nature & planetThe Public Domain Review
Aurora borealis has terrified and enchanted people for centuries, and science explained it only in the 20th century
The aurora forms about 50 miles or more above Earth, where particles from the solar wind are funnelled by the planet's magnetic field toward the poles. When these charged particles hit gases in the upper atmosphere, they excite the atoms, which release light as they return to their normal state. Oxygen at around 100 kilometres up produces the common green glow, while nitrogen adds pinks and purples. Many cultures interpreted the lights: the Norse saw Valkyries leading the fallen to Valhalla, and some Inuit groups saw them as spirits. Explaining their origin took centuries, and the Norwegian scientist Kristian Birkeland laid the foundations of the modern understanding in the early twentieth century.
The Sun constantly releases a stream of charged particles called the solar wind. Earth's magnetic field channels some of these particles toward the poles, where they collide with oxygen and nitrogen in the thermosphere. The collisions excite the gas atoms, which release light at specific colours, producing the shifting curtains of the aurora.
The aurora is a visible proof that the Sun's activity reaches Earth, linking the sky above the poles to space weather. Understanding it helped establish how the magnetosphere shields the planet from solar particles. Artists and explorers recorded these displays for centuries, so the history of the aurora is also a history of human wonder and fear.
The green colour of the aurora comes mainly from excited oxygen atoms, which is also why the glow looks different at different altitudes.
Image: The Public Domain Review
