The effects of external material on the chemistry and structure of Saturn's ionosphere

We have developed a one-dimensional coupled ion-neutral photochemical model for Saturn's upper atmosphere to better understand the structure and chemi stry of Saturn's ionosphere. In addition to modeling the chemistry of hydro gen and hydrocarbon ions, we investigate the effects of an oxygen and metal influx from ring or meteoric sources. The Infrared Space Observatory obser vations of H2O and CO2 in Saturn's stratosphere are used to constrain the i nflux of extraplanetary material, As expected, the topside ionosphere of Sa turn is dominated by H+, with H-3(+) prevailing just below the electron den sity peak. When micrometeoroid ablation is considered, we find that metal i ons, 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 cha racteristic double peak, with H+ creating the high-altitude peak and Mg+ th e low-altitude peak. A pronounced gap forms between the two peaks, especial ly at night, when H-3(+) 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, m ultiple sharp layers in the electron density profile can form in the lower ionosphere when oscillatory vertical drifts are introduced into the model t o simulate the effects of atmospheric gravity waves. The location and magni tude of the "main peak" as well as the sharper lower-ionospheric layers obs erved with the Voyager and Pioneer radio occultation experiments (and event ually with similar Cassini observations) can help constrain the atmospheric structure, wind profiles, or electric field properties in Saturn's upper a tmosphere.