Titan's ionosphere: A survey of solar EUV influences

Abstract

Effects of solar EUV on positive ions and heavy negative charge carriers (molecular ions, aerosol, and/or dust) in Titan's ionosphere are studied over the course of almost 12 years, including 78 flybys below 1400 km altitude between TA (October 2004) and T120 (June 2016). The Radio and Plasma Wave Science/Langmuir Probe‐measured ion charge densities (normalized by the solar zenith angle) show statistically significant variations with respect to the solar EUV flux. Dayside charge densities increase by a factor of ≈2 from solar minimum to maximum, while nightside charge densities are found to anticorrelate with the EUV flux and decrease by a factor of ≈3–4. The overall EUV dependence of the ion charge densities suggest inapplicability of the idealized Chapman theory below 1200 km in Titan's ionosphere. Nightside charge densities are also found to vary along Titan's orbit, with higher values in the sunward magnetosphere of Saturn compared to the magnetotail.

Re‐Analysis of the Cassini RPWS/LP Data in Titan's Ionosphere: 1. Detection of Several Electron Populations

“…However, one should also consider a possible effect of the active ion chemistry happening in these regions on the characteristics of the electron population P3. In particular, the ionosphere environment on the nightside close to the terminator is enabling the growth of large ions, which is otherwise disrupted under direct extreme UV fluxes (Shebanits et al, 2017b(Shebanits et al, , 2017aWellbrock et al, 2019).…”

Saturn’s near-equatorial ionospheric conductivities from in situ measurements

“…The most plausible explanation is therefore that the ion densities in the equatorial region are enhanced in the presence of neutral dust (which gets ionized locally by electron attachment), similar to Titan's ionosphere [26][27][28] .…”

“…In the dusty plasma of Titan's ionosphere, the ion densities have a second, larger peak at lower altitudes 28 . The ion densities are enhanced in electron-depleted regions of ionosphere due to ion-ion recombination being much slower than electron-ion one 26 , as mentioned above.…”

Titan's Variable Ionosphere During the T118 and T119 Cassini Flybys

“…The structure and variability of this ionosphere are sensitive to several factors. On the dayside of the moon, the ionosphere is created mainly through photoionization by extreme ultraviolet (EUV) radiation of the atmospheric N 2 and CH 4 (Ågren et al, ; Galand et al, ), which means that Titan's ionospheric properties will vary with the solar rotation and the phase of the solar cycle (Edberg et al, ; Madanian et al, ; Shebanits et al, ). Particle impact ionization also contributes and is the main ionization source on the nightside of the moon (Ågren et al, ; Cravens et al, , ; Vigren et al, ).…”