On the meaning of the dynamo radius in giant planets with stable layers

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 ~0.8R_J is obtained, where 1R_J is Jupiter's radius. This is rather deep, considering that the electrical conductivity inside Jupiter is expected to reach significant values at ~0.9R_J. Here we use 3-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 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 is at the base of the layer. For deeper stable layers, Lowes radii of ~0.87R_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 ~0.8R_J up to at least ~0.9R_J inside Jupiter. The physical origin of this extended stable layer and its connection to helium rain remain to be elucidated.