The Science Behind the Northern Lights
The Northern Lights, also known as Aurora Borealis, are a natural light display on the sky in the northern hemisphere. On the opposite end, in the southern hemisphere, there are Southern Lights, Aurora Australis. The auroras are predominantly seen in high-latitude regions such as areas near the Arctic and Antarctica. In this blog, I will talk about the science behind the Northern Lights and how to see them.
How are Auroras formed?
It all started in the sun. Solar activity causes the solar wind, which is a stream of charged particles released from the upper atmosphere of the Sun. The solar wind streams off of the Sun in all directions at speeds of about 400 km/s (about 1 million miles per hour). It usually takes around 3 days to reach Earth.
When these particles reach the Earth's magnetosphere, which is the region around Earth that is dominated by the Earth's magnetic field, they collide with atoms and molecules in Earth's atmosphere. The energy from these collisions is then released as light, causing the stunning display of colors we know as the Northern Lights. The different colors are determined by the type of gas particle that the solar wind particles are colliding with. For instance, the common green color is produced by oxygen molecules located about 60 miles above Earth, while red is produced by high-altitude oxygen, at heights of up to 200 miles. Nitrogen produces blue or purplish-red aurora.
The Auroras typically appear near the magnetic poles, where the planet’s magnetic field is the strongest. Since the magnetic field lines extend from one of the planet’s magnetic poles to the other and are concentrated near the poles, those magnetic field lines funnel charged particles in space toward the planet’s poles. Eventually, when the particles from the solar winds enter a planet’s atmosphere near the poles, they collide with the gas particles and form Auroras. This is why Auroras are most commonly seen in the Arctic and Antarctic regions. However, during periods of particularly high solar activity, the lights can be seen in areas much further from the poles. Auroras also occur on other planets that have a strong magnetic field, such as Venus, Mars, Jupiter, Saturn, Uranus, and Neptune from pretty much the same processes. Jupiter has the most Auroras because Jupiter's magnetic field is 20,000 times stronger than that of Earth, so the Auroras are far brighter than the ones that we see on Earth.
How to see the Auroras?
- There is a higher chance of seeing Auroras in the Arctic and Antarctic regions during winter time when the nights are longer and less clouds in the sky. For Arctic regions, the best time is between September and April, while the best time is between May to August in the Antarctic regions. It’s harder to see the Southern Light because there is less land in the Southern Hemisphere to see it from.
- Cameras can “see” the Auroras better than human eyes. There are two types of cells on the retina of our eyes that detect light. Cone cells allow us to see the different colors but it’s less sensitive to light. Rod cells are more sensitive to light but they give us a black and white picture instead of allowing us to see all the colors. This is why the Auroras are more vibrant in pictures than seeing them with our eyes. Each person also sees the Aurora differently. The pupil can open further for some people, which allows more light into the eyes, causing the cone cells to “kick in” and see the colors. Our pupil also gets smaller with age, so a younger person might be able to see more color in the Aurora than someone older.
- You can see the Northern Lights in the Northern half of the U.S. and all of Canada when the geomagnetic activity is high. Geomagnetic activity is the product of the interaction between the solar wind and magnetosphere, usually measured in Kp index ranging from 0-9. The Kp index is usually around 1-3 in the northern hemisphere, which is why we can only see the Northern Lights when we are closer to the Arctic regions.
