The structure and time variability of the ring atmosphere and ionosphere

The saturnian system is subject to constant bombardment by interplanetary meteoroids and irradiation by solar UV photons. Both effects release neutral molecules from the icy ring particles either in the form of impact water vapor or gas emission in the form of H2O, O-2 and H-2. The observations of the Cassini spacecraft during its orbit insertion have shown the existence of molecular and atomic oxygen ions. Subsequent modeling efforts have led to the picture that an exospheric population of neutral oxygen molecules is probably maintained in the vicinity of the rings via photolytic-decomposition of ice and surface reactions. At the same time, ionized products O+ and O-2(+) ions move along the magnetic field lines and, depending on the optical local thickness rings, can thread through the ring plane or impact a ring particle, the ion principal sink. In addition, collisional interactions between the ions and neutrals will change the scale height of the ions and produce a scattered component of O-2 molecules and O atoms which can be injected into Saturn's upper atmosphere or the inner magnetosphere. The ring atmosphere, therefore, serves as a source of O-2(+) ions throughout Saturn's magnetosphere. If photolysis of ice is the dominant source of O-2, then the complex structure of the ring atmosphere/ionosphere and the injection rate of neutral O-2 will be subject to modulation by the seasonal variation of Saturn along its orbit. In this work, we show how the physical properties of the ring oxygen atmosphere, the scattered component, and the magnetospheric O-2(+) ion source rate vary as the ring system goes through the cycle of solar insolation. In particular, it is shown that the magnetopheric O-2(+) ions should be nearly depleted at Saturn's equinox if O-2 is produced mainly by photolysis of the ring material. (C) 2009 Elsevier Inc. All rights reserved.