Determining ion production rates near Saturn's extended neutral cloud from ion cyclotron wave amplitudes

Recent Cassini observations of active venting of water molecules from Enceladus indicate that the moon is the primary source of Saturn's extended neutral cloud. Ionization of the neutrals through charge exchange creates a population of newborn ions with a velocity space distribution, which is highly unstable to the generation of electromagnetic ion cyclotron waves. Cassini observed such ion cyclotron waves, finding spatial and temporal variability in the wave amplitudes throughout the extended neutral cloud region. Since the amount of energy in the ion cyclotron waves is proportional to the number of newborn ions generating them, it is possible to infer the ion production rate in the region. To do so, we use two-dimensional electromagnetic hybrid (kinetic ions, fluid electrons) simulations to investigate the growth and nonlinear evolution of ion cyclotron waves. We focus on conditions near Enceladus' L shell and compare the simulated and observed ion cyclotron wave amplitudes to estimate the neutral densities and ion production rates. Our simulation results find a relatively linear relation between ion production rate and quasisteady wave energy level (delta B-2). For conditions near Enceladus' L shell, we find that water group ion production rates of 0.007-0.014/cc/s (which yield wave amplitudes of similar to 0.1-0.3 nT) are appropriate. For ion production within an annulus volume from 3.9 to 4 R-S, we obtain ion production rates of 3.8 x 10(26) to 7.6 x 10(26) ions/s or 10.2-20.4 kg/s.