The ring atmosphere is primarily generated by photolytic decomposition of water ice producing O2 and H2 (Johnson et al., 2006). In addition, it is predicted seasonal variations in the ring atmosphere and ionosphere due to the orientation of the ring plane to the sun (Tseng et al., 2010). The presence of an O2 atmosphere over Saturn’s main rings has been confirmed by the Cassini spacecraft at SOI (Waite et al., 2005; Tokar et al., 2005), and the seasonal dependence of the ring atmosphere and ionosphere is also showed in the following measurements (e.g., Elrod et al., 2012; 2014; Christon et al., 2013; 2014; Persoon et al., 2015).
The ring atmosphere and ionosphere play an important role in the coupling dynamics between the main rings and the Saturnian system. First, they are the sources of neutrals (i.e., O2, H2, H; Tseng et al., 2013a) and plasma (i.e., O2+ and H2+; Tseng et al., 2011) in the Saturnian system. Second, the main rings have strong interaction with Saturn’s atmosphere and ionosphere (i.e., the “ring rain”; O’Donoghue et al., 2013). Today I will focus on the complex plasma environment between the main rings and Enceladus (Tseng et al., 2013b) and the Cassini CAPS/RPWS data suggested the presence of negatively charged nanograin in this region (Johnson et al., submitted).