The conventional aurora borealis in the Arctic and aurora australis in the Antarctic are typically seen as curtains of brightly coloured light descending through the atmosphere near the poles. The light is generated when electrons from the solar wind become trapped and accelerated by the Earth's magnetic field to energies in excess of 1 keV.

The trapping process is complex because of the way in which the solar wind interacts with the earth's magnetic field. But eventually these electrons are channelled towards the poles where they collide with atoms in the atmosphere causing them to emit light.

But the distribution of the phenomenon is not even - the auroras form in a doughnut-shaped zone around each pole. So, conventional auroras do not form directly over the poles.

Now, Yongliang Zhang of the Johns Hopkins University Applied Physics Laboratory in Maryland, US, says he has evidence that these inner polar circles have their own kind of auroras.