Astronomy
What Causes the Aurora Borealis and Australis?
Quick fact
Auroras can occur on other planets too; Jupiter and Saturn have their own versions caused by interactions with their magnetic fields.
Why this is interesting
You've seen photos of the shimmering green curtains of the aurora borealis, but have you ever wondered what powers this celestial light show?
Read the full explanation
Understanding What Causes the Aurora Borealis and Australis?
Imagine a constant stream of particles—mostly electrons and protons—flowing from the Sun at about a million miles per hour. This is the solar wind. When it reaches Earth, our planet's magnetic field acts like a shield, deflecting most of the particles. However, near the poles, the magnetic field lines funnel these charged particles into the upper atmosphere. There, they collide with atoms and molecules of oxygen and nitrogen, giving them extra energy. When those atoms return to their normal state, they release that energy as light—the aurora. Different gases and altitudes produce different colors: oxygen gives green and red, nitrogen gives blue and purple.
A deeper explanation
The underlying mechanism is a complex interplay of electromagnetism and atomic physics. The solar wind carries a magnetic field of its own. When this field interacts with Earth's magnetosphere, it can reconnect with our field lines, a process called magnetic reconnection. This opens a pathway for solar-wind particles to enter the magnetosphere and accelerate along field lines toward the poles. The particles then collide with atmospheric gases, exciting electrons to higher energy levels. As the electrons drop back down, they emit photons in specific wavelengths characteristic of each gas. The altitude determines the color because at higher altitudes (around 300 km), atomic oxygen produces red auroras, while at lower altitudes (around 100 km), molecular nitrogen and oxygen produce green and blue. This process is not just beautiful—it's a visible sign of the energy transfer from the Sun to our planet, and it can intensify during geomagnetic storms when the solar wind is stronger, sometimes bringing auroras to lower latitudes.