jupiter blue spot magnetic pole

Above: Jupiter's magnetic field lines. (a) north polar view; (b) south polar view; (c) equatorial view

Juno observations reveal that Jupiter's magnetic field has a wacky plume.

Jupiter has the strongest magnetic field of any of the planets in the solar system. Like the field that shelters Earth, it's essentially dipolar, which means it has a north pole and a south pole, like the field created by a bar magnetic. A really, really big bar magnetic.

Earth's magnetic field is produced by churning liquid iron in the planet's outer core. Iron conducts electricity, and a changing electrical current creates a magnetic field. So as the liquid iron cycles up and down, carrying heat from the planet's center up to the mantle and then sinking again, it creates powerful electrical currents that in turn produce the planet's global field.

But Jupiter doesn't have an iron core. In fact, it's unclear if it has a core at all - Juno's observations suggest the core might be "fuzzy," a concentration of rock and ice that has dissolved (or is still dissolving) into the surrounding hydrogen. Instead, the source of the global field is the overlying mantle of metallic hydrogen, where hydrogen molecules trade electrons, creating currents. The planet's rotation organizes the resulting magnetic field into a dipole.

Or, at least it kind of does. Reporting in the September 6th Nature, Kimberly Moore (Harvard) and colleagues have discovered a strange plume of magnetic field shooting up from a region in Jupiter's northern hemisphere and reentering the planet at its equator. And it's three times stronger than the main dipole field.

Detecting the Invisible

As it flies around Jupiter, the Juno spacecraft measures the planet's magnetic field using two instruments called fluxgate magnetometers. At each magnetometer's core lie two rings, made of a material that soaks up magnetic field. Think of it like a magnetic sponge. Like a sponge, the material can only hold so much before it saturates.

The scientists can magnetically "fill up" the rings by running current through wires coiled around them, first one direction, then the other, explains John Connerney (NASA Goddard Space Flight Center), who heads up Juno's magnetometer investigations and is a coauthor on the new study. But if there's another magnetic field in the environment, the rings will soak it up, too. That will reduce how much of the applied field the rings can absorb from the wires in one direction, but increase the amount absorbed from current flowing the other direction. When the magnetometer cancels out this imbalance using another wire-wound structure around each of the rings, the instrument measures how strong the environmental field is based on how much current it takes to push the field in the rings back to zero. The coils' orientations give the external field's direction.

But the magnetometer only detects the magnetic field the spacecraft is flying through. The researchers have to extrapolate from those measurements, using detailed calculations to map the field at the planet's cloudtops and below.

Combining data from eight of Juno's flybys, the scientists confirmed the existence of the bizarre magnetic feature, hints of which had shown up in an analysis last year from Juno's first orbit. The structure looks like a ponytail shooting out from the planet's forehead and reentering through the nose, at a location the team is calling the Great Blue Spot (for its color in a map of the planet's field). There's nothing like this ponytail in the southern hemisphere.

"This was a very unexpected result," Moore says. "Why is the field so simple in one hemisphere and so complicated in the other?"

Juno does fly closer to Jupiter's north pole than it does to the south, but the scientists are confident that the difference isn't affecting their data: The resolution of their northern hemisphere map is only slightly better than that for the southern one, she explains.

Why does this magnetic ponytail exist? Scientists don't know. The team considers several ideas in their paper, the most likely being that there's some sort of layering in the metallic hydrogen mantle that's messing with the convection pattern. Layering could naturally arise with a dissolving core: Rock and ice mixed in with hydrogen would raise the density, and if that mixing isn't uniform, it could create layers of different density that could destabilize the cyclic convection patterns or spur different convection patterns in distinct layers.

Each scenario could lead to its own magnetic pattern at high latitudes. As Juno continues its dives, it will gather the polar observations needed to determine which theory best fits the data.


K. M. Moore et al. "A Complex Dynamo Inferred from the Hemispheric Dichotomy of Jupiter's Magnetic Field." Nature. September 6, 2018.