The Northern Lights could be mistaken for an iridescent green mist until they dance across the sky, dipping and pirouetting, swirling like a tornado, or hanging like curtains rippling in a breeze.
The Sun powers the performance via a stream of electrons and protons known as the solar wind that buffets atoms in the Earth’s atmosphere, causing them to release photons of light. The reason for the green colour is twofold. At an altitude of between 100 and 200 kilometres, nitrogen molecules are outnumbered by oxygen atoms, which are split from their molecular form by ultraviolet light. When buffeted by the solar wind, oxygen atoms glow green, a colour to which our eyes are more sensitive than the blue-violet glow of nitrogen molecules.
This dance was performed in Finish Lapland, just inside the Arctic Circle.
The fierce solar wind bumps other charged particles trapped in Earth’s magnetic field and sends them racing along the magnetic field lines to both poles.
At the North Pole the display goes by the name aurora borealis; at the South Pole it’s the aurora australis.
The solar wind normally blows at around 400 kilometres per second but explosions on the Sun’s surface known as coronal mass ejections create blizzards of 1,000 kilometres per second and dramatic auroras. On 11 September 2005, NASA’s IMAGE satellite captured a brilliant aurora australis created by one such storm.
The best place in the solar system to see aurorae is Jupiter where they are thousands of times brighter than those on Earth.
Here Jupiter’s aurorae are captured by NASA’s Chandra X-ray space telescope. The image is overlaid on a Hubble photograph of the gas giant.
Unlike Earth’s intermittent displays, Jupiter’s aurora permanently lights up its skies because, besides the Sun, the moon Io gets into the act.
Io’s volcanic eruptions spit jets of charged particles into space. They are captured by Jupiter’s magnetic field and catapulted toward the planet’s poles to create a never-ending show.
The Cassini space probe allowed us to see Saturn’s brilliant aurora.
Just as on Earth, it flares brightest when buffeted by solar storms. Cassini captured this powerful aurora 1,000 kilometres above the cloud tops around Saturn’s south pole.
Earth’s atmosphere is rich in oxygen and nitrogen but Saturn’s is mainly composed of hydrogen, which gives off strong ultra-violet emissions when excited. But the excited hydrogen does give off a little visible light too. Cassini’s spectrometers captured some of this light in this image. The green colouring is artificial. If it were possible to stand at Saturn’s poles you’d see faint pink-red ripples dancing across the sky.
James Mitchell Crow
James Mitchell Crow is a freelance writer and editor.