Longitude dependences of energetic H⁺ and O⁺ at Saturn

Abstract

The charge-energy-mass spectrometer instrument in the Cassini spacecraft measured differential fluxes of protons (2.8-236 keV) and oxygen ions (8.8-236 keV) from July 2004 to August 2007. The fluxes were bin-averaged in Saturn longitude system (SLS) longitude within ±5 R_S of the equator and between 8 and 12 R_S in radial distance (1 R_S = 60,238 km) to determine their global morphology. The 3-year time period is the range of validity of the SLS, which is based on a variable period of Saturn kilometric radiation. Fluxes at all energies of H⁺ and O⁺ display an essentially sinusoidal variation in longitude, often with peak-to-trough ratios of 2:1. For E < 77 keV, the maxima are consistently at ∼70° longitude (the minima are at ∼230°). For E > 77 keV; the maxima shift to ∼250°. The ion distributions closely resemble those of energetic neutral hydrogen and neutral oxygen atoms observed by the ion neutral camera onboard Cassini.