Energy-banded ions in Saturn's magnetosphere

Abstract

Using data from the Cassini Plasma Spectrometer ion mass spectrometer, we report the first observation of energy-banded ions at Saturn. Observed near midnight at relatively high magnetic latitudes, the banded ions are dominantly H⁺, and they occupy the range of energies typically associated with the thermal pickup distribution in the inner magnetosphere (L < 10), but their energies decline monotonically with increasing radial distance (or time or decreasing latitude). Their pitch angle distribution suggests a source at low (or slightly southern) latitudes. The band energies, including their pitch angle dependence, are consistent with a bounce-resonant interaction between thermal H⁺ ions and the standing wave structure of a field line resonance. There is additional evidence in the pitch angle dependence of the band energies that the particles in each band may have a common time of flight from their most recent interaction with the wave, which may have been at slightly southern latitudes. Thus, while the particles are basically bounce resonant, their energization may be dominated by their most recent encounter with the standing wave.<abstract type="synopsis"><title type="main">Plain Language SummaryDuring an outbound passage by the Cassini spacecraft through Saturn's inner magnetosphere, ion energy distributions were observed that featured discrete flux peaks at regularly spaced energies. The peaks persisted over several hours and several Saturn radii of distance away from the planet. We show that these "bands" of ions are plausibly the result of an interaction between the Saturnian plasma and standing waves that form along the magnetospheric magnetic field lines. These observations are the first reported evidence that such standing waves may be present in the inner magnetosphere, where they could contribute to the radial transport of Saturn's radiation belt particles.