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What Causes Auroras
The science behind the northern and southern lights
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Auroras are glowing displays of light that appear most often near Earth’s polar regions. They matter because they show a direct connection between the Sun, Earth’s magnetic field, and the upper atmosphere. Their bright green, pink, red, and violet colors are produced by energized atoms and molecules high above the ground. Studying auroras also helps scientists understand space weather that can affect satellites, radio communication, and power grids.
The process begins when the Sun releases charged particles in the solar wind or in stronger bursts called coronal mass ejections. Earth’s magnetosphere guides many of these particles along magnetic field lines toward the polar regions. When the particles collide with oxygen and nitrogen in the upper atmosphere, they transfer energy to those gases. As the gases release that energy as light, glowing auroral curtains and arcs form in the sky.
Key Facts
- Auroras form when charged particles from the Sun collide with gases in Earth’s upper atmosphere.
- Earth’s magnetic field guides many charged particles toward the north and south magnetic poles.
- Green auroras usually come from oxygen atoms about 100 km to 300 km above Earth.
- Red auroras often come from oxygen at higher altitudes, while blue and violet colors often involve nitrogen.
- Kinetic energy of a particle can be estimated with KE = 1/2 mv^2.
- Photon energy is related to light frequency by E = hf.
Vocabulary
- Aurora
- An aurora is a natural light display caused by charged particles exciting gases in a planet’s upper atmosphere.
- Solar wind
- Solar wind is a stream of charged particles flowing outward from the Sun.
- Magnetosphere
- The magnetosphere is the region around Earth controlled by Earth’s magnetic field.
- Ion
- An ion is an atom or molecule that has gained or lost electrons and therefore has an electric charge.
- Photon
- A photon is a small packet of electromagnetic energy, such as visible light.
Common Mistakes to Avoid
- Saying auroras are caused by sunlight reflecting off ice is wrong because auroras are produced by glowing gases in the upper atmosphere, not reflection from the surface.
- Placing auroras in the lower weather atmosphere is wrong because most auroral light comes from the thermosphere, far above clouds and normal weather.
- Assuming auroras only happen at the geographic poles is wrong because they follow magnetic field lines and are centered around the magnetic poles, which are not exactly the same as the geographic poles.
- Thinking stronger solar storms always make auroras brighter everywhere is wrong because visibility also depends on location, time of night, cloud cover, light pollution, and the shape of Earth’s magnetic field.
Practice Questions
- 1 A charged particle with mass 1.7 x 10^-27 kg moves at 4.0 x 10^5 m/s toward Earth’s magnetosphere. Use KE = 1/2 mv^2 to find its kinetic energy.
- 2 A green auroral photon has a frequency of 5.6 x 10^14 Hz. Using E = hf and h = 6.63 x 10^-34 J s, calculate the energy of the photon.
- 3 Explain why auroras are usually seen near the polar regions instead of evenly across the whole sky at all latitudes.