Using He++ and H2+ to Study the Two-way Passage of Suprathermal Ions through Saturn's Magnetopause

He⁺⁺ ions are an excellent tracer of solar wind entry into the magnetosphere of Saturn. H₂⁺ ions, on the other hand, originate entirely from within the magnetosphere and are an excellent tracer of the entry of magnetospheric particles into the magnetosheath. The mixing of the two species in the magnetosheath, boundary layer, and outer magnetosphere provides information on transport across the magnetopause.

The Cassini/CHEMS sensor measures the composition of suprathermal ions over the energy range 3-220 keV/e. CHEMS determines an ion's mass per charge ratio (m/q) via measurements of energy per charge (E/Q) and time-of-flight (TOF). A measurement of kinetic energy by solid-state detectors allows m and q to be separately determined, but only at energies above the SSD threshold of about 27 keV. Since both He⁺⁺ and H₂⁺ have m/q=2, this would imply that the two species cannot be distinguished below that energy. However, we have found that He⁺⁺ and H₂⁺ can largely, but not completely, be separated all the way down to 3 keV/e by making use of the small difference in TOF resulting from a similarly small difference in energy lost in the thin carbon foil at the start of the TOF path.

We have found that, typically, He⁺⁺ from the shocked solar wind dominates the m/q=2 particles in the magnetosheath below 15 keV/e. At higher energies H₂⁺ becomes more abundant, indicating escape of H₂⁺ into the sheath. We will examine these He⁺⁺/ H₂⁺ ratios in the sheath and nearby boundary layer and outer magnetosphere as a function of energy, magnetospheric conditions, and proximity of Titan, a strong source of H₂⁺.