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You have seen the photos – otherworldly shapes, dancing and undulating in the sky with hues of green, pink, purple and red. But what causes those colors? How common is it to see them? We get these questions a lot, so we thought we would help you understand them! 

What causes those colors in the northern lights?

The Aurora Borealis, also known as the Northern Lights, is one of the most mesmerizing and captivating natural phenomena that grace our planet’s skies. This breathtaking display of vibrant colors dancing across the heavens has captured the imagination of people for centuries. The colors of the Aurora Borealis are a result of a complex interplay between Earth’s magnetic field, charged particles from the Sun, and the gases in our atmosphere. In this article, we will delve into the science behind these enchanting hues that paint the polar night.

The Science Behind the Aurora Borealis: Solar Wind and Earth’s Magnetosphere

To truly appreciate the colors of the Aurora Borealis, it helps to understand the scientific processes that create this celestial masterpiece. At the heart of this phenomenon lies the interaction between the magnetically-charged solar wind and Earth’s magnetosphere.

The Sun constantly emits a stream of charged particles through the solar wind. This wind carries with it the magnetic charge of the sun, known as the Interplanetary Magnetic Field (IMF). When these high-energy particles reach Earth, they collide with the magnetosphere – the region surrounding our planet that is influenced by its magnetic field. Some of these charged particles manage to penetrate Earth’s magnetosphere, particularly near the polar regions, where the magnetic field lines converge.

Exciting the Atmosphere: Ionization and Energy Transfer

As these solar particles interact with Earth’s magnetosphere, they collide with atoms and molecules in the upper atmosphere. These collisions cause the atoms and molecules to become “excited,” temporarily elevating them to higher energy states. As these excited particles return to their normal energy levels, they release the excess energy in the form of light.

Different gases in the Earth’s atmosphere emit light at specific wavelengths when they are excited. The colors we observe in the Aurora Borealis are a direct result of the types of gases involved in these interactions.

Colors of the Aurora Borealis: A Spectral Kaleidoscope

  1. Green: The most common color observed in the Northern Lights is green. While most molecules or atoms can only store energy caused by a collision for about 1/1000 to 1/1,000,00 of a second, the excited state of oxygen that causes green can last 0.7 seconds. It is created when excited oxygen molecules at an altitude of around 100-250 kilometers emit light at a wavelength of 557.7 nanometers. This vibrant green glow is often the dominant hue in the auroral display.
  2. Red: Red hues in the Aurora Borealis are less common and occur at higher altitudes, generally 200-300 kilometers. They result from the emission of light by oxygen molecules at a wavelength of 630 nanometers. Red is often seen during intense solar activity.
  3. Purple and Pink: Nitrogen molecules contribute to the purplish and pinkish colors in the auroral display. These colors are produced at lower altitudes, typically around 100 kilometers or lower.
  4. Blue: While rarer than green and red, blue auroras do occur. They are caused by nitrogen molecules emitting light at around 427 nanometers. Blue auroras are often visible during periods of extremely high solar activity, and generally at 100km or lower.

How common is it to see color in the northern lights?

Here is why photographing the aurora borealis is the best way to experience it. Most commonly, we will not see the full colors of the aurora borealis with the naked eye. Here’s why.

Those darn cones!

The cones in our eyes are what transmit the light that the brain interprets as color, while the rods perceive light and dark and help with peripheral vision. At night, rods give us our night vision, but do not help with seeing color – that’s why things look in greyscale when it is dark. We need brighter light in order to perceive color because cones only perceive color. At night, it is too dark for our cones to work properly. 

So, with a quiet aurora borealis display, the light of the aurora will appear as a light grey band in the night sky. That is the work of our rods perceiving the light, but it is too dark for our cones to perceive color. As the aurora gets more active, it gets brighter. As the northern lights get brighter, then we begin to see color. For most aurora displays, we will typically only see green with the naked eye. During strong geomagnetic storms, the aurora is particularly bright. It is at those times that we can start to perceive colors like red and purple with the naked eye. 

Frequency of geomagnetic storms

Now you are wondering, “If I can only see red or purple with the naked eye during geomagnetic storms, who often do those occur?” Strong geomagnetic storms are typically caused by active sunspots that produce solar flares that fling the sun’s plasma out into space through a Coronal Mass Ejection, or through an active Coronal Hole. These do not occur as common as an aurora chaser might like. But, a study of historic aurora behavior shows that the months with the highest average of geomagnetic storms are March (9.55 in the month), April (8.65), September (8.40) and October (8.40). 

Camera exposures

Fortunately, our cameras can help us to overcome the biological limitations of our eyes. With longer shutter speeds, a camera can “brighten” any dark scene. The longer the exposure, the brighter the light. Many times a first exposure can reveal the presence of the aurora even though it is not visible at all to the naked eye, or what may appear grey to the naked eye will be green in the photo. And as the aurora gets more active, you can start to see pink, red, purple as well as green in the photo, while green or light grey hues are all that is visible to the naked eye. 

The Aurora Borealis is a vivid reminder of the intricate beauty and wonder that our natural world holds. The palette of colors that grace the polar skies is a direct result of the delicate interplay between solar wind particles, Earth’s magnetic field, and atmospheric gases. From the mesmerizing greens to the elusive reds and blues, each color tells a tale of charged particles on an ethereal journey, creating a captivating spectacle that continues to awe and inspire all who are fortunate enough to witness it. And you can appreciate it even more when you photograph it. 

Want to know more? Join us in the field for our Anchorage Aurora Quest or Private Aurora Tour!

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