Electrons ‘surf’ Alfvén waves to reach their high speeds
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Earth’s aurorae are caused when electrons emitted from the Sun hurtle towards Earth and are funnelled down Earth’s magnetic field lines, where they collide with oxygen and nitrogen molecules in the ionosphere – the upper atmosphere between 80 and 600 kilometres (50 and 370 miles). The absorption of energy by these ions causes them to move to an ‘excited’ high-energy state. To relax, the molecules reradiate the energy as light, producing impressive auroral displays.