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Friday 3 December 2021

What has Juno found on Jupiter? Part II – It’s magnetic

Built with a 20 radius and designed to spin, Juno is made to measure the magnetic field of Jupiter. Thanks to Juno, we now know that the planet’s dipole is the opposite way round (North and South) to Earth, and tilted ~10o from its rotational axis. The strength of the magnetic field (20 x that of Earth’s!) allows us to calculate how long a day is on Jupiter – because we can’t tell just by looking at the bands: they seem to move in opposite directions to each other and at different speeds! It also allows Jupiter to deflect solar winds as far out as 6 million km from the planet and hold onto its atmosphere. At this point, we also see weird effects that Juno is attempting to explain, such as ring-like features, known as Kelvin-Helmholtz instabilities, which scientists think may travel along the planet’s magnetic field lines. As well as the dipole, these include weaker quadrupoles and octupoles.
Jupiter's magnetosphere showing the Io Plasma Torus (in red). Yned via Wikipedia Commons.

Scientists also think the magnetic field of Jupiter holds its high energy particle plasma torus in place and rotates it round the planet. The plasma forms when the highly volcanic Jovian moon Io ejects as much as 1,000 kg sulfur dioxide gas into space per second. When Juno investigated, scientists thought its camera would be quickly destroyed, but somehow it survived, getting accurate readings of the flux tube of electrical current between the moon and planet: 400,000V or 1-5 million A. This phenomenon is known as a Van Allen radiation belt. So far the mechanisms behind the radiation belt are unknown, but the radiation is lower than might be expected from other measurements.

The magnetic field structure of Jupiter is also changing – and so far it’s the only planet that seems to do this. Known as secular variation, it’s a mystery why it happens, although scientists theorise it may be driven by the base of the atmosphere. It’s strongest in one particular spot – around the great blue spot, a weird blip where the magnetic field is super concentrated. We don’t know much about this spot, including how it formed, how it might change, nor what it’s lifetime is.

Internal structure of Jupiter.
We previously thought Jupiter had a solid core surrounded by metallic hydrogen – hydrogen at such high pressures that it falls apart, making a soup of protons (the hydrogen nucleus) and electrons. In this condition, it acts like a metal (although it is properly described as a plasma). If you look up models of the planet Jupiter's structure, this is still what you will find. But now scientists don't think this is true. If there even is a core (and scientists doubt it), it’s fuzzy, potentially mixing up with the metallic hydrogen layer. Measurements taken by Juno suggest it’s very light, and calculate that it’s a mixture of hydrogen and helium. It’s this core that scientists think cause the intensely bright aurora – so bright, in fact, that the direct current measured can’t account for it. This is why scientists speculate it forms because of an alternating current caused by turbulence in the magnetic field. Despite decades of observations, we still don’t understand the aurora, so Juno’s hard work continues.

For more information about Jupiter, check out our updated article, and if you haven’t already seen it, check out our part I blog post on Juno’s findings, featuring weather and water.a

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