Northern Lights, Explained by a Scientist
That’s why we decided to track down an expert in the field; someone who really knows a thing or two about the Aurora Borealis. And there’s probably no-one more knowledgeable on the subject than Dr Melanie Windridge, author of the book Aurora: In Search of the Northern Lights.
So before you head off, we recommend you read what the doctor has to say, commit it to memory and then casually wow your traveling companions with some scientific knowledge when the opportunity presents itself…
What are the Northern Lights & why do we see them?Our sun emits a constant stream of charged particles in all directions, known as solar wind, which sweeps across space at around one million miles per hour.
If these particles hit earth, they would expose us to harmful radiation and strip away our atmosphere. Fortunately the earth’s magnetic field protects us. When the charged particles reach us, they pump the magnetic field with energy, which catapults charged particles around the back of the earth down magnetic lines and into our upper atmosphere. Here they collide with oxygen and nitrogen atoms and cause them to emit light.
Because they come down field lines they end up near the poles, forming rings of aurora around the globe at high latitudes. The aurora you see is the result of billions of excited atoms emitting tiny flashes of light high up in the polar night sky.
Before scientific explanations, what did people think the lights were?Bright, mysterious lights in the sky have unsurprisingly given rise to all sorts of myths. The indigenous people of the arctic variously explained them as spirits of dead relatives, unborn children, or their enemies. The ancient stories of the Sami people warned that if you mock the lights they will seize you, and even now, parents tell their children about the aurora coming to get them if they misbehave.
Red aurorae are said to have foretold the death of Julius Caesar (44 BCE) and presaged the American Civil War (1860), so for millennia were interpreted as a bad omen.
Throughout their history the appearance of the aurora has combined wonder, mythology and fear. When you see them, even if you understand the science, it’s not hard to see why.
Who was first person to scientifically explain the Northern Lights?Whilst aurora sightings go back beyond written history, it was in 1908 that Norwegian, Kristian Birkeland, first scientifically explained what causes them, though his explanation wasn’t fully recognised until the 1960s, long after his death. He was nominated for the Nobel Prize several times, but never won.
If you’re aurora spotting in Norway, keep an eye out for Kristian on the 200 Kroner note, alongside one of his experiments – a small magnetised ball simulating the earth, called a terrella. Another of his experiments appears on the back, but is only visible under ultraviolet light – you might see it if you’re flying at low altitudes during a particularly strong solar storm, though finding an ultraviolet light bulb might be easier.
Why do we see so many different colors?The most common aurora is a green arch or band across the sky, perhaps rippling or twisting. This colour is emitted by oxygen. It’s impressive enough, but some lucky travellers see vivid reds and blues.
At very high altitudes (several hundred kilometres up) where oxygen atoms are more spread out, the light they emit has a longer wavelength, which appears red. Usually the red light is obscured by the lower green light. But during intense solar activity, strong solar winds make the aurora brighter and cause the rings of aurora around the earth’s poles to expand, so the lights can be seen at lower latitudes – even as far south as central Europe. In these lower latitude locations, viewers looking north will see the red top of the auroral curtain with the green lower part obscured by the earth’s curvature.
At lower altitudes incoming particles may hit nitrogen atoms. The wavelength of light from the different mix of atoms is shorter, creating a blue light (and ultraviolet, though this is invisible to the human eye). If solar activity is particularly strong, charged particles may make it far enough into the earth’s atmosphere to create blues and purples.
Are we the only planet with aurorae?Many planets in our solar system have magnetic fields, and would see similar auroral effects. If we could visit Mercury, Uranus, Jupiter, Saturn and Neptune, we would also have a chance of seeing spectacular auroral displays. For now, we have to settle for looking at spacecraft images such as from Nasa’s Juno. Then again, Iceland and Norway also offer pretty good options.
Venus and Mars don’t have magnetic fields though, so whilst we’re not alone in experiencing aurora, we can at least feel smug when we look over at our closest neighbors.
Author Melanie Windridge is a physicist, speaker & writer – click here to find out more about her.