Spatial variations of low mass negative ions in Titan's upper atmosphere

Abstract

Observations with Cassini's Electron Spectrometer discovered negative ions in Titan's ionosphere, at altitudes between 1400 and 950 km. Within the broad mass distribution extending up to several thousand amu, two distinct peaks were identified at 25.8-26.0 and 49.0-50.1 amu/q, corresponding to the carbon chain anions $CN^-$ and/or $C_2H^-$ for the first peak and $C_3N^-$ and/or $C_4H^-$ for the second peak. In this study we present the spatial distribution of these low mass negative ions from 28 Titan flybys with favourable observations between 26 October 2004 and 22 May 2012. We report a trend of lower densities on the night side and increased densities up to twice as high on the day side at small solar zenith angles. To further understand this trend, we compare the negative ion densities to the total electron density measured by Cassini's Langmuir Probe. We find the low mass negative ion density and the electron density to be proportional to each other on the dayside, but independent of each other on the night side. This indicates photochemical processes and is in agreement with the primary production route for the low mass negative ions being initiated by dissociative reactions with suprathermal electron populations produced by photoionisation. We also find the ratio of $CN^-/C_2H^-$ to $C_3N^-/C_4H^-$ highly constrained on the day-side, in agreement with this production channel, but notably displays large variations on the nightside.