Photoionization Modeling of Titan’s Dayside Ionosphere

Abstract

Previous modeling studies of Titan's dayside ionosphere predict electron number densities that are roughly a factor of 2 higher than those observed by the RPWS/Langmuir probe. The issue can equivalently be described as the ratio between the calculated electron production rates and the square of the observed electron number densities resulting in roughly a factor of 4 higher effective recombination coefficient than expected from the ion composition and the electron temperature.Here we make an extended reassessment of Titan's dayside ionization balance, focusing on 34 flybys between TA and T120. Using a recalibrated data set and by taking the presence of negative ions into account, we arrive at lower effective recombination coefficients compared with earlier studies. The values are still higher than expected from the ion composition and the electron temperature, but by a factor of ~2-3 instead of a factor of ~4. We have also investigated whether the derived effective recombination coefficients display dependencies on the solar zenith angle (SZA), the integrated solar EUV intensity (<80 nm), and the corotational plasma ram direction (RAM), and found statistically significant trends, which may be explained by a declining photoionization against the background ionization by magnetospheric particles (trends in SZA and RAM) and altered photochemistry (trend in EUV). We find that a series of flybys that occurred during solar minimum (2008) and with similar flyby geometries are associated with enhanced values of the effective recombination coefficient compared with the remaining data set, which also suggests a chemistry dependence on the sunlight conditions.

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).…”

“…Nevertheless, model results either predict too cold electrons (<0.02 eV; <200 K) at 900 km (Mukundan & Bhardwaj, 2018; Richard et al., 2011) or an overestimated electron density by a factor of two (Vigren et al., 2013, 2016). The addition of the negative ions to a photoionization model has been shown to decrease the discrepancy with the measurements (Shebanits, Vigren, Wahlund, Edberg, et al., 2017).…”

Re-analysis of the Cassini RPWS/LP data in Titan's ionosphere. Part I: detection of several electron populations

“…Nevertheless, model results either predict too cold electrons (< 0.02 eV; < 200 K) at 900 km (Mukundan & Bhardwaj, 2018;Richard et al, 2011) or an overestimated electron density by a factor of 2 (Vigren et al, 2013(Vigren et al, , 2016. The addition of the negative ions to a photoionisatoin model has been shown to decrease the discrepancy with the measurements (Shebanits et al, 2017a).…”

Re‐Analysis of the Cassini RPWS/LP Data in Titan's Ionosphere: 2. Statistics on 57 Flybys

“…Electron density and temperature were deduced from these measurements (Ågren et al., 2009; Edberg et al., 2010, 2013b, 2015; Wahlund et al., 2005). However, electron temperature measurements are not well reproduced by models of the ionosphere of Titan below 1,200 km: model results give electrons too cold by a factor of 2–3 (Galand et al., 2014; Mukundan & Bhardwaj, 2018; Richard et al., 2011; Shebanits, Vigren, Wahlund, Edberg, et al., 2017; Vigren et al., 2013, 2016).…”