The Juno spacecraft reached Jupiter in 2016 and has since been taking measurements of the gas giant’s magnetic and gravitational fields. Many interesting features on Jupiter’s surface, such as the intense isolated magnetic flux patch near the equator dubbed the “Great Blue Spot”, have been observed. Juno’s measurement also extends the Lowes spectrum—magnetic energy per spherical harmonic degree at the planet’s surface— considerably further than old data allowed. Using our numerical model of Jupiter’s dynamo, we investigate the connection between the shape of magnetic energy spectrum and the dynamo radius—the depth below the planetary surface at which dynamo action begins (Yue-Kin Tsang and Chris Jones, Earth Planet. Sci. Lett., vol. 530, 115879, Jan 2020).
Our dynamo model solves the MHD equations describing the behaviour of an electrically conducting fluid in a spherical shell driven by convection. A key point is that the electrical conductivity increases sharply but smoothly with depth so it is not known a priori where the dynamo radius is. The density of Jupiter varies greatly with depth, so an anelastic model is required to represent the convection accurately. Furthermore, the model parameters needs to be in the strong dynamo regime where the magnetic field strongly affects the flow. Both features are essential for realistic Jupiter models, but they increase computational cost significantly, making the use of the DiRAC facilities essential for our work.
The numerical model produces Jupiter-like dipolar dominant magnetic field. The Lowes spectrum at the surface, as well as the magnetic energy spectrum at different depths, are calculated. We find that within the region where magnetic field is being generated, the magnetic energy spectrum is shallow and maintains more or less the same shape. Outside this dynamo region, the spectrum steepens. This transition enables us to identify the dynamo radius in the model. We can compare our numerical spectrum with the spectrum observed by Juno, which shows that the dynamo radius of Jupiter cannot be deeper than 0.83 Jupiter radius.
Although Juno has revealed the intense flux patches predicted by our simulations, such as the great blue spot, the observed field is nevertheless smoother than expected at the high magnetic Reynolds numbers inside Jupiter. One possible explanation is the existence of a stably stratified layer just under the upper non-conducting molecular layer due to “helium rain-out”, high pressure making the helium in the gas giant form droplets which fall under gravity. This stable layer might also help explain the fierce zonal winds seen on Jupiter. We are currently including a stably stratified layer in our models, to see if this can give a dynamically consistent picture fitting the Juno data.
