Aurora

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.

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

Long before anyone understood the aurora as a solar-terrestrial physics problem, the people who lived where it was visible most often built it into their explanations of the world. Several Indigenous peoples of the North American Arctic and Subarctic, including Cree and Inuit communities, described the lights in terms of the spirits of the dead, sometimes playing a game, sometimes guiding the living; Norse tradition associated the lights with the Bifrost, the burning bridge connecting the world of gods to the world of humans, and later Scandinavian folklore offered more specific, sometimes contradictory readings, from a good omen to a warning best not whistled at, since a whistle was thought to draw the lights closer than was wise. None of these traditions had access to the magnetosphere, but many of them correctly linked the aurora's frequency and intensity to the seasons and to something changeable happening far away, an intuition modern space weather science would eventually formalize.

The aurora's practical consequences on the modern, electrified world are large enough that a dedicated forecasting effort now exists for it, run in the United States by the National Oceanic and Atmospheric Administration's Space Weather Prediction Center. A strong geomagnetic storm does not just brighten and lower the aurora, pushing displays visible normally only from Alaska or northern Scandinavia as far south as the middle latitudes; it can also induce currents in long conductors, power grids, pipelines and undersea cables, that were never designed to carry them. The most extreme documented case, the Carrington Event of 1859, triggered auroras visible near the equator and set telegraph systems on fire in some stations, sparking operators' equipment even after they had disconnected their batteries, powered instead by the storm's own induced current. A storm of that scale striking the modern power grid and satellite fleet is one of the scenarios space weather forecasters explicitly plan around, which is why the same agencies that predict the aurora's visibility for tourists also issue geomagnetic storm warnings to utilities and airlines.

Aurora tourism itself has become a substantial seasonal industry across the high-latitude north, from Tromso and Iceland to Fairbanks and Yellowknife, built almost entirely around the same forecasting data: the eleven-year solar cycle sets the general odds for a given year, while short-range space weather forecasts, tracking specific coronal mass ejections as they leave the Sun, can narrow a strong display down to a window of a few days, turning what used to be pure chance into something closer to a plannable, if still never guaranteed, trip.