Magnetic corridor
Zhang's auroras are generated in quite a different way from the traditional ones. Under certain conditions, the Sun's magnetic field can connect to the Earth's in a way that opens a direct magnetic corridor along which electrons can flow.These electrons have energies of only 100 electronvolt and arrive directly over the poles where they are known by scientists as polar rain. But because they have not been accelerated in the Earth's magnetic field, they are not normally energetic enough to generate light.However, Zhang has evidence that there may be more to polar rain than had been thought. His data comes from UV-sensitive cameras on a pair of Earth-observing satellites that have been watching the auroras since 2003. In data collected in July 2004, he noticed something unusual - a wave of polar rain with much higher energy than usual inside the aurora australis.
Energy mystery
Zhang says the electrons in this wave had energies of up to several KeV, more than enough to generate light. And although not observed from the ground, these particles must have produced a diffuse auroral glow directly over the south magnetic pole. He has christened this new phenomenon "polar rain aurora".One question is how these electrons reached such high energies. Zhang thinks they must have been emitted from the sun by a disturbance in its activity, such as a sunspot.The polar rain auroras move very quickly too: according to Zhang's measurements, the July 2004 event moved at up to 200 metres per second, or 720 km/h, about the speed of a commercial airliner.Zhang thinks polar rain auroras are likely to be common occurrences which nobody on the ground has distinguished from traditional aurora. Now the race is on to observe them from the ground.
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As in the days of Noah...