Ion Neutral Mass Spectrometer Measurements Research Articles

Wavelike perturbations in Titan’s ionosphere and their compositional variation: A preliminary survey of Cassini Ion Neutral Mass Spectrometer measurements

Wavelike perturbations in the ionosphere of Titan, the largest satellite of Saturn, are explored based on the Cassini Ion Neutral Mass Spectrometer (INMS) measurements. Strong wavelike perturbations are identified for more than twenty ion species, from simple ones such as N⁺ and CH₄⁺ to complex ones such as C₂H₃CNH⁺ and C₄H₇⁺. Simultaneous wavelike perturbations in background N₂, indicative of atmospheric gravity waves, are also observed, motivating us to speculate that the INMS-derived ion perturbations are wave-driven. The amplitudes of the ion perturbations are found to be larger than that of the N₂ perturbations. Clear compositional variation is revealed by the data: heavier ion species exhibit greater amplitudes. Such observations might be understood based on considerations either of force balance or chemical loss in Titan’s ionosphere.

Ion densities and composition of Titan's upper atmosphere derived from the Cassini Ion Neutral Mass Spectrometer: Analysis methods and comparison of measured ion densities to photochemical model simulations

The Cassini Ion Neutral Mass Spectrometer (INMS) has measured both neutral and ion species in Titan's upper atmosphere and ionosphere and the Enceladus plumes. Ion densities derived from INMS measurements are essential data for constraining photochemical models of Titan's ionosphere. The objective of this paper is to present an optimized method for converting raw data measured by INMS to ion densities. To do this, we conduct a detailed analysis of ground and in‐flight calibration to constrain the instrument response to ion energy, the critical parameter on which the calibration is based. Data taken by the Cassini Radio Plasma Wave Science Langmuir Probe and the Cassini Plasma Spectrometer Ion Beam Spectrometer are used as independent measurement constraints in this analysis. Total ion densities derived with this method show good agreement with these data sets in the altitude region (∼1100–1400 km) where ion drift velocities are low and the mass of the ions is within the measurement range of the INMS (1–99 Daltons). Although ion densities calculated by the method presented here differ slightly from those presented in previous INMS publications, we find that the implications for the science presented in previous publications is mostly negligible. We demonstrate the role of the INMS ion densities in constraining photochemical models and find that (1) cross sections having high resolution as a function of wavelength are necessary for calculating the initial photoionization products and (2) there are disagreements between the measured ion densities representative of the initial steps in Titan photochemistry that require further investigation.

Analysis of Titan's neutral upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements

In this paper we present an in-depth study of the distributions of various neutral species in Titan's upper atmosphere, between 950 and 1500 km for abundant species (N 2, CH 4, H 2) and between 950 and 1200 km for other minor species. Our analysis is based on a large sample of Cassini/INMS (Ion Neutral Mass Spectrometer) measurements in the CSN (Closed Source Neutral) mode, obtained during 15 close flybys of Titan. To untangle the overlapping cracking patterns, we adopt Singular Value Decomposition (SVD) to determine simultaneously the densities of different species. Except for N 2, CH 4, H 2 and 40Ar (as well as their isotopes), all species present density enhancements measured during the outbound legs. This can be interpreted as a result of wall effects, which could be either adsorption/desorption of these molecules or heterogeneous surface chemistry of the associated radicals on the chamber walls. In this paper, we provide both direct inbound measurements assuming ram pressure enhancement only and abundances corrected for wall adsorption/desorption based on a simple model to reproduce the observed time behavior. Among all minor species of photochemical interest, we have firm detections of C 2H 2, C 2H 4, C 2H 6, CH 3C 2H, C 4H 2, C 6H 6, CH 3CN, HC 3N, C 2N 2 and NH 3 in Titan's upper atmosphere. Upper limits are given for other minor species. The globally averaged distributions of N 2, CH 4 and H 2 are each modeled with the diffusion approximation. The N 2 profile suggests an average thermospheric temperature of 151 K. The CH 4 and H 2 profiles constrain their fluxes to be 2.6 × 10 9 cm − 2 s − 1 and 1.1 × 10 10 cm − 2 s − 1 , referred to Titan's surface. Both fluxes are significantly higher than the Jeans escape values. The INMS data also suggest horizontal/diurnal variations of temperature and neutral gas distribution in Titan's thermosphere. The equatorial region, the ramside, as well as the nightside hemisphere of Titan appear to be warmer and present some evidence for the depletion of light species such as CH 4. Meridional variations of some heavy species are also observed, with a trend of depletion toward the north pole. Though some of the above variations might be interpreted by either the solar-driven models or auroral-driven models, a physical scenario that reconciles all the observed horizontal/diurnal variations in a consistent way is still missing. With a careful evaluation of the effect of restricted sampling, some of the features shown in the INMS data are more likely to be observational biases.

The vertical structure of Titan's upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements

Data acquired by the Ion Neutral Mass Spectrometer (INMS) on the Cassini spacecraft during its close encounter with Titan on 26 October 2004 reveal the structure of its upper atmosphere. Altitude profiles of N 2, CH 4, and H 2, inferred from INMS measurements, determine the temperature, vertical mixing rate, and escape flux from the upper atmosphere. The mean atmospheric temperature in the region sampled by the INMS is 149 ± 3 K , where the variance is a consequence of local time variations in temperature. The CH 4 mole fraction at 1174 km is 2.71 ± 0.1 % . The effects of diffusive separation are clearly seen in the data that we interpret as an eddy diffusion coefficient of 4 −3 +4 × 10 9 cm 2 s −1 , that, along with the measured CH 4 mole fraction, implies a mole fraction in the stratosphere of 2.2 ± 0.2 % . The H 2 distribution is affected primarily by upward flow and atmospheric escape. The H 2 mole fraction at 1200 km is 4 ± 1 × 10 −3 and analysis of the altitude profile indicates an upward flux of 1.2 ± 0.2 × 10 10 cm −2 s −1 , referred to the surface. If horizontal variations in temperature and H 2 density are small, this upward flux also represents the escape flux from the atmosphere. The CH 4 density exhibits significant horizontal variations that are likely an indication of dynamical processes in the upper atmosphere.