Abs[ract. We have developed a one-dimensional coupled ion-neutral photochemical model for Saturn's upper atmosphere to better understand the structure and chemistry of Saturn's ionosphere. In addition to modeling the chemistry of hydrogen and hydrocarbon ions, we investigate the effects of an oxygen and metal influx from ring or meteoric sources. The Infrared Space Observatory observations of H20 and CO2 in Saturn's stratosphere are used to constrain the influx of extraplanetary material. As expected, the topside ionosphere of Saturn is dominated by H +, with H3 + prevailing just below the electron density peak. When micrometeoroid ablation is considered, we find that metal ions, represented here by Mg +, can take the place of hydrocarbon ions as the major ionic species in the lower ionosphere. The models then exhibit a characteristic double peak, with H + creating the high-altitude peak and Mg + the low-altitude peak. A pronounced gap forms between the two peaks, especially at night, when H3 + ions rapidly recombine. Neutral winds and electric fields in the presence of magnetic fields can cause vertical plasma motion that can shift the location of both electron density peaks. In addition, multiple sharp layers in the electron density profile can form in the lower ionosphere when oscillatory vertical drifts are introduced into the model to simulate the effects of atmospheric gravity waves. The location and magnitude of the "main peak" as well as the sharper lower-ionospheric layers observed with the Voyager and Pioneer radio occultation experiments (and eventually with similar Cassini observations) can help constrain the atmospheric structure, wind profiles, or electric field properties in Saturn's upper atmosphere.

A new method for the retrieval of the spectral refractive indices of micrometer-sized particles from infrared aerosol extinction spectra has been developed. With this method we use a classical damped harmonic-oscillator model of molecular absorption in conjunction with Mie scattering to model extinction spectra, which we then fit to the measurements using a numerical optimal estimation algorithm. The main advantage of this method over the more traditional Kramers-Kronig approach is that it allows the full complex refractive-index spectra, along with the parameters of the particle size distribution, to be retrieved from a single extinction spectrum. The retrieval scheme has been extensively characterized and has been found to provide refractive indices with a maximum uncertainty of approximately 10% (with a minimum of approximately 0.1%). Comparison of refractive indices calculated from measurements of a ternary solution of HNO3, H2SO4, and H2O with those published in J. Phys. Chem. A 104, 783 (2000) show similar differences as found by other authors.

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[1] Extinction spectra of H 2 SO 4 -HNO 3 -H 2 O supercooled ternary solution (STS) aerosol particles, fully covering the mid-and near-infrared region from 750 to 13,000 cm À1 (13.9 to 0.7 mm), have been measured in the laboratory using a 75 dm 3 volume coolable aerosol cell coupled with a Fourier transform spectrometer. The STS spectra were recorded at temperatures close to those encountered in the polar stratosphere and were fitted to a spectrum modelled by combining Mie scattering theory and published thin film refractive index data. Good agreement between model and laboratory data was found in the region 1300-13,000 cm