The northern lights, or aurora borealis, appear when energetic charged particles from near-Earth space collide with oxygen and nitrogen high in the atmosphere. Those gases release the absorbed energy as light. Earth’s magnetic environment guides many of the particles toward polar regions, while the gas involved and the altitude of the emission determine whether the glow looks green, red, blue, pink, purple or white.
What are the northern lights?
The northern lights are a natural light display called the aurora borealis. NASA uses that name for the aurora seen from the Northern Hemisphere. The Southern Hemisphere has the corresponding aurora australis; both are produced by related interactions between charged particles, Earth’s magnetic environment and the upper atmosphere. NASA’s aurora overview also describes auroras on planets such as Jupiter and Saturn, although the details can differ from world to world.
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An aurora is not sunlight reflected from snow, a cloud or a glowing layer of air. It is light generated in the atmosphere itself. The incoming particles supply energy, and atmospheric oxygen and nitrogen emit that energy as photons—the particles of light that reach an observer on the ground.
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The Sun sends out the solar wind
The Sun continuously releases a flow of charged particles known as the solar wind. Its speed, density and magnetic properties vary. Changes in that flow can transfer more energy into Earth’s magnetic environment and make auroral activity stronger. A dramatic solar eruption can intensify an aurora, but a single eruption is not required: the continuing solar wind powers the broader process. NASA explains the solar-wind connection.
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Earth’s magnetic environment redirects some particles
Earth’s magnetic field acts as a protective, dynamic shield. It deflects much of the incoming solar-wind flow, while also allowing some charged particles to enter along magnetic-field structures. Those structures guide particles toward the polar regions, which is why auroras are most common at high latitudes. The process is described in NASA’s heliophysics explanation.
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The particles collide with upper-atmosphere gases
As energetic particles travel into the upper atmosphere, they collide with atoms and molecules of oxygen and nitrogen. Each collision can transfer energy to the gas, leaving it in an excited state.
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The gases release that energy as light
Excited oxygen and nitrogen do not keep the extra energy indefinitely. When they release it, they emit photons. Billions of such emissions combine into the visible curtains, arcs and bands called an aurora. Rafael Mesquita, an EZIE Mission Scientist at Johns Hopkins Applied Physics Laboratory, compares the effect with a neon sign: charged particles flow along magnetic-field lines and “power up” gases in the sky, causing them to glow. NASA publishes the full analogy.
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Why are the northern lights concentrated near the poles?
Charged particles respond to magnetic fields, and Earth’s field has pathways that connect the near-Earth environment with the upper atmosphere around the magnetic poles. Particles entering along those pathways deposit energy in high-latitude atmospheric regions, creating broad auroral zones rather than an evenly illuminated global shell.
The display can expand toward lower latitudes when the solar wind transfers unusually large amounts of energy into the magnetic environment. That is why a strong geomagnetic storm can make the northern lights visible farther south than usual. It does not change the underlying mechanism: the same guided particles are exciting the same atmospheric gases, but over a larger area.
What determines an aurora’s colour?
Colour is mainly a question of which gas emits the light and where in the atmosphere that emission occurs. Oxygen is responsible for the familiar green and can also produce red. Nitrogen contributes blue and pink. The altitude figures below are NASA’s broad educational ranges, not sharp borders; the bands overlap and a single auroral display can contain emissions from several heights. See NASA’s aurora colour guide and its more detailed atmosphere overview.
| Observed colour | Main emitting gas | Approximate altitude association | How to interpret it |
|---|---|---|---|
| Green | Oxygen | Roughly 100–200 km; NASA’s overview also describes green around 100 km | The most commonly reported auroral colour. It indicates oxygen emissions in a broad upper-atmosphere layer, not a precise height reading. |
| Red | Oxygen | About 200 km and higher | Higher-altitude oxygen emissions can add red to the display, sometimes as a diffuse upper glow. |
| Blue | Nitrogen | Roughly 100–200 km | Nitrogen emissions can appear blue, especially where the display is intense enough for the colour to stand out. |
| Pink | Nitrogen | At or below roughly 100 km | Lower-altitude nitrogen emissions may produce pink or magenta-looking edges and streaks. |
Why green is so common
Green is often the colour people associate with the aurora because oxygen emissions in the commonly observed altitude range are especially prominent to human observers. A green arc is therefore not a separate phenomenon from a red or blue aurora; it is one visible result of the same particle-and-atmosphere process.
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Why red can appear above green
Red is also produced by oxygen, but NASA associates it with higher altitudes—about 200 kilometres and above. An auroral curtain can therefore show a green lower region with a red upper glow when emissions from both altitude ranges are visible along the same line of sight.
Where blue and pink come from
Nitrogen emissions add blue and pink. These colours are often less obvious than green in a weak display, but they can become conspicuous in brighter or more complex auroral structures. The colour does not identify a single isolated stripe of gas: overlapping emissions and viewing conditions affect what the eye sees.
How purple, pinkish-white and white appearances arise
NASA notes that combinations of emissions can look purple, pink or white. A mixed colour may mean that oxygen and nitrogen are glowing together, or that red, green and blue light overlap along the observer’s line of sight. Consequently, a perceived colour is not always evidence of one gas or one exact altitude.
Why altitude matters without creating hard colour boundaries
Earth’s atmosphere changes with height, and auroral particles can excite gases in more than one layer. The light from those layers travels toward the observer at once. The result is a three-dimensional display projected onto the sky: a single ribbon may contain several colours, and adjacent curtains may be produced at different heights.
For that reason, NASA’s altitude values should be read as approximate associations. Seeing green does not let you calculate an exact altitude, and seeing pink does not prove that every visible particle collision occurred below 100 kilometres. The ranges are useful for understanding the relationship between colour, gas and height, but they are not rigid colour-coded floors and ceilings.
Why auroras look like moving curtains and arcs
The familiar shapes reflect the organisation of charged particles along Earth’s magnetic-field structures. As the incoming particle flow and the magnetic environment change, the regions receiving energy brighten, fade, shift and ripple. Long arcs can develop into folded bands or curtain-like forms because different parts of the auroral zone are being energized at different times.
Those motions do not mean that a physical sheet of glowing gas is blowing through the atmosphere like a cloud. They are changing patterns of emissions produced as energy enters different magnetic and atmospheric regions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Do the northern lights happen every night?
Auroral activity occurs continually somewhere, but it is not continuously visible from every location. Daylight overwhelms the relatively faint visible glow, and the auroral zone may be far from your latitude. Darkness, a suitable viewing location and sufficient activity all have to coincide.
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Are the northern lights caused only by solar storms?
No. The solar wind is always present, and its interaction with Earth’s magnetic environment continually supplies the basic source of auroral energy. Solar storms are periods of especially disturbed solar-wind conditions that can inject more energy, brighten the display and extend auroral visibility toward lower latitudes. Treating every aurora as the aftermath of one spectacular eruption misses the continuous background process described by NASA.
Northern lights, southern lights and auroras on other planets
“Northern lights” refers specifically to the Northern Hemisphere aurora borealis. A related aurora australis appears around the Southern Hemisphere’s polar region. The names identify viewing hemisphere, not different colours or a separate light-producing mechanism.
Auroras are not unique to Earth. NASA has observed auroral phenomena on planets including Jupiter and Saturn. The general idea—particles transferring energy to an atmosphere or surrounding gas—can be shared, but each planet’s magnetic field, atmosphere and particle sources change the details. Earth-based colour and altitude ranges should not be applied automatically to another world.
Common misconceptions to avoid
- “The aurora is a reflection.” The visible light is emitted by excited atmospheric gases, not reflected sunlight.
- “Every display requires one giant solar flare.” The ongoing solar wind powers auroras; storms can intensify them but are not the only cause.
- “Each colour maps to one exact altitude.” NASA’s ranges are approximate, emissions overlap and line-of-sight mixing can create intermediate colours.
- “The northern lights only exist in the north.” The Southern Hemisphere has auroras too, and other planets can produce their own forms.
- “A forecast guarantees a show.” Solar-activity indications are probabilistic; local darkness, cloud and the actual evolution of the magnetic disturbance determine what you see.
The simplest way to remember the science
The Sun supplies the particles and upstream energy. Earth’s magnetic environment steers some of those particles toward polar regions. Oxygen and nitrogen in the upper atmosphere absorb the energy and release it as light. Oxygen commonly produces green and red; nitrogen contributes blue and pink; mixtures can look purple, pink or white. Altitude changes which emissions are present, but the published ranges are broad and overlapping rather than hard boundaries.
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