Magnetic signatures of ion cyclotron waves during Cassini's high-inclination orbits of Saturn

Abstract

Based on magnetic field data from Cassini's high-inclination orbits of Saturn (radius RS=60,268km), we analyze the latitudinal distribution of ion cyclotron waves in the giant planet's magnetosphere. Our survey takes into account magnetic field data from all high-inclination orbits between 2004 and 2015. We analyze the dependency of the occurrence rate and amplitude of the ion cyclotron waves on radial distance ρ to Saturn's rotation axis, vertical distance z to Saturn's equatorial plane, and magnetic latitude λ. The occurrence rate of ion cyclotron waves is approximately 100% in Saturn's equatorial plane between the orbits of Enceladus and Dione and decreases to 50% at altitudes of |z|≈0.6RS. Ion cyclotron waves were detected up to |z|=2.0RS. The occurrence rate displays strong, non-monotonic variations with respect to ρ, z, and λ. The vertical amplitude profile of the waves exhibits an M-like pattern with two distinct peaks near z=±0.3RS and the central minimum at z=0. Compared to earlier observations, we find this M-like structure to be inflated in±z direction by a factor of three. The available magnetic field data provides only weak evidence for a local impact of Enceladus and Dione on the ion cyclotron wave field. Using the observed Doppler shift of the ion cyclotron wave frequency during Cassini's high-inclination orbits, we demonstrate the existence of a narrow band of bidirectional wave propagation. This band is centered around Saturn's equatorial plane and possesses a half-width of |z|=0.15RS, which agrees well with the vertical scale height of Saturn's neutral cloud. To the north of this band, all ion cyclotron waves propagate towards the north (z>0); and to the south, all waves propagate towards the south (z<0). In companion with our previous study (Meeks et al., 2016), this survey provides the complete three-dimensional picture of the ion cyclotron wave field between the orbits of Enceladus and Rhea during the Cassini era.