The Northern Lights (Aurora Borealis) Explained

If you live far enough north or south on planet Earth, you may be lucky enough to see the northern/southern lights (aurora borealis/aurora australis). You'll likely be starstruck by these dazzling light and color displays if you're fortunate enough to find yourself outside on a clear night while the aurora borealis occur. These dancing bands of light are indeed one of nature's most stunning sights. What are they, and how do they form? The answers to these questions help us understand the Earth's relationship with the Sun. They also provide insight into how the interior of our world and its atmosphere interact with the Sun's energy

Northern Light, Aurora borealis at Kirkjufell in Iceland.
Northern Light, Aurora borealis at Kirkjufell in Iceland.

How The Aurora Are Produced 

The Sun is the nearest star to us at 93 million miles. Despite that vast distance, the sheer energy from the Sun is powerful enough to warm our world and sustain all life. The Sun is constantly emitting a flow of radiation called the solar wind. The solar wind is composed of countless particles that have been charged with high amounts of energy from the Sun. These high-energy particles are deadly to life on Earth and can damage the very structure of our DNA. Thankfully, the Earth has developed natural barriers that protect us from solar wind. The Earth's magnetic field is our primary form of defense against the solar wind. This field is produced due to Earth having an iron core that spins. Iron can conduct electricity, and as it turns, the moving electric charge creates a magnetic field. The magnetic field produced by the Earth's core moves outwards and forms a web-like structure that encompasses our world. When solar wind encounters the Earth's magnetic field, it is redirected away from our world. Without a magnetic field, the solar wind would slowly erode our atmosphere until nothing but a thin veil of air remains.

Solar Wind Illustration
Solar Wind Illustration

However, not all of the particles in the solar wind are redirected away. Instead, the magnetic field redirects some charged particles towards the Earth's poles. Even when the solar wind manages to reach our world, life is still protected by an ozone layer. However, there are two large holes in the Earth's ozone, one located at either pole. When charged particles are redirected towards the North and South Poles, they interact with the atoms in our atmosphere. The amount of energy in the solar wind causes some of the atoms in the Earth's atmosphere to lose their electrons, a process called ionization. By ionizing particles in our atmosphere, the solar wind causes them to release light beams at high energy. The aurora are simply ionized atoms in our atmosphere that have interacted with charged particles from the Sun, causing them to glow and move through the air. 

The Many Colors Of The Aurora 

Aurora Borealis in Iceland
Aurora Borealis in Iceland

Although the aurora are generally green, they can come in a multitude of different colors. The color of the aurora is determined by the atoms that are being ionized along and the altitude at which this is happening. Since oxygen and nitrogen are the most abundant chemicals in the Earth's atmosphere, the colors of the aurora are determined by how these two chemicals interact with the solar wind. When oxygen is ionized at a high altitude, the aurora glow red. At a lower altitude, ionized oxygen is green. The reason why the same chemical can give off two different colors has to do with the fact that at higher altitudes, the density of oxygen is far lower than at lower altitudes. The high altitude allows the solar wind to ionize oxygen at a higher frequency and produce a red glow. Likewise, at lower altitudes the frequency at which the solar wind interacts with oxygen is lower and the product is green light. In addition to red and green, the aurora can be blue or purple. If you happen to see these two colors, it means that nitrogen is being ionized in the atmosphere. The process by which the aurora form and produce their extraordinary colors only adds to their beauty and complexity. 

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