Ionization balance in Titan’s nightside ionosphere

Abstract

Ionization balance in Titan's nightside ionosphere, Icarus (2014), doi: http://dx.doi.org/10.1016/j.icarus. 2014.11.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ABSTRACTBased on a multi-instrumental Cassini dataset we make model versus observation comparisons of plasma number densities, n P =(n e n I ) 1/2 (n e and n I being the electron number density and total positive ion number density, respectively) and short-lived ion number densities (N + , CH 2 + , CH 3 + , CH 4 + ) in the southern hemisphere of Titan's nightside ionosphere over altitudes ranging from 1100 and 1200 km and from 1100 to 1350 km, respectively. The n P model assumes photochemical equilibrium, ionelectron pair production driven by magnetospheric electron precipitation and dissociative recombination as the principal plasma neutralization process. The model to derive short-lived-ion number densities assumes photochemical equilibrium for the short-lived ions, primary ion production by electron-impact ionization of N 2 and CH 4 and removal of the short-lived ions through reactions with CH 4 . It is shown that the models reasonably reproduce the observations, both with regards to n P and the number densities of the short-lived ions. This is contrasted by the difficulties in accurately reproducing ion and electron number densities in Titan's sunlit ionosphere.

Suprathermal Electrons in Titan’s Sunlit Ionosphere: Model–observation Comparisons

“…At present the modeled plasma number densities on the dayside are higher than the RPWS/LP-derived values by a factor of ∼1.92 ± 0.08. On the nightside the level of agreement is better, the modeled values being on average a factor of ∼1.05 ± 0.16 higher than observed (Vigren et al 2013(Vigren et al , 2015. 2.…”

“…Based on ambient suprathermal electron fluxes measured by the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) and N 2 and CH 4 number densities measured by the Ion Neutral Mass Spectrometer in the Closed Source Neutral mode (INMS/CNS), Vigren et al (2015) calculated electron production rates at different locations along the T55-T59 Titan flybys by the Cassini spacecraft. The derived electron production rates were combined with estimates of the effective recombination coefficient, and the resulting plasma number densities (which were defined as the geometric mean of the electron and positive ion number densities) agreed well with values derived from RPWS/LP observations.…”

“…Negative ions, when present, typically contaminate the measurements in the central anodes, 4-5. For this reason and others (see discussion in Vigren et al 2015) we have in the present study utilized the measurements from anode 2 of the CAPS/ELS instrument. By instead using anode 6, we obtain similar (within 15%) results to those presented in Tables 2-3 (see Section 3).…”

“…In this subsection we focus explicitly on CAPS/ELS-derived electron production rates (P e,EI,O ) and view the present dayside results against results associated with the nine nightside locations (sampled from the T55-T59 flybys) investigated in the study by Vigren et al (2015). In Figure 3(a) we display the contribution to the total electron-impact electron production rate of electrons with energies <60 eV.…”

“…Shown as a function of ambient N 2 + CH 4 number densities (derived from INMS/CSN measurements) are (a) the relative contribution of <60 eV electrons to the CAPS/ELS-derived electron-impact ionization frequencies (electron energies up to 10 keV considered), (b) electron-impact ionization frequencies when limited to electrons with energies <60 eV, and (c) electron-impact ionization frequencies when limited to electrons with energies <10 keV. The nightside points (shown by magenta triangles) are taken from the study by Vigren et al (2015) and are associated with the T55-T59 flybys.…”