This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
The aurora begins ninety three million miles away, in the outer atmosphere of the Sun. The Sun constantly sheds a stream of charged particles, mostly electrons and protons, called the solar wind, and during a solar flare or a coronal mass ejection it sheds far more of them at once, aimed in a specific direction. Most of that stream never reaches the ground: the magnetic field of the Earth acts as a shield, deflecting the bulk of the solar wind around the planet in a long teardrop shaped cavity called the magnetosphere. But the shield is not sealed. Some particles leak in along the magnetic field lines that converge at the two magnetic poles, and it is along those funnels that the solar wind finally reaches the upper atmosphere, sixty to six hundred miles up, colliding with oxygen and nitrogen molecules and knocking their electrons briefly into a higher energy state. When an electron falls back down, it releases that extra energy as a photon of light, and it is billions of these tiny releases happening together that a viewer on the ground sees as a moving curtain of color.
The color depends on which gas is struck and at what altitude. Oxygen produces the aurora's signature green at lower altitudes, roughly sixty to one hundred and eighty miles up, where collisions are frequent enough for the excited oxygen atom to shed its energy quickly; the same oxygen produces a rarer, deep red at higher altitudes, above about one hundred and eighty miles, where collisions are rare enough that the atom has time to undergo a slower, different kind of transition. Nitrogen contributes the blues and purples sometimes visible along the lower fringe of a display. Because the magnetic funnels sit at both poles at once, the same storm produces a mirrored display in each hemisphere simultaneously, aurora borealis in the north and aurora australis in the south, a fact confirmed only once satellites could observe both poles at the same time.
It took physics far longer to work this out than it took observers to notice the lights themselves. Norwegian physicist Kristian Birkeland proposed the mechanism in the early 1900s and built a device called a terrella, a small magnetized sphere in a vacuum chamber, bombarding it with an electron beam to recreate a miniature aurora in the laboratory. His theory that charged solar particles, guided by a planetary magnetic field, produced the aurora was correct in its essentials, though it was not confirmed by direct measurement in space until decades after his death, once satellites could finally observe the particle streams and field lines Birkeland had only been able to model on a tabletop.