Auroras appear in many different shapes. The aurora in the early evening sky forms green arcs that stretch across the sky in an east-west direction (Image 1). The longitudinal extent (length) of an auroral arc can be as large as several thousand kilometres, but its width can be as small as 100 metres.
The basic mechanism behind auroral arcs has been known for decades. When (sheets of) highly energetic electrons manage to precipitate into the upper atmosphere, they are stopped by collisions with neutral atoms and molecules, primarily oxygen and nitrogen, at altitudes of a few hundreds kilometres down to 80 km. Each collision transfers part of the electron energy to these atmospheric particles. In turn, they get rid of this energy excess by emitting visible light at specific wavelengths, producing characteristic colours such as green (oxygen) or purple (nitrogen).
