Deep convection-driven vortex formation on Jupiter and Saturn

A rapidly time-varying equatorial jet in Jupiter's deep interior

Planetary magnetic fields provide a window into the otherwise largely inaccessible dynamics of a planet's deep interior. In particular, interaction between fluid flow in electrically conducting interior regions and the magnetic field there gives rise to observable secular variation (time dependency) of the externally observed magnetic field. Secular variation of Jupiter's field has recently been revealed1-3 and been shown to arise, in part, from an axisymmetric, equatorial jet2. Whether this jet is time dependent has not previously been addressed, yet it is of critical importance for understanding the dynamics of the planet's interior. If steady, it would probably be a manifestation of deep dynamo convective flow (and jets are anticipated as part of that flow4-9) but if time dependent on a timescale much shorter than the convective turnover timescale of several hundred years, it would probably have a different origin. Here we show that the jet has a wavelike fluctuation with a period of roughly 4 years, strongly suggestive of the presence of a torsional oscillation10 (a cylindrically symmetric oscillating flow about the rotation axis) or a localized Alfvén wave in Jupiter's metallic hydrogen interior. This opens a pathway towards revealing otherwise hidden aspects of the magnetic field within the metallic hydrogen region and hence constraining the dynamo that generates Jupiter's magnetic field.

Polar vortex crystals: Emergence and structure

Vortex crystals, geometric arrays of like-signed vortices, are observed in natural systems with vastly different space and time scales: at the poles of Jupiter (∼10,000-km radius and lifetime of at least 5 y) and in laboratory experiments with pure-electron plasma (∼3.5-cm radius, lifetime of about 1.7 s). We follow the adage “less is more” and show that minimal physics is required for polar vortex crystals formation and persistence. Crystals, resembling those of Jupiter, form from the free evolution of an unstratified and rapidly rotating fluid in an axisymmetric geometry. An essential ingredient in this minimal model is the decrease of the vertical component of the Coriolis force with distance from the pole. Once formed, the crystal seems to survive indefinitely.

Vortex crystals are quasiregular arrays of like-signed vortices in solid-body rotation embedded within a uniform background of weaker vorticity. Vortex crystals are observed at the poles of Jupiter and in laboratory experiments with magnetized electron plasmas in axisymmetric geometries. We show that vortex crystals form from the free evolution of randomly excited two-dimensional turbulence on an idealized polar cap. Once formed, the crystals are long lived and survive until the end of the simulations (300 crystal-rotation periods). We identify a fundamental length scale, Lγ=(U/γ)1/3, characterizing the size of the crystal in terms of the mean-square velocity U of the fluid and the polar parameter γ=fp/ap2, with f_p the Coriolis parameter at the pole and a_p the polar radius of the planet.

The depth of Jupiter's Great Red Spot constrained by Juno gravity overflights

[Figure: see text].

Microwave observations reveal the deep extent and structure of Jupiter's atmospheric vortices

[Figure: see text].