2 observations of the solar wind, we suggest a test-charge approach as an additional mechanism to explain the ion loss caused by the solar wind (SW) interaction with the upper ionosphere of Titan (at altitudes of 1600–1700 km). This approach consists of assuming that a test particle that is inserted into the plasma system and moves with a speed that is higher than the acoustic speed can form a wakefield. This wakefield can drag the ionosphere particles and can thus cause them to escape from the upper ionosphere of Titan. In the upper ionosphere of Titan, most of the plasma species consist of three positive planetary ions (HCNH + , C 2 H 5 + , and CH 5 + ) with Maxwellian electrons and streaming SW protons with isothermal electrons. We deduced the electrostatic Debye screening and wakefield potentials caused by a moving test charge, as well as the modified dielectric constant of the ion acoustic waves (IAWs). Using the spacecraft measurements of the plasma configuration at Titan, we carried out a parametric analysis of these fields and found that the normalized Debye potential decreases exponentially with the axial distance. Computational calculations demonstrate, however, that ionosphere particle concentrations and temperatures increase the potential amplitude of the wakefield. Denser and hotter regions provide ionosphere particles with energy and push them to follow the test particle and escape from Titan. Furthermore, the increased density of SW protons amplifies the magnitude of the wakefield potential. The velocity and temperature of the SW protons remain unaffected, however, because their velocity is much higher than the acoustic speed of the plasma system. For ionosphere particles to interact with SW particles, their velocity ranges must therefore be comparable for them to be able to sense and respond to each other. Moreover, we determined the characteristics of the IAWs in the upper ionosphere of Titan for minimum and maximum plasma parameters, where the electric field amplitude of solitary waves ranges from 0.5 to 50 mV/m, the frequency range is 10–500 Hz, and the pulse time duration is 0.01–0.8 s in addition to the test particle. At a distance of z > 100 λ D from the test-charge particle, however, the bipolar electric field pulse reaches ≈0.05 mV/m. This agrees well with observed data from the Cassini mission.