A Multi‐Fluid Model of Plasma Transport in the Equatorial Ionosphere of Saturn

Abstract The Cassini mission made the first in situ measurements of the ionosphere of Saturn during its Grand Finale in 2017. The Ion and Neutral Mass Spectrometer (INMS) instrument found molecular hydrogen and helium as well as minor species including water, methane, ammonia, and organics. INMS also made measurements of light ion species (H + , H 2 + , H 3 + , He + ). For a single orbital pass through the ionosphere, INMS measured the energies of ionospheric protons and demonstrated the existence of flow along magnetic field lines at some latitudes and at high altitudes. Flow speeds of a few km/s were found. This work applies a multi‐fluid transport model to the equatorial ionosphere of Saturn in order to explore plasma flow along magnetic field lines. Two field line cases were considered: one for an L‐shell of 1.1 and the other for L = 1.2. For the second case, the field line enters the shadow of the B‐ring on the atmosphere and ionosphere. Simple ion chemistry is adopted for the main ionosphere, but the focus of this work is on ion transport. Diurnal variation of the ionosphere is studied by varying the solar photoionization rate. Model results are generally within a factor of 2–3 of INMS ionospheric data. The role of ring shadowing of the ionosphere is shown to be important for determining plasma flow. Without ring shadowing, proton velocities are on the order of 0.1 km/s and are symmetric across hemispheres. With ring shadowing, proton velocities can exceed 8 km/s flowing southward for northern summer.