Sheets of green light dancing across a black polar sky — the aurora borealis is one of nature's most magical displays. Travelers spend thousands of dollars chasing them in winter at the Arctic Circle. Yet despite their beauty, auroras have a surprisingly violent origin: explosions on the Sun that send streams of particles into Earth's magnetic field, eventually colliding with our atmosphere to produce the spectacular light show.
The Short Answer
The northern lights (aurora borealis) are caused by charged particles from the Sun, channeled by Earth's magnetic field to the polar regions, where they collide with atoms in the upper atmosphere and excite them to glow. Different atmospheric gases produce different colors when excited — oxygen gives off green and red light, nitrogen produces blue and purple. The same phenomenon at the South Pole is called aurora australis.
The Sun's Role
Auroras originate millions of kilometers away on the Sun. The Sun constantly emits a stream of charged particles called the "solar wind" — protons, electrons, and other ions traveling at 400-800 km/s. Solar storms (coronal mass ejections, solar flares) eject even more intense bursts of particles, sometimes traveling at over 2,000 km/s and arriving at Earth in less than a day.
The solar wind's intensity varies with the 11-year solar cycle. During "solar maximum," when sunspot activity peaks, auroras are more frequent and intense. During "solar minimum," auroras may be sporadic and faint.
Earth's Magnetic Field
Earth has a magnetic field generated by molten iron flowing in its outer core. This field extends thousands of kilometers into space, forming the magnetosphere — a vast protective bubble. The magnetosphere deflects most solar wind particles around Earth, like water flowing around a rock.
But the magnetic field isn't uniform. It funnels some charged particles toward the magnetic poles, where the field lines converge. Particles spiral down these field lines into the upper atmosphere over the polar regions. This is why auroras appear in oval-shaped bands around the magnetic poles, not uniformly worldwide.
The Aurora Process
The sequence of events that produces an aurora:
- Sun ejects charged particles (solar wind or coronal mass ejection).
- Particles travel through space toward Earth.
- Earth's magnetic field deflects most particles but channels some toward the poles.
- Captured particles spiral down magnetic field lines.
- Particles enter the upper atmosphere at 100-300 km altitude.
- Particles collide with atmospheric atoms (oxygen, nitrogen).
- The collisions excite atoms, knocking their electrons into higher energy states.
- When electrons return to lower states, they emit photons of light.
- We see the resulting glow as the aurora.
The Colors of Auroras
Different atmospheric gases produce different aurora colors:
- Green (557.7 nm): The most common color. Produced by oxygen atoms at 100-300 km altitude. Most visible to human eyes.
- Red (630.0 nm): Produced by oxygen at very high altitudes (300-400 km), where the air is thin enough for the slow-emitting red transition.
- Blue and purple: Produced by nitrogen molecules. Often seen at the bottom of aurora curtains.
- Pink: A mixture of red oxygen with high-altitude blue nitrogen.
- Yellow: Mixture of red and green oxygen emissions.
Why Green Is Most Common
The green wavelength from oxygen is the dominant aurora color for several reasons:
- Oxygen is abundant in the lower thermosphere where auroras typically occur
- The green transition is relatively quick (~0.7 seconds)
- Human eyes are most sensitive to green wavelengths
Red oxygen emissions are slower (over 100 seconds), so they can only happen at very high altitudes where collisions don't interrupt the process. This is why red is rarer.
Aurora Shapes
Auroras come in many distinct shapes:
- Diffuse aurora: A faint background glow that's often present.
- Arcs: Smooth bands stretching across the sky.
- Bands: Folded curtains that ripple along their length.
- Rays: Vertical pillars or beams of light.
- Coronas: Aurora directly overhead, with rays converging at the zenith.
- Patches: Pulsating areas that brighten and dim cyclically.
- STEVE: A recently identified aurora-like ribbon (Strong Thermal Emission Velocity Enhancement) discovered to be different from typical aurora.
The Aurora Oval
Auroras occur in oval-shaped regions around each magnetic pole, called the "auroral oval." These ovals are typically:
- Centered on the magnetic poles (not geographic poles)
- About 3,000-4,000 km across in diameter
- Visible from latitudes within and just outside the oval
The oval expands during geomagnetic storms, pushing auroras toward lower latitudes. During extreme events, auroras have been seen as far south as Mexico or Cuba.
Best Places to See Auroras
The best aurora viewing happens in the "auroral zone" — typically 65-72° latitude:
- Tromsø and Northern Norway: Accessible, often clear winter skies.
- Reykjavik and Iceland: Convenient mid-Atlantic location.
- Yellowknife, Canada: Considered one of the best places worldwide.
- Fairbanks, Alaska: Long aurora season.
- Murmansk, Russia: Kola Peninsula auroras.
- Finnish Lapland: Famous "aurora hunting" tours.
- Greenland and Svalbard: Polar locations with active aurora oval.
For aurora australis (southern lights), Antarctica is the obvious location, but accessible viewing exists in Tasmania and the southern tip of New Zealand.
Best Time to See Auroras
Aurora viewing depends on multiple factors:
- Season: Winter (long dark nights). Some say equinox months (September, March) have better auroras due to alignment of Earth's magnetic field with solar wind.
- Time of night: 10 PM to 2 AM are peak hours.
- Sky conditions: Need clear, dark skies. New moon is ideal.
- Solar activity: More auroras during solar maximum (the current cycle peaks around 2024-2025).
- Forecast: Aurora forecasts based on Kp index (geomagnetic activity).
Aurora Australis
The southern lights mirror the northern lights but are much harder to access. Most of the aurora australis appears over Antarctica or the Southern Ocean. Tasmania and Stewart Island (New Zealand) offer the best populated viewing. Aurora australis is visually identical to aurora borealis.
Auroras on Other Planets
Aurora-like phenomena occur on other planets with magnetic fields:
- Jupiter: Has the brightest auroras in the solar system, especially around its strong magnetic poles.
- Saturn: Spectacular auroras visible from Hubble Space Telescope.
- Uranus and Neptune: Both have auroras.
- Mars: Has localized auroras around magnetized crustal patches, plus diffuse auroras from solar particles striking the thin atmosphere.
- Venus: No strong magnetic field, but some aurora-like phenomena occur.
The Solar Connection
Major solar events trigger major auroras:
- Solar flares: Bursts of electromagnetic radiation and particles.
- Coronal mass ejections (CMEs): Massive ejections of magnetized plasma. Cause the strongest auroras.
- High-speed solar wind streams: Cause moderate aurora activity.
- Coronal holes: Open regions on the Sun that release fast-moving solar wind.
The famous Carrington Event of 1859 was a massive solar storm that caused auroras visible from Cuba, set telegraph systems on fire, and would cause catastrophic damage to modern electronics if it happened today.
Aurora Sounds?
For centuries, indigenous peoples and explorers reported hearing sounds during auroras — crackling, hissing, swishing. Scientists long doubted this since auroras occur 100+ km up, too far for sound to travel. But recent research has detected actual sound near aurora displays, possibly produced by electrical phenomena at low altitudes during very active auroras. The mechanism is still being studied.
Cultural Significance
Auroras have profound mythological and cultural meaning:
- Vikings believed they were reflections from the shields of Valkyries
- Inuit traditions interpreted them as spirits of the dead playing games
- Finnish folklore: caused by a magical fox (revontulet, "fox fires")
- Sami people: ancestral spirits
- Many cultures regarded auroras as omens, often unsettling
Key Facts
- Auroras are caused by solar particles colliding with atmospheric gases.
- Earth's magnetic field channels particles to the polar regions.
- Oxygen produces green and red light; nitrogen produces blue and purple.
- Auroras occur 100-400 km above Earth's surface.
- The 11-year solar cycle affects aurora frequency and intensity.
Fun Facts
- The Carrington Event of 1859 caused auroras visible from Cuba and ignited telegraphs.
- Jupiter has the largest auroras in the solar system.
- STEVE (Strong Thermal Emission Velocity Enhancement) was identified as a new phenomenon in 2018.
- Some northern lights observers claim to hear them — recent research suggests this is real.
- Vikings believed auroras came from Valkyrie shields.
- The word "aurora" comes from the Roman goddess of dawn.
- Galileo Galilei coined the term "aurora borealis" in 1619.
The Bottom Line
The northern lights are caused by charged particles from the Sun being channeled by Earth's magnetic field into the polar atmosphere, where they collide with oxygen and nitrogen atoms and excite them to glow. The result is one of nature's most beautiful spectacles, dependent on the interplay of solar activity, Earth's magnetic field, and atmospheric chemistry. The next time you see (or chase) the aurora, you're witnessing a direct connection between Earth and the violent processes happening 150 million kilometers away on the Sun.