The magnetic field of Jupiter has a strange structure

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

This enhanced color image of Juno shows the swirling cloud bands of the southern hemisphere of Jupiter. Measurements of the magnetic field of the planet indicate that the magnetic field in this hemisphere is part of the overall dipolar structure, like a magnetic bar, but the field in the northern hemisphere is more complex.
NASA / JPL-Caltech / SwRI / MSSS / Kevin M. Gill

Jupiter has the strongest magnetic field of all planets in the solar system. As the field which shelters the Earth, it is essentially dipolarwhich means that it has a north pole and a south pole, like the field created by a magnetic bar. A very, really big magnetic bar.

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

But Jupiter does not have an iron core. In fact, it is not clear if it has a nucleus at all – Juno's observations suggest that the nucleus could be "fuzzy", a concentration of rock and ice dissolved (or still dissolving) in the surrounding hydrogen . Instead, the source of the global field is the overlay coat of metallic hydrogen, where hydrogen molecules negotiate electrons, creating currents. The rotation of the planet organizes the resulting magnetic field into a dipole.

Or at least that's the case. Report to September 6 NatureKimberly Moore (Harvard) and her colleagues discovered a strange magnetic field plume emerging from an area of ​​the northern hemisphere of Jupiter and returning to the planet at its equator. And it is three times stronger than the main dipolar field.

Detect the invisible

While flying over Jupiter, the Juno spacecraft measures the planet's magnetic field using two instruments called fluxgate magnetometers. At the center of each magnetometer are two rings made of a material that absorbs the magnetic field. Think of it as a magnetic sponge. Like a sponge, the material can not contain as much material before saturating.

John Connerney (NASA Goddard Space Flight Center), who leads the investigation of the Juno magnetometer and co-author of the new study. But if there is another magnetic field in the environment, the rings also permeate it. This will reduce the amount of field applied as the rings can absorb wires in one direction, but will increase the amount absorbed by the current flowing in the other direction. When the magnetometer cancels this imbalance using another structure wrapped around each of the rings, the instrument measures the strength of the environmental field as a function of the amount of current required to bring the field back into the rings at zero. The orientations of the coils indicate the direction of the external field.

But the magnetometer detects only the magnetic field traversed by the spacecraft. Researchers must extrapolate from these measurements, using detailed calculations to map the field on the planet's clouds and below.

By combining data from eight Juno flyovers, scientists confirmed the existence of this bizarre magnetic feature, whose clues were revealed last year by an analysis of Juno's first orbit. The structure looks like a ponytail gushing from the front of the planet and returning through the nose, to a place where the team calls the big blue spot (for its color in a map of the field of the planet). There is nothing like this ponytail in the southern hemisphere.

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

Juno is closer to the north pole of Jupiter than the south, but scientists are convinced that the difference does not affect their data: the resolution of their map of the northern hemisphere is only slightly better than that of the south, explains -t it. .

Why does this magnetic ponytail exist? Scientists do not know. The team considers several ideas in its article, the most likely being that there is some kind of superposition in the metallic hydrogen mantle that disrupts the convection pattern. Stratification could naturally occur with a dissolving core: rock and ice mixed with hydrogen would raise the density, and if this mixture were not uniform, it could create layers of different density that would destabilize the convection configurations cyclic or distinct layers.

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

Reference:

K. M. Moore et al. "A complex dynamo deduced from the hemispherical dichotomy of Jupiter's magnetic field." Nature. September 6, 2018.

Learn more about the Earth's magnetic field in The March 2018 issue of Sky & Telescope.