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A magnetic insight into Jupiter’s striped appearance.

A magnetic insight into Jupiter’s striped appearance

26 May 2023

Torsional oscillations at layers below the clouds are key to understanding Jovian weather cycles.

An animated GIF showing glowing bands shifting over time.
Jupiter in IR light, as seen by NASA’s Infrared Telescope Facility. The animation showcases different bands and zones across the atmosphere. Credit: NASA/JPL-Caltech

Jovian weather is primarily tracked by studying the planet’s belts and zones. Those striped regions are what gives Jupiter its familiar appearance, but they are also indications of the planetwide weather (See Physics Today, May 2018, page 19). The clouds become lighter or darker, sometimes even appearing redder than usual. The changes are caused by long-term and quasiperiodic cycles, which alter how much heat rises from deep levels of Jupiter. The origin of those cycles, however, has been a mystery. IR observations at 5 μm (such as the IR image seen above) from the past 30 years have shown that the cycles have four- to nine-year intervals. Such long periods are unlikely to be a result of fluid dynamics alone, which typically creates changes on the scale of days, so something more must be driving the changes.

Kumiko Hori (Kobe University), Christopher Jones (University of Leeds), and colleagues have been able to look for clues deeper into the planet than previous researchers by using recent magnetic field data from NASA’s Juno spacecraft. The observations show gradual magnetic variations, which suggest a rotation of the fluid velocity in the east–west direction. The rotation creates an oscillation in Jupiter’s dynamo region, where the planet’s magnetic field is generated and sustained. Known as axisymmetric torsional oscillations, they resemble the way a bag of bagels rotates clockwise and counterclockwise a couple times when you let it untwist.

The torsional oscillations in Jupiter’s magnetic field affect the weather around it. Gases at different depths flow in opposite directions, creating a wave of alternating magnetic field lines that disrupts the heat flow. As that wave goes around the planet, it creates shear forces that push or pull the convective loops and alter the efficiency of heat transfer. Instead of looking like a rubber band looped around two pegs to form a simple ellipse, it resembles one that has been passed to the left and right of multiple pegs to make a longer and more curved path. As a result, heat transfer to higher levels isn’t even or constant. The uneven heat transport could cause the changing appearance of clouds and aerosols and trigger storms in Jupiter’s topmost layer.

Theoretical estimates connecting magnetic field fluctuation with the surface appearances are a good match to existing data. The computed periodicity of the cycles is a close match to the observed activity at northern latitudes, where most of the data are from. The researchers’ simulations are consistent with the observed drift of the Great Blue Spot—a region where the magnetic fields of the planet point inward, as seen in the illustration below.

Looping lines represent magnetic field lines around a sphere.
A still image from an animation illustrating Jupiter’s magnetic field at a moment in time. The Great Blue Spot—an-invisible-to-the-eye concentration of magnetic field near the equator—features prominently. Credit: NASA/JPL-Caltech/Harvard/Moore et al.

The authors point out that their work may unlock another way to study the interiors of gaseous planets. Just as seismologists use earthquakes to understand the interior composition of Earth, studying interior waves on Jupiter and other planets will give clues about the material they pass through. (K. Hori et al., Nat. Astron., 2023, doi:10.1038/s41550-023-01967-1.)

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