The Science Behind the Northern and Southern Lights - Explained

One of nature's most breathtaking light displays, the auroras – the aurora borealis (the northern lights) in the Northern Hemisphere and aurora australis (the southern lights) in the Southern Hemisphere – have enchanted humans for generations. Unlike meteors or comets that can flash across the night sky in moments, auroras bring stunning and colorful displays that can last for hours with waves of color painted in the sky, with greens, reds, and purples. These spectacular displays in the sky are beautiful, but they also have specific ties to the dynamic physical interactions taking place between Earth and the Sun. These displays are not simply atmospheric; when they occur they are the product of powerful events in space weather brought on by our own star. If we could get to the bottom of what brings them on and the reason why we see them in polar regions, we could possible understand their potential better.

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How Does the Sun Trigger the Auroras?

Even though they happen within the atmosphere of Earth, auroras are formed as a result of solar activity. The outer layer of the Sun's atmosphere, the corona, continually releases streams of charged particles that travel through space as the solar wind. These particles are made up of high-energy protons and electrons that are emitted from the Sun at very high speeds.

When these solar particles encounter the magnetic field of the Earth, they react. This invisible shield protects us from dangerous cosmic radiation, but some of these particles can still be channeled into the magnetic poles, which are the north and south poles. Some properties of the magnetic field can guide solar wind into the upper atmosphere of the Earth, which is the polar region.

What Happens When Solar Particles Reach Earth?

When these energetic particles enter the atmosphere, they collide with the gaseous envelope of the Earth consisting of primarily oxygen and nitrogen. These collisions cause enough energy to eject electrons from the atom and create ions in an excited state. As the ions drop back to their original energy state they release light, i.e. neon lights. 

• Oxygen produces green and red light. 

• Nitrogen produces purple and blue shades. 

The colors and intensity depends on the gas, the altitude, and the energy from collision.

Why Are Auroras Mostly Seen Near the Poles?

The Earth's magnetic field is strongest at the poles, which is why auroras are most prevalent in polar regions. It also explains why Alaska, Canada, Norway, and Antarctica are recognized for their auroral displays.

However, during periods of high solar activity (such as solar flares or coronal mass ejections) the volume of charged particles released from the Sun increases. This enhances auroral activity, allowing the lights to shift farther away from the poles and be seen at mid-latitude regions such as the northern United States or southern Australia and New Zealand.

How Do Solar Cycles Affect the Visibility of Auroras?

The Sun has an 11-year activity cycle which ebbs and flows from solar minimum (low levels of activity) to solar maximum (high levels of activity). During solar minimum there are fewer sunspots, and solar wind is diminished, auroras will compress in latitude and become fewer. During solar maximum, auroras will become brighter, occur more frequently, and volume will expand geographically. 

Historically, during large solar storms, aurora borealis has been reported as far south as 40° latitude in the U.S. in states such as Illinois and Pennsylvania.

At What Altitude Do Auroras Occur?

Auroras usually happen in the upper atmosphere around 80 km to 250 km (50 to 155 miles) above the Earth's surface. The majority of auroras are seen at altitude of around 100 km (60 miles) where they produce beautiful bands or curtains of light that ripple and dance through the night sky. 

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Conclusion

Despite seemingly being magical streaks of colour in the sky, auroras are the result of complex physical processes connecting the Sun to space and its connection to the Earth's magnetic field. These works of art created by nature bring to our attention how we are connected to the solar system and all of the many forces that are affecting the Earth just beyond the atmosphere. If you happen to see an aurora in a frozen northern landscape or are lucky enough to catch a rare glimpse further south, these are both scientifically fascinating events as well as visually stunning experiences.