J. H. Westlake

Titan's thermospheric response to various plasma environments

[1] TheCassini‐HuygensmissionhasbeenobservingTitansinceOctober2004,resultingin over 70 targeted flybys. Titan’s thermosphere is sampled by the Ion and Neutral Mass Spectrometer (INMS) during several of these flybys. The measured upper atmospheric density varies significantly from pass to pass. In order to quantify the processes controlling this variability, we calculate the nitrogen scale height for a variety of parameters related to the solar and plasma environments and, from these, we infer an effective upper atmospheric temperature. In particular, we investigate how these calculated scale heights and temperatures correlate with the plasma environment. Measured densities and inferred temperatures are found to be reduced when INMS samples Titan within Saturn’s magnetospheric lobe regions, while they are enhanced when INMS samples Titan in Saturn’s plasma sheet. Finally the data analysis is supplemented with Navier‐Stokes model calculations using the Titan Global Ionosphere Thermosphere Model. Our analysis indicates that, during the solar minimum conditions prevailing during the Cassini tour, the plasma interaction plays a significant role in determining the thermal structure of the upper atmosphere and, in certain cases, may override the expected solar‐driven diurnal variation in temperatures in the upper atmosphere. Citation: Westlake, J. H., J. M. Bell, J. H. Waite Jr., R. E. Johnson, J. G. Luhmann, K. E. Mandt, B. A. Magee, and A. M. Rymer (2011), Titan’s thermospheric response to various plasma environments, J. Geophys. Res., 116, A03318,

Titan's ionospheric composition and structure: Photochemical modeling of Cassini INMS data

Titans upper atmosphere produces an ionosphere at high altitudes from photoionization and electron impact that exhibits complex chemical processes in which hydrocarbons and nitrogen-containing molecules are produced through ion-molecule reactions. The structure and composition of Titans ionosphere has been extensively investigated by the Ion and Neutral Mass Spectrometer (INMS) onboard the Cassini spacecraft. We present a detailed study using linear correlation analysis, 1-D photochemical modeling, and empirical modeling of Titans dayside ionosphere constrained by Cassini measurements. The 1-D photochemical model is found to reproduce the primary photoionization products of N2 and CH4. The major ions, CH5+, C2H5+, and HCNH+ are studied extensively to determine the primary processes controlling their production and loss. To further investigate the chemistry of Titans ionosphere we present an empirical model of the ion densities that calculates the ion densities using the production and loss rates derived from the INMS data. We find that the chemistry included in our model sufficiently reproduces the hydrocarbon species as observed by the INMS. However, we find that the chemistry from previous models appears insufficient to accurately reproduce the nitrogen-containing organic compound abundances observed by the INMS. The major ion, HCNH+, is found to be overproduced in both the empirical and 1-D photochemical models. We analyze the processes producing and consuming HCNH+ in order to determine the cause of this discrepancy. We find that a significant chemical loss process is needed. We suggest that the loss process must be with one of the major components, namely C2H2, C2H4, or H2.

THE 12C/13C RATIO ON TITAN FROM CASSINI INMS MEASUREMENTS AND IMPLICATIONS FOR THE EVOLUTION OF METHANE

We have re-evaluated the Cassini Ion Neutral Mass Spectrometer (INMS) 12 C/ 13 C ratios in the upper atmosphere of Titan based on new calibration sensitivities and an improved model for the NH3 background in the 13 CH4 mass channel. The INMS measurements extrapolated to the surface give a 12 C/ 13 Ci n CH4 of 88.5 ± 1.4. We compare the results to a revised ratio of 91.1 ± 1.4 provided by the Huygens Gas Chromatograph Mass Spectrometer and 86.5 ± 7.9 provided by the Cassini Infrared Spectrometer and determine implications of the revised ratios for the evolution of methane in Titan’s atmosphere. Because the measured 12 C/ 13 C is within the probable range of primordial values, we can only determine an upper boundary for the length of time since methane began outgassing from the interior, assuming that outgassing of methane (e.g., cryovolcanic activity) has been continuous ever since. We find that three factors play a crucial role in this timescale: (1) the escape rate of methane, (2) the difference between the current and initial ratios and the rate of methane, and (3) production or resupply due to cryovolcanic activity. We estimate an upper limit for the outgassing timescale of 470 Myr. This duration can be extended to 940 Myr if production rates are large enough to counteract the fractionation due to escape and photochemistry. There is no lower limit to the timescale because the current ratios are within the range of possible primordial values.

An empirical approach to modeling ion production rates in Titan's ionosphere I: Ion production rates on the dayside and globally

Titans ionosphere is created when solar photons, energetic magnetospheric electrons or ions, and cosmic rays ionize the neutral atmosphere. Electron densities generated by current theoretical models are much larger than densities measured by instruments on board the Cassini orbiter. This model density overabundance must result either from overproduction or from insufficient loss of ions. This is the first of two papers that examines ion production rates in Titans ionosphere, for the dayside and nightside ionosphere, respectively. The first (current) paper focuses on dayside ion production rates which are computed using solar ionization sources (photoionization and electron impact ionization by photoelectrons) between 1000 and 1400 km. In addition to theoretical ion production rates, empirical ion production rates are derived from CH4, CH3+, and CH4+ densities measured by the INMS (Ion Neutral Mass Spectrometer) for many Titan passes. The modeled and empirical production rate profiles from measured densities of N2+ and CH4+ are found to be in good agreement (to within 20%) for solar zenith angles between 15 and 90°. This suggests that the overabundance of electrons in theoretical models of Titans dayside ionosphere is not due to overproduction but to insufficient ion losses.

High-Altitude Production of Titan's Aerosols

Measurements with the Cassini Ion and Neutral Mass Spectrometer (INMS) and two Cassini Plasma Spectrometer (CAPS) sensors, the Ion beam Spectrometer (IBS) and the Electron Spectrometer (ELS), have revealed the presence of a significant population of heavy hydrocarbon and nitrile species well above the homopause, with masses as large as several thousand Daltons (Da). The INMS ion and neutral spectra cover the mass range 1–100 Da. The IBS has measured positive ions up to 350 Da, while the ELS has detected concentrations of negative ions as high as 20% of the total negatively charged ionosphere component extending to over 13,000 Da. These measurements have motivated the development of new atmospheric models and have significant implications for our knowledge and understanding of Titans haze layers.

Observations of Energetic Particle Escape at the Magnetopause: Early Results from the MMS Energetic Ion Spectrometer (EIS)

Energetic (greater than tens of keV) magnetospheric particle escape into the magnetosheath occurs commonly, irrespective of conditions that engender reconnection and boundary-normal magnetic fields. A signature observed by the Magnetospheric Multiscale (MMS) mission, simultaneous monohemispheric streaming of multiple species (electrons, H+, Hen+), is reported here as unexpectedly common in the dayside, dusk quadrant of the magnetosheath even though that region is thought to be drift-shadowed from energetic electrons. This signature is sometimes part of a pitch angle distribution evolving from symmetric in the magnetosphere, to asymmetric approaching the magnetopause, to monohemispheric streaming in the magnetosheath. While monohemispheric streaming in the magnetosheath may be possible without a boundary-normal magnetic field, the additional pitch angle depletion, particularly of electrons, on the magnetospheric side requires one. Observations of this signature in the dayside dusk sector imply that the static picture of magnetospheric drift-shadowing is inappropriate for energetic particle dynamics in the outer magnetosphere.

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

In this study, we develop a best fit description of Titans 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 Titans 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 Titans 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.

Electrodynamic context of magnetopause dynamics observed by magnetospheric multiscale

Magnetopause observations by Magnetospheric Multiscale (MMS) and Birkeland currents observed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment are used to relate magnetopause encounters to ionospheric electrodynamics. MMS magnetopause crossings on 15 August and 19 September 2015 occurred earthward of expectations due to solar wind ram pressure alone and coincided with equatorward expansion of the Birkeland currents. Magnetopause erosion, consistent with expansion of the polar cap, contributed to the magnetopause crossings. The ionospheric projections of MMS during the events and at times of the magnetopause crossings indicate that MMS observations are related to the main path of flux transport in one case but not in a second. The analysis provides a way to routinely relate in situ observations to the context of in situ convection and flux transport.

The source of heavy organics and aerosols in Titan's atmosphere

Ion-neutral chemistry in Titans upper atmosphere (∼ 1000 km altitude) is an unex- pectedly prodigious source of hydrocarbon-nitrile compounds. We report observations from the Cassini Ion Neutral Mass Spectrometer (INMS; Waite et al. 2004) and Cassini Plasma Spec- trometer (CAPS; Young et al. 2004) that allow us to follow the formation of the organic material from the initial ionization and dissociation of nitrogen and methane driven by several free en- ergy sources (extreme ultraviolet radiation and energetic ions and electrons) to the formation of negative ions with masses exceeding 10,000 amu.

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

The Ion and Neutral Mass Spectrometer (INMS) and Cassini Plasma Spectrometer (CAPS) have observed Titans 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 (Ci 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 drop-off 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 C2H2 and C2H4, which have been shown to be energetically favorable [Ghesquiere et al., 2014] and that have already been identified as ion growth patterns for the lighter ions detected by the INMS [Westlake et al. 2012]. 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 C6H7+ of 2.4 × 10-16 g cm-2 s-1 and a total production rate for large molecules of 7.1 × 10-16 g cm-2 s-1.