It’s not every day we get the chance to see the aurora borealis, let alone in Chicago of all
places. On November 11th and 12th, the aurora was visible across the United States, including in states in the south like Texas and Florida. Lights of a vibrant red and green hue were visible along the horizon. While not as picturesque as what might be visible at the North Pole, the lights seen were mesmerizing all the same.
The biggest obstacle in viewing the aurora is light pollution; the bright city lights saturate our eyes and the sky, making seeing the aurora with the naked eye difficult. It is possible to see the lights using a camera with a long exposure time, but to see them with your own eyes, it’s best to find a place away from the city lights. Many people congregated on Chicago’s lakefront and got a spectacular view of the lights. Chicago and its suburbs were blessed with wonderfully clear skies, allowing for great views in the evening. As for myself, I was able to see them outside my home in the suburbs.
How do auroras happen?
The aurora occurs because of the Earth’s magnetosphere interacting with solar wind. Let’s break it down: Our sun is a large ball of plasma and charged ions. These charged particles create electric and magnetic fields. Regions of intense activity are called sunspots (those black dots you see in pictures of the sun). A sunspot may erupt, ejecting plasma, in what is called a coronal mass ejection (CME). While rare, it is possible that the earth can be in the path of this eruption, and that ejected plasma cascades towards earth at high speeds over250km/s. Scientists can predict that there will be an aurora a few days in advance after an eruption, as well as predict the intensity of its impact. These events are more common during the peak of the sun’s solar cycle, which is currently happening, and occurs around
every 11 years.
That wave of plasma is referred to as solar wind and contains electrons, protons, and other light ions like helium, along with trace heavy elements. When this wave hits the Earth, the shape of the Earth’s magnetic field changes. Charged particles from the wind will enter the upper atmosphere. These particles will excite, or give energy to, the contents of the atmosphere, such as oxygen or nitrogen. When the atoms deexcite, they emit particular wavelengths of light.
The light we see in the aurora is the light that is emitted by the deexcitation of oxygen and nitrogen in our upper atmosphere. For instance, the red hue seen was due to the 630 nm emitted by excited oxygen, and the green hue was due to a 557.7 nm emission by excited oxygen. While not visible in Chicago last week, colors like blue are also possible due to nitrogen emissions at 428 nm.
As an aside, I saw a fake news post on Facebook depicting a white aurora. This is not possible. As you may recall, white light is due to emissions at a spectrum of visible light: red, orange, yellow, etc. The light emitted in an aurora is emitted at only a few particular wavelengths. It is possible these colors can mix, but that would result in hues like pink or yellow, not white.
Side Effects of a CME
While we might enjoy the beauty of the aurora, there are possible side effects of a CME hitting Earth. The changing magnetic field on Earth will affect electronics by inducing currents in circuits. It was reported that radio technology in Africa was affected as a result of this particular storm. This is a common theme for strong solar storms.
The strongest recorded geomagnetic storm occurred in 1859 and is known as the Carrington Event. This resulted in auroras being seen worldwide, in places like Mexico, the Caribbean, Hawaii, and even countries close to the equator like Columbia. It was reported that telegraph lines were significantly impacted. Telegraph lines arced, and operators could feel an electric shock when operating. Users could even use their telegraphs with the power disconnected.
Many scientists warn of the potential of a modern Carrington Event. Our world is much more dependent on electronics, and these events can cause serious disruptions to the power grid. A large storm could devastate the world, with key infrastructure failing and supply lines collapsing.
But all that worry is for another time. Personally, I found the event exhilarating, and it was fun to describe the physics behind it to my parents. Sightings of aurora in this part of the country, no less the city, are very rare, and could quite possibly be a once-in-a-lifetime experience for us. It brings to scale that we are just a ball floating around a star, and it can sometimes grace us with something as beautiful as an aurora.
My view of the aurora from the Chicago suburbs. A strong band of red was seen low in the sky, along with a green haze along the horizon.