NASA, ESA, and J. Nichols (University of Leicester)
Not all auroras are created equal. Jupiter’s auroral glow is much stronger than Earth’s, so researchers assumed it was caused by the same process that generates our planet’s brightest auroras. But new observations from NASA’s Juno spacecraft show that’s not true.
“For many years we thought we understood Jupiter’s aurora,” says John Clarke at Boston University. “But then Juno got there and it went through these magnetic fields right above an active aurora, and it didn’t see what we thought it would.”
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Now, Barry Mauk at Johns Hopkins University in Maryland and his colleagues have analysed Juno’s data and found that the likely cause of powerful auroras on Jupiter is one does something quite different on Earth.
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On our planet, most intense auroras are the result of powerful electric fields building up along Earth’s magnetic field lines. This creates wells of electric potential – areas where the electric field changes sharply – that accelerate electrons from the solar wind toward the ground.
Twisting light
Those electrons collide with atoms in the upper atmosphere and give up some of their energy. When the atoms release that energy, they emit tiny bursts of light that together make up the aurora.
“On Earth, these electric potentials cause the most intense auroras – these are the beautiful twisting snake-like undulations that people marvel at when they go see the aurora in northern regions,” says Mauk.
But even though Jupiter’s electric potential wells are up to 30 times stronger than Earth’s, Mauk and his team found that they don’t align with the auroras.
“These electric potentials are not the source of the most intense aurora at Jupiter. That’s a surprise,” Mauk says. Instead, Jupiter’s auroras might come from a process that only causes dim auroras on Earth.
These weaker auroras can be caused by ripples in Earth’s magnetic field that accelerate electrons just a smidge. Some of these electrons can build up enough energy to light up the sky when they hit gas in the upper atmosphere.
Because Jupiter is so large, the electric potentials there could get so strong that they become unstable, turning into waves and random turbulence. It is these phenomena that can accelerate electrons so much that they produce a dazzling display, he says.
We often use models of Earth’s atmosphere and magnetic field as jumping-off points for describing other planets, but the Juno observations could force us to re-examine our assumptions, especially when it comes to understanding exoplanets – many of which are more like Jupiter than Earth.
Journal reference: Nature, DOI: 10.1038/nature23648
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