Abstract Current structure models of Jupiter and Saturn suggest that helium becomes immiscible in hydrogen in the outer part of the planets’ electrically conducting regions. This likely leads to a layer in which overturning convection is inhibited due to a stabilizing compositional gradient. The presence of such a stably stratified layer impacts the location and mechanism of convectively driven dynamo action. Juno’s measurements of Jupiter’s magnetic field enabled an estimate of its dynamo radius based on the magnetic Lowes spectrum. A depth of ∼0.8 R J is obtained, where 1 R J is Jupiter’s radius. This is rather deep, considering that the electrical conductivity inside Jupiter is expected to reach significant values at ∼0.9 R J . Here, we use three-dimensional numerical dynamo simulations to explore the effects of the existence and location of a stably stratified helium rain layer on both the inferred Lowes radius and the location of the radial extent of dynamo action. We focus on a Jupiter-like internal structure and electrical conductivity profile. We find that for shallower stable layers, there is no magnetic field generation occurring above the stable layer, and the effective dynamo radius and the inferred Lowes radius are at the base of the layer. For deeper stable layers, Lowes radii of ∼0.87 R J are inferred as a shallow secondary dynamo operates above the stable layer. Our results strongly suggest the existence of a stable layer extending from ∼0.8 R J up to at least ∼0.9 R J inside Jupiter. The physical origin of this extended stable layer and its connection to helium rain remain to be elucidated.
Wulff et al. (2025) studied this question.