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

Abstract

Water group ions W⁺ (O⁺, OH⁺, H₂O⁺, and H₃O⁺), along with H⁺ and H₂⁺, dominate Saturn's near-equatorial magnetospheric suprathermal ion populations. The singly charged, minor heavy ions O₂⁺ and ²⁸M⁺ 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 O₂⁺/W⁺ transient and seasonal responses to variable insolation of Saturn's ring atmosphere prior to mid-2012. A similar seasonal variation in ²⁸M⁺/W⁺ (²⁸M⁺ ~27-30 amu/e molecular minor ions) was suggested but inconclusive. Now with data from mid-2012 through 2013, we find that both O₂⁺ and ²⁸M⁺ clearly exhibit seasonal recoveries from mid-2012 onward. Prominent radial partial number density peaks at ~9 R_s identify W⁺, O₂⁺, and ²⁸M⁺ as clear ring current participants. It is presently unclear which part of Saturn's magnetosphere produces the seasonally varying ²⁸M⁺ component. Dissimilar ²⁸M⁺/W⁺ and O₂⁺/W⁺ responses to a strong late 2011 solar UV burst suggest different seasonal ring-based photolytic processes.