Science Puzzle
What Makes the Northern Lights Glow?
On dark nights in the far north, and now and then as far south as England, the sky fills with shimmering curtains of green light, sometimes with red above. These are the Northern Lights, or aurora borealis, and they glow about 100 kilometres up and higher, far above any cloud.
What makes them glow?
The Answer
Particles from the Sun. Charged particles blown out by the Sun are steered by Earth's magnetic field towards the polar regions, where they crash into oxygen and nitrogen high in the air and make them glow.
The Sun constantly streams out charged particles, the solar wind, and now and then it hurls out a huge burst of them in a solar storm. They take a day or more to reach us. Earth's magnetic field turns most of them aside, but near the poles the field lines curve down into the atmosphere, and particles are guided in along them. That's why the lights usually appear in a ring around each pole: the Northern Lights in the north and the Southern Lights, aurora australis, in the south.
When a fast particle hits an atom of oxygen or nitrogen, it gives the atom extra energy, and the atom quickly gets rid of it as a flash of light. Each gas glows in its own colours, rather like the gas in a neon sign. Oxygen gives the common green glow, about 100 to 250 kilometres up, and a deep red higher still, where the air is thinnest. Nitrogen adds blue and purple at the lower edge.
A big solar storm pushes the lights much further from the poles. In May 2024 they were seen right across Britain and even in southern Europe.
The principle: Excited gases give out light. Charged particles from the Sun, steered by Earth's magnetic field, crash into oxygen and nitrogen high in the atmosphere, and each gas glows in its own colours.