What Causes the Northern Lights? The Science Explained
Understand the fascinating science behind aurora borealis—from solar activity to atmospheric collisions—explained in simple terms for beginners.
What Causes the Northern Lights? The Science Explained
The northern lights have inspired myths and legends for millennia. Ancient peoples saw gods, spirits, and omens dancing in the sky. Today, we understand the science—and it's arguably more wondrous than any myth.
This guide explains what actually creates aurora, step by step, without requiring a physics degree.
The Short Answer
Aurora occurs when charged particles from the sun collide with gases in Earth's atmosphere. These collisions excite atoms of oxygen and nitrogen, causing them to release energy as light—the colorful displays we see dancing across polar skies.
But this simple explanation hides a remarkable chain of events that begins 93 million miles away.
Step 1: It Starts with the Sun
The sun isn't just a ball of fire—it's a churning nuclear reactor with a complex magnetic field. This activity produces two key phenomena that create aurora on Earth.
Solar Wind: The Constant Stream
The sun continuously emits a stream of charged particles called solar wind. These particles—mostly electrons and protons—travel through space at 400-800 km/s (about 1-2 million mph). At typical speeds, they reach Earth in about 3-4 days, though fast solar wind streams and CMEs can arrive in as little as 1-2 days.
| Solar Wind Property | Typical Value | Aurora Impact |
|---|---|---|
| Speed | 400-800 km/s | Faster = more intense aurora |
| Density | 3-10 particles/cm³ | Higher density = brighter aurora |
| Magnetic field (Bz) | -10 to +10 nT | Negative Bz = much better aurora |
Solar wind alone produces background aurora visible at high latitudes. But for the spectacular displays that make headlines, we need something more powerful.
Solar Storms: The Aurora Generators
The sun periodically releases massive bursts of energy that dramatically enhance aurora activity:
Coronal Mass Ejections (CMEs) Billions of tons of magnetized plasma erupting from the sun's surface. When Earth-directed, CMEs can trigger geomagnetic storms lasting hours to days. These produce the strongest aurora events—visible at unusually low latitudes.
Solar Flares Intense bursts of electromagnetic radiation. While flares themselves don't create aurora directly, they often accompany CMEs and indicate heightened solar activity.
Coronal Holes Regions where the sun's magnetic field opens into space, allowing faster solar wind to escape. These create recurring aurora opportunities every ~27 days (one solar rotation).
Step 2: Earth's Magnetic Shield
When solar wind reaches Earth, it encounters our planet's magnetosphere—a protective magnetic bubble extending thousands of kilometers into space.
Why We Have a Magnetosphere
Earth's liquid iron core generates a magnetic field similar to a giant bar magnet. This field deflects most solar wind around our planet, protecting life from harmful radiation.
But the magnetosphere isn't impenetrable. Here's what happens:
The Magnetosphere-Solar Wind Interaction
| Process | What Happens | Result |
|---|---|---|
| Deflection | Most particles flow around Earth | Magnetosphere shaped like a teardrop |
| Compression | Solar wind pressure compresses the dayside | Stronger storms = more compression |
| Tail stretching | Night side extends into a long magnetotail | Stores energy like a stretched rubber band |
| Reconnection | When Bz is negative, magnetic fields merge | Particles funnel toward the poles |
The Bz component is critical. When the solar wind's magnetic field points south (negative Bz), it can merge with Earth's north-pointing field. This "magnetic reconnection" opens gaps that allow particles to enter our magnetosphere—fuel for aurora.
Step 3: Particles Enter the Atmosphere
Once particles breach the magnetosphere, Earth's magnetic field guides them toward the poles. Think of magnetic field lines as highways funneling traffic toward two destinations: the Arctic (northern lights) and Antarctic (southern lights).
The Auroral Oval
Particles don't just hit the North or South Pole—they form an oval ring around each magnetic pole. This auroral oval is where aurora occurs most frequently.
| Geomagnetic Activity | Auroral Oval Position | Visible At |
|---|---|---|
| Quiet (Kp 0-2) | 67-70°N latitude | High Arctic only |
| Active (Kp 3-4) | 64-67°N | Northern Scandinavia, Alaska, Canada |
| Storm (Kp 5-6) | 60-64°N | Scotland, northern US states |
| Severe Storm (Kp 7+) | 55-60°N or lower | Northern Europe, northern US |
During intense storms, the oval expands equatorward, bringing aurora to locations that rarely see it.
Step 4: Atmospheric Collisions Create Light
The final step occurs 80-500 km above Earth's surface. Here, incoming particles collide with atmospheric gases—and these collisions produce the light we see.
How Atoms Produce Light
When a charged particle hits an oxygen or nitrogen atom, it transfers energy. This "excites" the atom—its electrons jump to higher energy states. But atoms don't like being excited; they quickly release this extra energy as photons (light particles).
Different atoms release different wavelengths (colors) of light:
| Gas | Altitude | Color Produced | Wavelength |
|---|---|---|---|
| Oxygen | Above 300 km | Red | 630.0 nm |
| Oxygen | 100-300 km | Green | 557.7 nm |
| Nitrogen | 100-200 km | Blue/Purple | 391-470 nm |
| Nitrogen (ionized) | Lower edges | Pink/Magenta | Various |
Green is most common because oxygen atoms at 100-300 km are abundant and efficiently produce visible light. Red aurora requires the oxygen atoms to remain excited longer (about 2 minutes) without colliding with other particles, which only happens at high altitudes above 300 km. Blue and purple from nitrogen emissions often appear at the lower edges of auroral curtains.
Why Aurora Dances
Aurora constantly shifts and moves because:
- Solar wind varies — Density and speed fluctuate, changing particle flow
- Magnetic reconnection pulses — Energy releases in bursts, not steady streams
- Substorms — Sudden releases of stored magnetotail energy create rapid intensification
- Atmospheric dynamics — Winds and temperature gradients affect where collisions occur
The result: aurora that ripples, pulses, and flows like living curtains of light.
The 11-Year Solar Cycle
Solar activity isn't constant—it follows an approximately 11-year cycle between solar minimum (few sunspots, less activity) and solar maximum (many sunspots, frequent storms).
| Cycle Phase | Characteristics | Aurora Opportunities |
|---|---|---|
| Solar Minimum | Few sunspots, weak storms | Aurora mostly at high latitudes |
| Rising Phase | Increasing activity | More frequent strong aurora |
| Solar Maximum | Peak activity, frequent CMEs | Best chances, even at lower latitudes |
| Declining Phase | Recurrent coronal holes | Regular, predictable aurora cycles |
The Sun entered Solar Maximum in late 2024, meaning we're currently in a prime period for aurora viewing. NASA and NOAA announced this milestone in October 2024, and the maximum phase typically lasts 1-2 years before activity begins declining.
Common Questions About Aurora Science
Why only at the poles?
Earth's magnetic field lines converge at the magnetic poles, funneling charged particles to these regions. At lower latitudes, the field lines don't connect to the particle-injection zones.
Can aurora happen during the day?
Yes! Aurora occurs on the dayside too, but sunlight makes it invisible. Only during extreme storms can faint dayside aurora be detected with special instruments.
Is aurora dangerous?
No. Aurora occurs far above the surface (80-500 km). The particles creating it never reach ground level—Earth's atmosphere absorbs them. The beautiful light show is the safe byproduct.
Do other planets have aurora?
Absolutely. Any planet with an atmosphere and magnetic field can produce aurora. Jupiter and Saturn have spectacular aurora, far more powerful than Earth's, visible in ultraviolet and infrared wavelengths.
Why do cameras see aurora better than my eyes?
Two reasons:
- Exposure time — Cameras collect light over seconds; your eyes "refresh" constantly
- Sensitivity — Camera sensors detect fainter light than human rod cells
During strong displays, your eyes will see vivid colors. During weaker aurora, cameras reveal colors invisible to the naked eye.
Predicting Aurora: Using the Science
Understanding aurora science helps you predict when displays will occur:
Watch for CME Arrivals
When solar observatories detect Earth-directed CMEs, aurora hunters get 1-3 days' notice. The larger and faster the CME, the stronger the potential storm.
Monitor the Bz Component
Real-time solar wind data shows whether Bz is negative (good) or positive (poor). Even high solar wind speed won't produce strong aurora if Bz stays positive.
Track Substorm Activity
Magnetometers detect substorms—sudden energy releases that create the brightest aurora. These can occur even during modest overall conditions.
Consider Multiple Factors
No single indicator guarantees aurora. The best predictions combine:
- Solar wind speed and density
- Bz direction and strength
- Current Kp index
- Local magnetometer readings
- Historical patterns
Conclusion
The northern lights are a visible reminder that Earth exists within the sun's influence. Every aurora display represents a chain of events spanning 93 million miles—from nuclear fusion in the sun's core, through the violent release of solar storms, to gentle collisions in our upper atmosphere.
Understanding this science doesn't diminish aurora's beauty; it deepens appreciation for the cosmic forces creating these displays. Next time you watch green curtains ripple across the sky, you'll know you're seeing the sun's energy transformed into light, right before your eyes.
Aurora Go tracks all the scientific indicators: Solar wind data, Bz direction, Kp index, and substorm alerts—giving you the complete picture of what's happening between the sun and Earth.
References
- NOAA Space Weather Prediction Center. "Aurora." Altitude ranges and auroral oval information.
- NOAA Space Weather Prediction Center. "Aurora Tutorial." Wavelengths and emission altitudes.
- NOAA Space Weather Prediction Center. "Solar Wind." Solar wind speed and density data.
- NASA. "Solar Cycle 25." Solar maximum announcement (October 2024).
- NASA Marshall Space Flight Center. "The Solar Wind." Solar wind characteristics.