JUPITER’S MAGNETOSPHERE

As the largest structure in the solar system, the magnetosphere is about 21 million kilometers (12 million miles) wide. That’s 150 times wider than the planet itself and almost 15 times wider than the sun. If the magnetosphere were visible from Earth, it would appear as big as the moon. The solar wind, composed of charged particles ejected from the Sun, streams past Jupiter, elongating the shape of the magnetosphere into a teardrop. Its tail stretches all the way to Saturn’s orbit, twice as far from the Sun as Jupiter.    

Juno’s orbit takes it over Jupiter’s poles, allowing the spacecraft to avoid most of the harmful radiation that encircles the planet. Juno’s trajectory also provides a spectacular tour over an unexplored part of Jupiter. The spacecraft will journey through a region called the polar magnetosphere, where charged particles pour into the planet’s atmosphere, creating a dazzling display of energy and light called auroras.

The unique orbit allows Juno to watch the auroras in high resolution with infrared, ultraviolet and visible-light cameras. The craft will also sample the streams of charged particles that generate these polar light shows. Additionally, it will measure the magnetic and electric fields that guide and power these particles. Juno will also listen to radio signals produced by the processes that spark the auroras.    

Surrounding Jupiter is a belt of radiation and charged particles - protons, electrons, and ions – like an electric doughnut. These particles fly around and can damage electronics, posing a hazard for any spacecraft visiting Jupiter. Earth has similar - albeit weaker - rings of radiation called the Van Allen Belts.

To keep Juno safe from radiation, its most sensitive electronics are protected in a shielded container.    

A mixture of freely flowing electrons and ions, a plasma is a distinct state of matter, in addition to gases, liquids and solids. Plasma is also the most common form of matter, making up more than 99 percent of the visible universe. It’s in stars and in the tenuous wisps of gas that fills the voids between them.

A plasma forms when gas becomes ionized – when atoms gets stripped of their electrons, producing an electrically charged soup of electrons and ions. This can happen inside Jupiter, where the tremendous pressure and scalding temperatures squeeze electrons off atoms. In space, stray electrons zipping around can knock an electron off a neutral atom. Ultraviolet light, in the form of high-energy photons, can also ionize gases to make plasma.

Since plasma is electrically charged, it’s influenced by electric and magnetic fields. Jupiter’s magnetic field, for example, confines plasma inside its magnetosphere, channeling and pumping it through space, which generates various phenomena like Jupiter’s auroras, radiation belts and radio signals.    

On Earth, auroras – or the northern and southern lights – appear as brilliant ribbons of shimmering light across the sky. These displays are triggered when Earth’s magnetic field funnels electrically charged particles into the planet’s atmosphere near the poles. To see the light shows, you have to be close enough to the poles.

Since Jupiter has the strongest magnetic field of any planet, its auroras are the brightest in the solar system – sometimes 10 times brighter and 100 times more energetic than Earth’s. In fact, the charged particles that bombard Jupiter’s poles dump more energy onto the atmosphere than sunlight. The auroras are visible as a glowing ring around Jupiter’s poles, and they’re especially brilliant in ultraviolet and infrared light.

As Jupiter spins, so does its magnetic field, sweeping through the electrically charged gas that fills the magnetosphere. It turns out that the magnetic field’s interaction with the charged gas – called plasma – slows down Jupiter’s rotation. Because of this interaction, Jupiter’s rotation energy is transferred into the plasma and eventually takes the form of the bright auroras. A similar process happens with other astronomical objects with magnetic fields, such as the Sun early in its history and other young stars.

The rotation of the plasma in Jupiter’s magnetosphere is the main engine that drives the auroras. Earth’s auroras, on the other hand, are fueled by the energy of the solar wind – gusts of charged particles flowing from the sun at millions of miles per hour.

Jupiter’s moons are another energy source that powers the auroras. Jupiter’s magnetic field interacts with Io, Europa and Ganymede in such a way that it channels charged particles between the planet and its moons, creating imprints of bright spots along the glowing auroral rings. The solar wind may also play a role in changing the patterns of light near Jupiter’s poles.

Needless to say, the exact processes that produce Jupiter’s auroras are complex and not well understood. But Juno aims to change that.