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What causes the aurora borealis?

The aurora borealis, or northern lights, are an ethereal display of colored lights shimmering across the night sky. What causes them?

View larger. | Photo taken March, 2016, by Mike Taylor in Maine.  Visit MikeTaylorPhoto.com

View larger. | Photo by Mike Taylor in Maine. Visit MikeTaylorPhoto.com

Those who live at or visit high latitudes might at times experience colored lights shimmering across the night sky. Some Inuit believed that the spirits of their ancestors could be seen dancing in the flickering aurora. In Norse mythology, the aurora was a fire bridge to the sky built by the gods. This ethereal display – the aurora borealis or aurora australis, the northern or southern lights – is beautiful. What causes these lights to appear?

Our sun is 93 million miles away. But its effects extend far beyond its visible surface. Great storms on the sun send gusts of charged solar particles hurtling across space. If Earth is in the path of the particle stream, our planet’s magnetic field and atmosphere react.

When the charged particles from the sun strike atoms and molecules in Earth’s atmosphere, they excite those atoms, causing them to light up.

What does it mean for an atom to be excited? Atoms consist of a central nucleus and a surrounding cloud of electrons encircling the nucleus in an orbit. When charged particles from the sun strike atoms in Earth’s atmosphere, electrons move to higher-energy orbits, further away from the nucleus. Then when an electron moves back to a lower-energy orbit, it releases a particle of light or photon.

What happens in an aurora is similar to what happens in the neon lights we see on many business signs. Electricity is used to excite the atoms in the neon gas within the glass tubes of a neon sign. That’s why these signs give off their brilliant colors. The aurora works on the same principle – but at a far more vast scale.

When charged particles from the sun strike air molecules in Earth's magnetic field, they cause those molecules' atoms to become excited. The molecules give off light as they calm down. Image via NASA

The aurora often appears as curtains of lights, but they can also be arcs or spirals, often following lines of force in Earth’s magnetic field. Most are green in color but sometimes you’ll see a hint of pink, and strong displays might also have red, violet and white colors. The lights typically are seen in the far north – the nations bordering the Arctic Ocean – Canada and Alaska, Scandinavian countries, Iceland, Greenland and Russia. But strong displays of the lights can extend down into more southerly latitudes in the United States. And of course, the lights have a counterpart at Earth’s south polar regions.

The colors in the aurora were also a source of mystery throughout human history. But science says that different gases in Earth’s atmosphere give off different colors when they are excited. Oxygen gives off the green color of the aurora, for example. Nitrogen causes blue or red colors.

So today the mystery of the aurora is not so mysterious as it used to be. Yet people still travel thousands of miles to see the brilliant natural light shows in Earth’s atmosphere. And even though we know the scientific reason for the aurora, the dazzling natural light show can still fire our imaginations to visualize fire bridges, gods or dancing ghosts.

Photo by Ruslan Merzlyakov. Visit him on Facebook.

Photo by Ruslan Merzlyakov. Visit him on Facebook.

Jennifer Khordi captured this aurora last night (September 7) over the Catskills of New York state.

Jennifer Khordi captured this aurora over the Catskills of New York. Visit Jennifer Khordi on Facebook.

Aurora just west of Saskatoon, by Colin Chatfield. He said,

Aurora just west of Saskatoon, by Colin Chatfield. He said, “We were just about to leave as the aurora was just a dull band, then it came alive for about an hour or so … It is not very visible, but I caught Comet Lovejoy at centre left of this photo.”

“This photo was captured a couple of hours ago in Nordreisa, Norway. I was dressed in my very best winter clothes and I can easily admit that I was freezing most of the time anyways, 22 below (-7.6 Fahrenheit) kinda has that effect.” (January 2015). © 2015 Tor-Ivar Næss

Aurora in Vesterlålen, Norway, January 2015 by Stig Hansen

Aurora in Vesterlålen, Norway by Stig Hansen

Mike Taylor in Maine caught this photo of last night's aurora.  More about Mike and this photo.

Mike Taylor in Maine caught this photo in September 2014. More about Mike and this photo.

Photo © 2014 Tor-Ivar Næss

Photo © Tor-Ivar Næss

Photo credit: Hallvor Hobbyfotograf Lillebo

View larger. | Aurora borealis over Norway’s Steinvikholmen Castle by Hallvor Hobbyfotograf Lillebo.

Reisafjorden, Norway bathing in auroras on January 2, 2014.  Copyright 2014 Tor-Ivar Næss.

Reisafjorden, Norway bathing in auroras. Image copyright 2014 Tor-Ivar Næss. Image via WaitForIt

View larger. | Mike Taylor calls this photo Moonlight Aurora II.  He captured it on February 19, 2014.

View larger | | Mike Taylor calls this photo Moonlight Aurora II. Visit Taylor Photography

View larger. | Aurora over Mount Hood in Oregon as captured by Ben Coffman Photography during the night of May 31-June 1, 2013.

View larger. | Aurora over Mt. Hood in Oregon as captured by Ben Coffman Photography. Visit Ben on Facebook.

Aurora on January 1, 2014 by Geir-Inge Bushmann.  See more photos from Geir-Inge Bushmann

Aurora on January 1, 2014 by Geir-Inge Bushmann. The lights typically are seen in the far north – the nations bordering the Arctic Ocean – Canada and Alaska, Scandinavian countries, Iceland, Greenland and Russia. See more photos from Geir-Inge Bushmann

View larger. | Aurora seen by Colin Chatfield in Saskatchewan, Canada.

View larger. | Spectacular aurora, or northern lights, seen by Colin Chatfield in Saskatchewan, Canada.

Bottom line: When charged particles from the sun strike atoms in Earth’s atmosphere, they cause electrons in the atoms to move to a higher-energy state. When the electrons drop back to a lower energy state, they release a photon: light. This process creates the beautiful aurora, or northern lights.

Eleanor Imster

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