# Characterising Jupiter's dynamo radius using its magnetic energy spectrum

## TL;DR

This study uses a numerical model of Jupiter's dynamo to identify a dynamo radius based on magnetic energy spectrum analysis, comparing it with observational data and discussing implications for Jupiter's interior structure.

## Contribution

The paper introduces a method to estimate Jupiter's dynamo radius using magnetic energy spectrum analysis and compares model predictions with Juno spacecraft observations.

## Key findings

- The dynamo radius is identified where the magnetic energy spectrum shape becomes invariant.
- The Lowes radius provides a lower bound to the dynamo radius in Jupiter.
- Juno observations suggest a smaller Lowes radius than models, possibly due to a stably stratified layer.

## Abstract

Jupiter's magnetic field is generated by the convection of liquid metallic hydrogen in its interior. The transition from molecular hydrogen to metallic hydrogen as temperature and pressure increase is believed to be a smooth one. As a result, the electrical conductivity in Jupiter varies continuously from being negligible at the surface to a large value in the deeper region. Thus, unlike the Earth where the upper boundary of the dynamo---the dynamo radius---is definitively located at the core-mantle boundary, it is not clear at what depth dynamo action becomes significant in Jupiter. In this paper, using a numerical model of the Jovian dynamo, we examine the magnetic energy spectrum at different depth and identify a dynamo radius below which (and away from the deep inner core) the shape of the magnetic energy spectrum becomes invariant. We find that this shift in the behaviour of the magnetic energy spectrum signifies a change in the dynamics of the system as electric current becomes important. Traditionally, a characteristic radius derived from the Lowes--Mauersberger spectrum---the Lowes radius---gives a good estimate to the Earth's core-mantle boundary. We argue that in our model, the Lowes radius provides a lower bound to the dynamo radius. We also compare the Lowes--Mauersberger spectrum in our model to that obtained from recent Juno observations. The Lowes radius derived from the Juno data is significantly lower than that obtained from our models. The existence of a stably stratified region in the neighbourhood of the transition zone might provide an explanation of this result.