No abstract

We analyse combined electron spectra across the dynamic range of both Cassini electron sensors in order to characterise the background plasma environment near Titan for 54 Cassini‐Titan encounters as of May 2009. We characterise the encounters into four broad types: Plasma sheet, Lobe‐like, Magnetosheath and Bimodal. Despite many encounters occurring close to the magnetopause only two encounters to date were predominantly in the magnetosheath (T32 and T42). Bimodal encounters contain two distinct electron populations, the low energy component of the bi‐modal populations is apparently associated with local water group products. Additionally, a hot lobe‐like environment is also occasionally observed and is suggestively linked to increased local pick‐up. We find that 34 of 54 encounters analysed are associated with one of these groups while the remaining encounters exhibit a combination of these environments. We provide typical electron properties and spectra for each plasma regime and list the encounters appropriate to each.

Solar 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.A model of Titan's nightside ionosphere will be described and the results compared with data from the Ion and Neutral Mass Spectrometer (INMS) and the Langmuir probe (LP) part of the Radio and Plasma Wave (RPWS) experiment for the T5 and T21 nightside encounters of the Cassini Orbiter with Titan.Electron impact ionization associated with the precipitation of magnetospheric electrons into the upper atmosphere is assumed to be the source of the nightside ionosphere, at least for altitudes above 1000 km. Magnetospheric electron fluxes measured by the Cassini electron spectrometer (CAPS ELS) are used as an input for the model. The model is used to interpret the observed composition and structure of the T5 and T21 ionospheres. The densities of many ion species (e.g., CH 5 + and C 2 H 5 + ) measured during T5 exhibit temporal and/or spatial variations apparently associated with variations in the fluxes of energetic electrons that precipitate into the atmosphere from Saturn's magnetosphere.

Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan's upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.planetary sciences | heterogeneous chemistry | particles charging | plasma P hotochemical aerosols are observed in many atmospheres of our solar system (1-3), and their presence is also detected in exoplanet atmospheres (4). Apart from their direct influence on the atmospheric properties through their interaction with the radiation field, aerosols have further astrobiological implications; their presence in the early Earth's atmosphere could have protected the surface and any life evolving there from UV radiation (5), and laboratory studies of Titan aerosol analogs have identified an in vitro formation of amino acids during aerosol production (6). However, the general mechanisms involved in the production and growth of photochemical aerosols from atmospheric gases have remained elusive. Titan, as the most extreme example of an aerosol-dominated atmosphere, provides a unique opportunity to investigate these mechanisms.Cassini observations revealed the presence of aerosols in multiple regions of the atmosphere, from the troposphere (7), stratosphere (8), and mesosphere (9, 10) up to the thermosphere where the detection of large mass positive and particularly negative ions has been suggested to be the signature of aerosol formation (11)(12)(13)(14). During a recent flyby (T70), the Cassini spacecraft penetrated to deeper regions of Titan's thermosphere than usual, reaching altitudes close to 880 km that had not previously been sampled with in situ measurements. Measurements from the Langmuir probe (LP) during this unique flyby reveal a high abundance of negative ions at closest approach, comparable to the positive ion density, and a corresponding decrease in the electron density (15). This conclusion is supported by the analysis of multiple Cassini observations, which reveals that the observed electron density is smaller than the density anticipated from photochemical equilibrium, implying that the photochemical models are missing a significant electron loss mechanism (16, 17). Thus, current observations demonstrate a significant decrease of the electron density relative to the positive ion density in the lower ionosphere with a concurrent increase in the density of the negative ions.Aerosols, or dust particles in general, are known to interact with free electr...