Saturn Magnetosphere Plasma Density Budget: Enceladus Neutral Source, Ion Production and Cassini Plasma Measurements

This paper computes a quantitative budget for the magnetosphere of Saturn from published spacecraft values, using a deliberately transparent calculation whose every step can be checked from the deposited script. The headline results are: neutral_source_kg_s = 300; ion_source_rate_kg_s = 60; plasma_inventory_kg = 1.56e+08; mean_ion_density_per_m3 = 1.93e+09; residence_time_days = 30. The computed mean density, converted, lies below the peak densities Cassini measured near the source orbit, as it should: the calculation spreads plasma through the full annular volume while measurements peak where the plasma concentrates near the equator and near the source longitude sector. The agreement at order-of-magnitude level, from a source rate measured by entirely different instruments, supports the picture in which Enceladus alone sustains the inner magnetosphere's plasma content, with ring and Titan contributions secondary in this region. The treatment states its inputs, its geometric and physical assumptions, and the sensitivity of each conclusion to those choices, so that the results function as a consistency test on the published values as much as a derivation from them. Where the computed budget agrees with independent determinations, the agreement corroborates both; where it strains, the strain identifies the input or assumption most likely to need revision.