Simulating the one‐dimensional structure of Titan's upper atmosphere: 1. Formulation of the Titan Global Ionosphere‐Thermosphere Model and benchmark simulations

Bell, J. M.; Bougher, S. W.; Waite, J. H.; Ridley, A. J.; Magee, B.; Mandt, K.; Westlake, J. H.; DeJong, A. D.; Bar‐Nun, A.; Jacovi, Ronen; Tóth, G.; Haye, V. de La

doi:10.1029/2010je003636

Cited by 40 publications

(112 citation statements)

References 93 publications

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“…Our efforts here differ significantly from our work in Bell et al [2010aBell et al [ , 2010bBell et al [ , 2011b. Previously, we accounted for both diurnal and sun-Saturn orbital distance variations over the course of our simulations, which are not accounted for in the collective works of Cui et al [2008], Strobel [2008Strobel [ , 2009Strobel [ , 2010Strobel [ , 2012, and Yelle et al [2008].…”

Section: Introductioncontrasting

confidence: 44%

“…We approximate a diurnal average by dividing the solar fluxes by a factor of 2.0. This approach differs from the settings in Bell et al [2010aBell et al [ , 2010bBell et al [ , 2011b, but it matches other 1-D investigations of Titan's upper atmosphere [e.g., Krasnopolsky, 2009Krasnopolsky, , 2010Strobel, 2010;Yelle et al, 2008].…”

Section: Introductionmentioning

confidence: 95%

“…In order to correct for a systematic underestimate of the neutral densities relative to other investigations, the INMS neutral densities have been multiplied by a uniform factor of 2.7 [Bell et al, 2010b]. Moreover, we average the flyby densities between TA and T40 to create a prime mission average, properly propagating both the counting statistical and geophysical variabilities into our averages [Bell et al, 2010a[Bell et al, , 2010b.…”

Section: The Data Sets Usedmentioning

confidence: 99%

“…We will do this by building upon our previous work in Bell et al [2010aBell et al [ , 2010bBell et al [ , 2011b], where we identified three key aspects of our 1-D simulations that determined the amount of atmospheric escape of CH 4 and H 2 needed to match INMS measurements: (1) the amount of turbulence used in the model, (2) the net chemical destruction of methane (net production of H 2 ) included in the model (i.e., the sum over direct photolytic, ion-neutral, and neutral-neutral chemical losses), and (3) the inclusion of a self-consistent thermal balance calculation. We have designed a series of numerical experiments that explore each of these aspects sequentially, building from highly simplified simulations to fully coupled simulations that combine all three aspects into a global mean description of Titan's upper atmosphere.…”

Section: Introductionmentioning

confidence: 99%

“…Tucker et al [2013], using a direct simulation Monte Carlo method, have reproduced the INMS measurements in the upper atmosphere while using only thermal escape rates of methane (less than 1.0 × 10 9 CH 4 m −2 s −1 ). Similarly, Bell et al [2010aBell et al [ , 2010bBell et al [ , 2011b] used a 1-D version of the Titan Global Ionosphere-Thermosphere Model (T-GITM) to reproduce the INMS densities and mixing ratios using a methane homopause near 1000 km and escape rates of ∼1.0 × 10 8 CH 4 m −2 s −1 .…”

Section: Introductionmentioning

confidence: 99%

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Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

et al. 2014

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In this study, we develop a best fit description of Titan's upper atmosphere between 500 km and 1500 km, using a one‐dimensional (1‐D) version of the three‐dimensional (3‐D) Titan Global Ionosphere‐Thermosphere Model. For this modeling, we use constraints from several lower atmospheric Cassini‐Huygens investigations and validate our simulation results against in situ Cassini Ion‐Neutral Mass Spectrometer (INMS) measurements of N2, CH4, H2, 40Ar, HCN, and the major stable isotopic ratios of 14N/15N in N2. We focus our investigation on aspects of Titan's upper atmosphere that determine the amount of atmospheric escape required to match the INMS measurements: the amount of turbulence, the inclusion of chemistry, and the effects of including a self‐consistent thermal balance. We systematically examine both hydrodynamic escape scenarios for methane and scenarios with significantly reduced atmospheric escape. Our results show that the optimum configuration of Titan's upper atmosphere is one with a methane homopause near 1000 km and atmospheric escape rates of 1.41–1.47 ×1011 CH4 m−2 s−1 and 1.08 ×1014 H2 m−2 s−1 (scaled relative to the surface). We also demonstrate that simulations consistent with hydrodynamic escape of methane systematically produce inferior fits to the multiple validation points presented here.

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Section: Introductioncontrasting

confidence: 44%

Section: Introductionmentioning

confidence: 95%

Section: The Data Sets Usedmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

et al. 2014

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The role of ion‐molecule reactions in the growth of heavy ions in Titan's ionosphere

et al. 2014

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The Ion and Neutral Mass Spectrometer (INMS) and Cassini Plasma Spectrometer (CAPS) have observed Titan's ionospheric composition and structure over several targeted flybys. In this work we study the altitude profiles of the heavy ion population observed by the Cassini Plasma Spectrometer-Ion Beam Spectrometer (CAPS-IBS) during the nightside T57 flyby. We produce altitude profiles of heavy ions from the C6-C13 group (C i indicates the number, i, of heavy atoms in the molecule) using a CAPS-IBS/INMS cross calibration. These altitude profiles reveal structure that indicates a region of initial formation and growth at altitudes below 1200 km followed by a stagnation and dropoff at the lowest altitudes (1050 km). We suggest that an ion-molecule reaction pathway could be responsible for the production of the heavy ions, namely reactions that utilize abundant building blocks such as C 2 H 2 and C 2 H 4 , which have been shown to be energetically favorable and that have already been identified as ion growth patterns for the lighter ions detected by the INMS. We contrast this growth scenario with alternative growth scenarios determining the implications for the densities of the source heavy neutrals in each scenario. We show that the high-mass ion density profiles are consistent with ion-molecule reactions as the primary mechanism for large ion growth. We derive a production rate for benzene from electron recombination of C 6 H 7 + of 2.4 × 10 À16 g cm À2 s À1and a total production rate for large molecules of 7.1 × 10 À16 g cm À2 s À1 .

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Solar cycle variations in ion composition in the dayside ionosphere of Titan

et al. 2016

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One Titanian year spans over two complete solar cycles, and the solar irradiance has a significant effect on ionospheric densities. Solar cycle 24 has been one of the quietest cycles on record. In this paper we show data from the Cassini ion and neutral mass spectrometer (INMS) and the radio and plasma wave science Langmuir probe spanning the time period from early 2005, at the declining phase of solar cycle 23, to late 2015 at the declining phase of solar cycle 24. Densities of different ion species measured by the INMS show a consistent enhancement for high solar activity, particularly near the ionospheric peak. The density enhancement is best seen in primary ion species such as CH3+ rather than heavier ion species such as HCNH+. Unlike at Earth, where the ionosphere and atmosphere thermally expand at high solar activity, at Titan the altitude of the ionospheric peak decreases, indicating that the underlying neutral atmosphere was less extensive. Among the major ion species, CH5+ shows the largest decrease in peak altitude, whereas heavy ions such as C3H5+ show very little decrease. We also calculate the ion production rates using a theoretical model and a simple empirical model using INMS data and show that these effectively predict the increased ion production rates at high solar activity.

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Simulating the one‐dimensional structure of Titan's upper atmosphere: 1. Formulation of the Titan Global Ionosphere‐Thermosphere Model and benchmark simulations

Cited by 40 publications

References 93 publications

Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

The role of ion‐molecule reactions in the growth of heavy ions in Titan's ionosphere

Solar cycle variations in ion composition in the dayside ionosphere of Titan

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