Both a bar magnet (left) and Earth are surrounded by magnetic fields with north and south poles. Earth’s field is shaped by charged particles – electrons and protons – flowing from the sun called the solar wind. Credit: Andy Washnik (left) and NASA
On Earth the aurora is intimately connected to solar activity. High speed electrons and protons from the sun find their way into the upper atmosphere by following invisible lines of magnetic force that surround our planet much like the those around an old fashioned horseshoe magnet. You can render the invisible visible by placing a magnet on a sheet of paper and sprinkling iron filings around it. Immediately they’ll align themselves in series of arcs defining the magnetic lines of force.
Solar wind particles are a bit like guided missiles. Under the right conditions, they spiral down the field lines and crash into Earth’s atmosphere, temporarily dislodging electrons in oxygen and nitrogen atoms. When the sprung electrons meet up with their parent atoms an instant later, those billions of oxygens and nitrogens emit tiny flashes of green and red light. It’s this sub-microscopic activity that’s behind a spectacular display of northern lights.
Nature often dazzles by numbers. We don’t notice a few snowflakes, but trillions of them can be whipped into a storm powerful enough to stop us in our tracks.
Jupiter’s aurora photographed in ultraviolet light in March 2007 by the Hubble Space Telescope. The bright dot at right is Io’s auroral “footprint” described below. Click to enlarge. Credit: NASA/ESA/J.Clarke
Jupiter also possesses a magnetic field or technically, a magnetosphere, but as you might guess, it’s far larger and more powerful than Earth’s. This is due both to Jupiter’s size and rapid rotation rate of just 10 hours. We can picture planets with magnetospheres as spinning magnets. Spinning a small magnet creates a small electric current but spinning a huge magnet like Jupiter at a rapid speed creates a current of 10 million volts at its north and south poles. Powerful electric fields coupled with the planet’s “animal magnetism” grab hold of any particles in the neighborhood and dash them into Jupiter’s upper atmosphere, where they spark extensive auroras.
Northern and southern lights on seen by Hubble on September 20, 1997. Electrified sulfur and oxygen atoms from Io are primarily responsible for Jupiter’s auroras, but the sun and possibly material in the planet’s high atmosphere also play a role. Click to enlarge. Credit: NASA/ESA/J.Clarke
On Earth, particles from the sun are the chief cause of the aurora, but on Jupiter they play only a small role. The planet relies largely on its moon-sized moon Io, the most volcanically active body in the solar system.
Io is the innermost of the Jupiter’s four brightest moons and orbits the planet in just 1.8 days. Astronomers have mapped more than 300 active volcanoes on this small world that spew lava across the landscape and volcanic gases into outer space at the rate of one ton per second.
Sulfur and oxygen atoms in the expelled gas are electrified (ionized) by Jupiter’s magnetic field and eventually make their way down the field lines headed for the poles. As they crash into molecules in the planet’s atmosphere, their electrons are temporarily stripped off. When the sulfur and oxygen ions eventually slow down, they snatch back their electrons and emit tiny bursts of ultraviolet and X-ray light in the process. Voila – auroras bloom over Jove’s poles!
Like blurry images in a mirror, Io, Ganymede and Europa leave an electrified impression of themselves in Jupiter’s polar aurora in this photo taken in 2000 by Hubble. Io even has a ‘tail’ that sweeps around the top of Jupiter as the planet rotates. Click to enlarge. Credit: NASA/ESA/J. Clarke
Jupiter auroras, which show up best in UV and X-rays, are thousands of times more intense than anything here on Earth. Buried within their curtains and curls are features never seen in earthly auroras. As Jupiter’s magnetic field sweeps past Io, powerful electric currents connect the moon directly with the planet’s magnetic poles. Billions of electrified sulfur and oxygen ions are swept along by the field, slamming into the polar atmosphere to create a set of bright dots or “footprints” of aurora at both poles.
Particles from Io, shown as the glowing red donut around Jupiter, get pulled into the planet’s magnetic field (blue arcs) and slam into the atmosphere to create auroras. The electrical conduits (pink) from Io, Ganymede and Europa lead to their individual “footprints”. Credit: LASP/NASA
Ganymede, the only moon in the moon in the solar system with its own magnetic field, and Europa are also connected to Jupiter’s magnetic field and sport their own polar footprints. While it’s understood how Ganymede can hook up with the planet’s field, it’s less clear with Europa. You need something to conduct electricity like Io’s ions or a magnetic field to make the connection to Jupiter. Maybe we’ll learn the answer come 2016, when the Juno space probe, launched in August 2011, is expected to arrive at Jupiter. One of its mission’s goals is to examine and take close-up photos of the planet’s mighty auroras.
(Note: Thanks to Jan Karon’s question for the inspiration for this blog.)