Abstract. X-ray emission due to charge transfer collisions between heavy solar wind ions and neutrals has been predicted to exist both in the heliosphere and in the geocorona. The heliospheric X-ray emission can account for roughly half of the observed soft X-ray background intensity. It was also suggested that temporal variations in the heliospheric and geocoronal soft X-ray intensities will result from solar wind variations. In this paper, a simple model of the charge exchange X-ray emission mechanism is combined with measured solar wind parameters as a function of time and used to generate predictions of the temporal variation of the X-ray intensity observed at Earth for the time periods 1990 -1993 and 1996-1998. Measured solar wind proton fluxes are also directly compared with the "long-term enhancement" part of the soft X-ray background measured by the R/3ntgen Satellite (ROSAT). A significant positive correlation exists, which supports the existence of X-ray emission associated with the solar wind interaction with either interstellar neutrals and/or with geocoronal neutral hydrogen.

[1] We present Cassini Ion and Neutral Mass Spectrometer (INMS) measurements of ion densities on the nightside of Titan from April 16, 2005, and show that a substantial ionosphere exists on the nightside and that complex ion chemistry is operating there. The total ionospheric densities measured both by the INMS and the Cassini Radio and Plasma Wave (RPWS) experiments on Cassini suggest that precipitation from the magnetosphere into the atmosphere of electrons with energies ranging from 25 eV up to about 2 keV is taking place. The absence of ionospheric composition measurements has been a major obstacle to understanding the ionosphere. Seven ''families'' of ion species, separated in mass-to-charge ratio by 12 Daltons (i.e., the mass of carbon), were observed and establish the importance of hydrocarbon and nitrile chains in the upper atmosphere. Several of the ion species measured by the INMS were predicted by models (e.g., HCNH + and C 2 H 5 + ). But the INMS also saw high densities at mass numbers not predicted by models, including mass 18, which we suggest will be ammonium ions (NH 4 + ) produced by reaction of other ion species with neutral ammonia.

Energetic protons and oxygen ions have been observed in Saturn's outer magnetosphere and can precipitate into Titan's atmosphere where they deposit energy, ionize, and drive ionospheric chemistry. Ion production rates caused by this precipitation are calculated using fluxes of incident 27 keV to 4 MeV protons measured by the Cassini MIMI instrument. We find that significant ion production rates exist in the 500 km to 1000 km altitude range and estimate associated electron densities of about 200–2000 cm−3 in reasonable agreement with measured densities. We demonstrate that energetic oxygen ions do not penetrate below about 650 km, but they can also generate significant ionization. We suggest that a few percent of the oxygen flux is converted to negative O ions as a consequence of charge exchange collisions, which might help explain the negative ions observed near 960 km by the Cassini CAPS instrument.

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.

[1] The neutral nitrogen and methane measurements made by Ion and Neutral Mass Spectrometer during Cassini flybys T A , T B , and T 5 in Titan's upper atmosphere and exosphere are presented. Large horizontal variations are observed in the total density, recorded to be twice as large during T A as during T 5 . Comparison between the atmospheric and exospheric data show evidence for the presence of a significant population of suprathermal molecules. Using a diffusion model to simultaneously fit the N 2 and CH 4 density profiles below 1500 km, the atmospheric structure parameters are determined, taking into account recent changes in the calibration parameters. The best fits are obtained for isothermal profiles with values 152.8 ± 4.6 K for T A , 149.0 ± 9.2 K for T B , and 157.4 ± 4.9 K for T 5 , suggesting a temperature '5 K warmer at night than at dusk, a trend opposite to that determined by solar-driven models. Using standard exospheric theory and a Maxwellian exobase distribution, a temperature of 20 to 70 K higher would be necessary to fit the T A , T B , and egress-T 5 data above 1500 km. The suprathermal component of the corona was fit with various exobase energy distributions, using a method based on the Liouville theorem. This gave a density of suprathermals at the exobase of 4.4 ± 5.1 Â 10 5 cm À3 and 1.1 ± 0.9 Â 10 5 cm À3 , and an energy deposition rate at the exobase of 1.1 ± 0.9 Â 10 2 eV cm À3 s À1 and 3.9 ± 3.5 Â 10 1 eV cm À3 s À1 for the hot N 2 and CH 4 populations, respectively. The energy deposition rate allowed us to roughly estimate escape rates for nitrogen of '7.7 ± 7.1 Â 10 7 N cm À2 s À1 and for methane of '2.8 ± 2.1 Â 10 7 CH 4 cm À2 s À1 . Interestingly, no suprathermal component was observed in the ingress-T 5 data.