Investigating Jupiter's Deep Zonal Flows using Juno Gravitational Data

Abstract

In this thesis, I use the high-precision Juno gravitational data to investigate zonal flows within the dynamo region, where the magnetic field is generated, and the deep atmosphere. I start by determining the gravitational contribution from the slow zonal flows in the dynamo region. I consider two types of fluid motions: steady zonal flows that satisfy Ferraro's law of isorotation and time-varying torsional oscillations (i.e., oscillatory cylindrical flows). For each zonal flow profile, I use the thermal wind equation to calculate the dynamical density perturbation and, from this, obtain the zonal gravitational harmonics. I find that steady dynamo zonal flows with RMS velocities of 10 cm s$^{-1}$ produce $J_{3}$ values that are on the same order as the Juno inferred value and $J_{2}$ and $J_{4}$ values that are on the same order as $\sim 3,000$ km deep atmospheric zonal flows, but do not contribute significantly to the zonal gravitational harmonics above $J_{4}$. Torsional oscillations with peak RMS velocities of 10 cm s$^{-1}$ generally produce $J_{2}$ and $J_{4}$ signals that are above the noise level of the Juno gravitational measurements. Since the period of these flows should be about $10$ years, it is possible that their gravitational signals could be detected by Juno during the extended mission. These results show that the dynamo region provides an important contribution to Jupiter's low-degree gravitational harmonics.

In the second part of my thesis, I test physically motivated models of deep atmospheric zonal flow against the odd zonal gravitational harmonics measured by Juno. First, I consider a model where the flow is barotropic and satisfies the Taylor-Proudman constraint (i.e., is $z$-invariant) until it is truncated at depth by some dynamical process. The model is agnostic on the truncation mechanism, although possibilities include a stably stratified layer or magnetohydrodynamic processes. I find that $\sim 1000$ km deep barotropic zonal flows involving the observed surface winds between $20.9^{\circ}$S-$26.4^{\circ}$N and a few smooth mid/high latitude jets produce odd zonal gravitational harmonics that are consistent with the Juno values. I then move to evaluate another physically motivated model where the entropy is homogenized along the spin axis in each hemisphere until it is disrupted at depth by the magnetic field. This model is unable to produce odd zonal gravitational harmonics that match the Juno measurements. I conclude that the Juno gravitational data is consistent with a model where the deep atmospheric zonal flow is barotropic, but inconsistent with a model where entropy is homogenized along the spin axis.