Structure of Titan's ionosphere: Model comparisons with Cassini data

doi:10.1016/j.pss.2009.07.011

Planetary and Space Science

Volume 57, Issues 14–15, December 2009, Pages 1834-1846

https://doi.org/10.1016/j.pss.2009.07.011 Get rights and content

Abstract

Solar extreme ultraviolet and X-ray radiation and energetic plasma from Saturn's magnetosphere interact with the upper atmosphere producing an ionosphere at Titan. The highly coupled ionosphere and upper atmosphere system mediates the interaction between Titan and the external environment. New insights into Titan's ionosphere are being facilitated by data from several instruments onboard the Cassini Orbiter, although the Ion and Neutral Mass Spectrometer (INMS) measurements will be emphasized here. We present dayside ionosphere models and compare the results with both Radio and Plasma Wave–Langmuir Probe (RPWS/LP) and INMS data, exploring the sensitivity of models to ionospheric chemistry schemes and solar flux variations. Modeled electron densities for the dayside leg of T18 and all of T17 (dayside) had reasonable agreement with the measured RPWS electron densities and INMS total ion densities. Magnetospheric inputs make at best minor contributions to the ionosphere for these flybys, at least for altitudes above about 1000 km. At lower (<1100 km) altitudes, the total ion densities measured by the INMS are less than the electron densities measured by the RPWS/LP which could be due to heavy (>100 daltons) ions, which the INMS is not able to detect. Qualitatively, INMS spectra exhibit the same ion species and 12 amu family separations for the dayside ionospheres of T17 and T18 as were seen in the mass spectra measured during T5 (nightside). However, the relative abundance of high-mass (m>50) ion species is about 10 times less for the dayside T17 and T18 passes than it was for the polar nightside T5 flyby, which can perhaps be explained in several ways including differences in neutral composition, less dissociative recombination on the nightside than on the dayside (due to lower electron densities and affecting heavier ion species more than lighter ones), and transport of longer-lived high-mass species from day-to-night.

Introduction

Solar radiation and energetic particles from Saturn's magnetosphere interact with neutrals in Titan's atmosphere, producing an ionosphere. The electron density profile in Titan's ionosphere was measured by Voyager 1 in 1980 using the radio occultation technique (Bird et al., 1997). Prior to the Cassini mission a large number of models of the ionosphere and its composition were created (Keller et al., 1992; Banaskiewicz et al., 2000; Galand et al., 1999; Fox and Yelle, 1997; Wilson and Atreya, 2004; Cravens et al., 2004). The Langmuir probe (LP) part of the Radio and Plasma Wave Spectrometer (RPWS) instrument measured in-situ electron densities and temperatures in Titan's ionosphere during the Ta (and Tb) close encounter in October 2004 (Wahlund et al., 2005). During this first flyby the Ion and Neutral Mass Spectrometer (INMS) measured densities of important neutral species in the upper atmosphere, including N₂ (95%) and CH₄ (5%) (Waite et al., 2005). But the INMS open source ion (osi) mode was not operated during this encounter and the ion composition was not measured. During the Ta flyby, the spacecraft entered the atmosphere and ionosphere on the dayside, crossed the terminator at closest approach (at an altitude of 1176 km and solar zenith angle of 91.1°), and continued onto the nightside (Waite et al., 2005). Comparisons between a theoretical model of the ionosphere and the measured electron densities demonstrated that most of the dayside ionosphere is produced by photoionization by solar radiation (Cravens et al., 2005; Galand et al., 2006).

The first measurements of the ionospheric composition at Titan were made by INMS during the T5 Cassini flyby in April 2005 (Cravens et al., 2006). This flyby took place entirely on the nightside and solar radiation could not be the source of at least the short-lived ion species (Cravens et al., 2009). Pre-Cassini models did investigate the role of impact ionization by energetic electrons from Saturn's outer magnetosphere transported along magnetic field lines as an important ionization source (cf., Atreya, 1986; Gan et al., 1992; Cravens et al., 2009). Cravens et al. (2008) explored the role of impact ionization by energetic ion precipitation from Saturn's magnetosphere, which might be especially important for the ionosphere detected remotely at lower altitudes (below 1000 km and down to 400 km) by the Cassini Radio Science (RSS) experiment (Kliore et al., 2008).

The composition of Titan's ionosphere as observed by the Cassini INMS was partly explained by the pre-Cassini chemical models (see the references listed above). For example, it was correctly recognized that HCNH⁺, C₂H₅⁺, and CH₅⁺ are abundant; however, a number of other ion species were seen in the measured mass spectra (Cravens et al., 2006) but were not predicted to be present with significant abundances (e.g., species at mass numbers 18 and 30). Vuitton et al., 2006, Vuitton et al., 2007 re-examined Titan's ion–neutral chemistry and introduced many new ion species (e.g., CH₂NH₂⁺ at m=30), as well as new minor neutral species which have low abundances, yet have effects on the ion chemistry. Krasnopolsky (2009) created a photochemical model of Titan's atmosphere and ionosphere using a coupled ion and neutral chemistry, as did De La Haye et al. (2008) earlier, and obtained reasonable results when compared to Cassini data. Hörst et al. (2008) created a photochemical model demonstrating that O⁺ deposition is probably at the origin of the O-bearing species observed on Titan. Ågren et al. (2007) and Cravens et al. (2009) quantitatively analyzed the ionosphere using electron transport codes and ion chemistry schemes for T5 conditions, demonstrating that magnetospheric electrons are able to reproduce the observed ionospheric structure on the nightside.

The current paper presents INMS data and some model comparisons for the ionosphere both on the dayside and on the part of the nightside close to the terminator (i.e., solar zenith angles, SZA, up to ≈105°). INMS data from the T17 and T18 Titan flybys are presented, as well as some comparison data from the RPWS/LP experiment. The RPWS data were presented and described by Ågren et al. (2009). The T17 flyby took place on September 7, 2006, and was entirely on the dayside (SZA from 31° to 71° for altitudes below about 1600 km) and the T18 flyby took place on September 23, 2006, and was on both the day and nightside (SZA ranging from 75° to 104° for altitudes less than 1600 km, with a SZA at closest approach of about 90°). The Ta ionosphere is also revisited in this paper. Both the ion composition and the vertical and horizontal structure of the ionosphere will be explored in the paper. Only ionization due to solar radiation is included in the model calculations carried out for this paper because we found that for the T17 and T18 ionospheres, magnetospheric particle precipitation was not required for the model to produce electron densities in agreement with measured values.

Section snippets

Description of the model

The theoretical model of Titan's ionosphere that we use to interpret the Cassini data has evolved over many years and was described by Keller et al., 1992, Keller et al., 1994, Gan et al. (1992), Keller et al. (1998), and Cravens et al., 2004, Cravens et al., 2005, Cravens et al., 2009. The model includes three main components: (1) a code that accounts for (primary) ion production (and photoelectron production) due to the absorption of solar radiation, (2) an electron transport code that

Cassini measurements: INMS and RPWS/LP

The introduction provided a brief review of Cassini ionospheric measurements but we provide a few more details here. The INMS measures both neutral and ion species with charge to mass ratios (m/q) ranging from 0.5 to 8.5 daltons and from 11.5 to 99.5 daltons, using a radio-frequency quadrupole mass analyzer. In its open source ion (osi) mode, ions enter a narrow aperture before being guided to the mass analyzer (see the instrument descriptions by Kasprzak et al., 1996, and Waite et al., 2004).

Results of the ionospheric model and comparisons with cassini data—the Ta flyby

Cravens et al. (2005) generated electron densities from an earlier version of the ionospheric model and compared these densities with the density time history measured by the RPWS/LP experiment (Wahlund et al., 2005). Since INMS only measured neutral densities and ion composition was not measured during this flyby, the modeled results could only be compared with RPWS measurements. Cravens et al. considered several cases including: (a) ionization just from solar radiation and (b) ionization both

Model and data—the T18 flyby

The T18 flyby took place on September 23, 2006, during which the Cassini spacecraft traveled over the north polar region of Titan, entering the atmosphere on the nightside at a latitude of ~63° north and 255° west longitude. Closest approach (CA) was at an altitude of 960 km and was at high latitudes near the terminator.

Model and data—the T17 flyby

The September 7, 2006, T17 flyby covered a region just north of the equator and the part of the flyby with altitudes of 2000 km or less was all on the dayside. The solar zenith angle ranged from about 30° to 70°, making T17 a purely dayside flyby.

Comparison of INMS data for T5, T17, and T18 at 1100 km

The mass spectra measured by the INMS during T5 (high latitude nightside), T17 (low latitude dayside) and T18 (high latitude dayside) are compared in Fig. 19. Note that the measured T17 low latitude dayside spectrum near this altitude is similar to the T18 spectrum. Qualitatively, the T5, T17 and T18 spectra are similar in that the same major species (mass numbers 28, 29, 30, for example) are present, as is the “family” structure (spacing of about 12 daltons) extending up to 100 daltons. On the

Discussion and conclusions

This paper has presented ion densities versus time and ion mass spectra as measured by the Cassini ion and neutral mass spectrometer for Titan's dayside ionosphere for two flybys. The dayside data have also been compared with the nightside T5 ionospheric data and with Cassini RPWS Langmuir probe data. In order to help interpret the dayside ion density data we used a photochemical model that included both primary and secondary ionization and a large number of ion–neutral reactions. The model did

Acknowledgements

Support from the NASA Cassini project (Grant NFP45280 via subcontract from Southwest Research Institute) is acknowledged. Model development at the University of Kansas was also supported by the NASA Planetary Atmospheres Grant NNX07AF47G. Solar Irradiance Platform historical irradiances are provided courtesy of W. Kent Tobiska and Space Environment Technologies. These historical irradiances have been developed with partial funding from the NASA UARS, TIMED, and SOHO missions.

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Structure of Titan's ionosphere: Model comparisons with Cassini data

Abstract

Introduction

Section snippets

Description of the model

Cassini measurements: INMS and RPWS/LP

Results of the ionospheric model and comparisons with cassini data—the Ta flyby

Model and data—the T18 flyby

Model and data—the T17 flyby

Comparison of INMS data for T5, T17, and T18 at 1100 km

Discussion and conclusions

Acknowledgements

Icarus

Planet. Space Sci.

Planet. Space Sci.

Adv. Space Res.

Icarus

Icarus

Icarus

Planet. Space Sci.

Icarus

Planet. Space Sci.

J. Atmos. Sol. Terr. Phys.

Icarus

Icarus

Icarus

Deriving solar X-ray irradiance from Yohkoh observations

J. Geophys. Res.

On magnetospheric electron impact ionisation and dynamics in Titan's ram-side and polar ionosphere—a Cassini case study

Ann. Geophys.

Atmospheres and Ionospheres of the Outer Planets and their Satellites

A coupled model of Titan's atmosphere and ionosphere

Icarus

Toward a reduction of the bimolecular reaction model for Titan's ionosphere

Int. J. Chem. Kinet.

Discovery of heavy negative ions in Titan's ionosphere

Geophys. Res. Lett.

Titan's ionosphere: model comparisons with Cassini Ta data

Geophys. Res. Lett.

The composition of Titan's ionosphere

Geophys. Res. Lett.

Coupled ion and neutral rotating model of Titan's upper atmosphere

Icarus