Suprathermal magnetospheric minor ions heavier than water at Saturn: Discovery of 28 M + seasonal variations

Water group ions W + (O + , OH + , H 2 O + , and H 3 O + ), along with H + and H 2 + , dominate Saturn's near‐equatorial magnetospheric suprathermal ion populations. The singly charged, minor heavy ions O 2 + and 28 M + were also observed in the suprathermal energy range, but at much lower densities, having ≤10 −2 the abundance of W + . From 2004 through 2013, Cassini's charge‐energy‐mass ion spectrometer has measured suprathermal 83–167 keV/ e heavy ions at ~4–20 R s (1 Saturn radius, R s  = 60,268 km). Christon et al. (2013) found apparent O 2 + /W + transient and seasonal responses to variable insolation of Saturn's ring atmosphere prior to mid‐2012. A similar seasonal variation in 28 M + /W + ( 28 M + ~27–30 amu/ e molecular minor ions) was suggested but inconclusive. Now with data from mid‐2012 through 2013, we find that both O 2 + and 28 M + clearly exhibit seasonal recoveries from mid‐2012 onward. Prominent radial partial number density peaks at ~9 R s identify W + , O 2 + , and 28 M + as clear ring current participants. It is presently unclear which part of Saturn's magnetosphere produces the seasonally varying 28 M + component. Dissimilar 28 M + /W + and O 2 + /W + responses to a strong late 2011 solar UV burst suggest different seasonal ring‐based photolytic processes.