Aurora is the light produced when energetic electrons rain down Earth’s magnetic field lines and collide with oxygen and nitrogen in the upper atmosphere. Those collisions happen in a ring, or oval, centred on each magnetic pole, which is why the display is called the northern lights in one hemisphere and the southern lights in the other. How bright it becomes, and how far from the poles it reaches, depends on how disturbed the magnetic field is at that moment.
What actually creates the glow
The electrons that make the visible aurora come from Earth’s own magnetosphere, the magnetic bubble around the planet, rather than straight from the Sun. swpc.noaa.gov explains the chain: solar wind energy is transferred into the magnetosphere, electrons are accelerated along field lines, and they strike atoms and molecules in the upper atmosphere. The impact lifts an atom into a higher energy state, and as it settles back it releases that energy as a photon, in the same way a neon tube glows.
The same page notes that the aurora typically forms roughly 80 to 500 km above the surface, well above the weather and far above any aircraft. It also exists almost all the time somewhere; what changes is brightness, shape and latitude.
Why it forms a ring, not a blanket
Earth’s magnetic field lines converge towards the magnetic poles. Electrons travelling along them therefore arrive in a concentrated band rather than spread across the whole planet, producing the auroral oval. Because the field is roughly symmetrical, the northern and southern ovals brighten and fade at the same time.
The practical consequence is geographic. Aurora is a high-latitude phenomenon: the swpc.noaa.gov overview puts the best viewing zone between about 60 and 75 degrees latitude, where clear skies can show aurora on more than half the nights of a year. Mid-latitude sightings are the exception, not the rule.
What decides the colour
Colour comes from which gas is hit and at what altitude. science.nasa.gov sets out the pattern below.
| Colour | Altitude | Source gas |
|---|---|---|
| Red | Above 200 km | Oxygen |
| Green | 100 to 200 km | Oxygen |
| Blue | 100 to 200 km | Nitrogen |
| Pink | Below 100 km | Nitrogen |
Green is the colour most people actually see, because it forms at altitudes where the air is still dense enough for frequent collisions. The deep red from oxygen needs the much thinner air above 300 km, since that excited state survives only where collisions are rare, which is why red usually appears during stronger disturbances or as a faint high-altitude glow.
Why the oval moves on stormy nights
The driver is geomagnetic activity, summarised by the planetary K index, or Kp, which runs from 0 to 9. NOAA’s tutorial states that a high Kp, between 7 and 9, brings a bright aurora that pushes the oval towards lower latitudes, while a moderate Kp of 5 or 6 pulls it back towards the poles. Even Kp 3 or 4 can give a reasonable display, simply closer to the magnetic poles.
Two solar events raise Kp. A coronal mass ejection throws out a large cloud of magnetised plasma and can produce the biggest storms and the southernmost aurora. A coronal hole streams faster solar wind and produces more moderate activity. The direction of the magnetic field carried by that wind matters as well: activity rises when it points opposite to Earth’s field.
How far ahead it can be predicted
Space weather forecasting has an unusual property. Satellites parked upstream in the solar wind measure what is heading towards us, which allows a fairly accurate aurora forecast with a lead time of only about 15 to 45 minutes. Longer range outlooks exist, but the strength and orientation of the magnetic field inside a storm cloud is hard to measure before it arrives, so they carry much more uncertainty.
What a forecast cannot fix is the ground conditions. You need darkness and clear sky, which is why summer at high latitudes is a poor season for watching. Around local midnight is usually the brightest window, and a faint aurora often looks merely pale white to the eye, since night vision resolves brightness before colour.
The bottom line
The northern lights are not weather in the usual sense. They are the visible part of a magnetic system being pushed by the Sun, drawn into rings around the poles, and coloured by whichever gas is excited at which altitude. A high Kp does not guarantee a sighting, but it does move the oval, which is what makes a mid-latitude display worth the trip outside.