What can be learned from the absence of auroral X-ray emission from Saturn?

To understand the origin and magnitude of the present upper limit observations of Saturn's auroral X‐ray emission, we use simple models based on the mechanism that leads to analogous emission at Jupiter, charge transfer between ion precipitation and atmospheric gas. Several putative sources and characteristics of the precipitation are considered, namely, (1) highly charged solar wind ions with additional acceleration and (2) ambient, thermal ion population originating, for example, from Saturn's satellites, and then accelerated to high energies. Estimates obtained for each of these sources show the need for acceleration, either to focus the highly charged solar wind ions into the atmosphere or to enable stripping of the initially low‐charge state ambient ions to higher charges. The former yields a constraint on the existing accelerating potentials present at Saturn but can only account for about a tenth of the observed upper limit to the auroral luminosity, while the latter requires extremely low limits on the area (i.e., less than 100 km2) over which field‐aligned potentials are active and needed to produce the acceleration to generate the observational upper limit on the X‐ray luminosity. We therefore narrow the range of possible ion sources, the accelerating potentials required that are consistent with the present understanding of the magnetosphere, and model upper limit of X‐ray emission from ion precipitation.