Why are there different colours in aurorae?
The different colours are due to the different atoms and their molecular states when they are excited. The high-altitude emissions come typically from oxygen atoms excited by lowenergy electrons, and low-altitude emissions come typically from nitrogen excited by highenergy electrons. The high-energy electrons from space make it to lower altitudes in the atmosphere because they are energetic enough to penetrate deeper and reach denser layers of the atmosphere, where atomic nitrogen exists.
The low-intensity red colours come from oxygen excitation by low-energy electrons reaching roughly 250 kilometres (155 miles) altitude. The most common colour is green, which comes from nitrogen and oxygen excitation by higher energy electrons reaching roughly 100 kilometres (60 miles). The least common, but most intense, is purple, which comes from nitrogen excitation by even higher energy electrons reaching approximately 80 kilometres (50 miles). They all occur during an intense period of geomagnetic activity, such as magnetic substorms and storms.
These colours appear in layers, or bands, that we call ‘aurorae’. This is because they line up along constant magnetic field intensity layers at the equator. You can imagine a bar magnet at the centre of Earth and field lines moving out from the equator into space.
The higher the latitude, the further away the equatorial projection is from Earth. The constant-field equatorial regions lie roughly in a circle of constant distance from Earth, and their ionospheric projections – the aurorae – lie on a constant circle around the magnetic pole. This is how aurorae are arranged.