The past several posts I’ve written about my friend Kevin and his visit to Iceland over the New Year. He stayed for about two weeks, and although we had a lot of entertaining adventures (only a few of which involved mortal peril – really, driving across mountains in white-out conditions is a bad idea; you don’t even get any good photos) we had one major disappointment … or rather, the same disappointment repeated a dozen times. The northern lights eluded us, night after long, Arctic night.
Lots of visitors come to Iceland during the summer. The weather is mild, the days are long, and there are concerts and festivals and lots of things to see and do. But there are trade-offs, and when you choose to visit in the summer, one of those is that you will not see the northern lights. You need darkness to see the lights, and from May through July it never really gets dark here.
The northern lights begin with the brightest light in the sky, the Sun. Although it’s relatively small in comparison to other stars, the Sun is nonetheless almost unimaginably huge by human standards. Sunlight is produced in the core of the Sun through the fusion of hydrogen atoms into helium: each reaction converts a minuscule fraction of mass into energy. Roughly 600 million tonnes of hydrogen are fused into helium every second – about the same as 1,000 Mount Everests per hour, or one Earth every 70,000 years.
The fusion reactions in the Sun’s core convert about 4 million tonnes per second of mass into energy in the form of highly-energetic photons and, eventually, visible light. In essence, the Sun is a vast hydrogen bomb detonating every second of every day, with only its own immense gravitational field holding it together. And it’s been going off now for about four and a half billion years.
Not surprisingly, the outer layers of the Sun are constantly seething with the heat being released in the core. The outermost layer, called the corona, is hot enough (and far enough from the core) that some of it escapes the Sun’s gravity well and surges outward into space. This is the solar wind, a perpetual stream of charged particles that ebbs and flows with the swirling and boiling of the star beneath. The solar wind gusts between 300-800 kilometers (180-480 miles) per second, but even at those speeds it takes several days for the particles in the wind to reach the Earth.
Sometimes, solar activity is so intense it produces a coronal mass ejection, a large blob of highly-energetic particles surging into space. When the ejection is in the direction of the Earth, it’s particularly likely to produce an intense display of northern lights.
When the charged particles of the solar wind reach the Earth, they interact with our planet’s magnetosphere. Earth has a core of molten iron that creates a powerful magnetic field as it rotates. Most of the charged particles from the Sun follow the magnetic field and flow around the Earth, distorting the shape of the field somewhat but never interacting with us in any meaningful way. Some of the particles, though, follow the field toward the magnetic poles of the Earth and reach our upper atmosphere.
The Earth’s atmosphere attenuates with altitude. From a density exerting a pressure of about 100 kilopascals (kPa) (about 14.7 pounds per square inch) at sea level, our air decreases in density and pressure the higher you go: at about 10 kilometers (6 miles) above the Earth’s surface, the pressure has dropped to less than 30 kPa, and the decline continues on a gradual curve toward nothing as you head toward space.
Neon lights are made from glass tubes filled with neon, argon, and other gases at extremely low pressures: typically around 1 kPa. The gases are made to glow by firing charged particles – electrons – into the tube. The particles interact with the gases, imparting some of their energy to the electrons in the gases, forcing them into a higher energy state than they would ordinarily occupy. The higher energy state isn’t stable, and when the electrons return to their normal state they give up the imparted energy by releasing photons. The same process takes place when charged particles from the Sun encounter the thin atmosphere high above the Earth’s surface. Our upper atmosphere starts to behave like a giant neon light – without the glass tubing, wires, and cheesy commercial messaging.
The Earth’s atmosphere is comprised mainly of nitrogen and oxygen, and interactions between solar particles and various forms of those two elements are the most common. Very high-altitude interactions with oxygen tend to involve more energy and to produce reddish glows; interactions with denser layers of oxygen produce more greenish glows. Interactions with nitrogen produce blue or purple light. As the charged particles plunge deeper into the atmosphere, the air becomes thicker, and the particles are more likely to interact. At about 90 kilometers (55 miles), the air is too dense for all but a very few particles to penetrate, and so no more light is produced below that altitude.
The term aurora borealis, from Aurora, the Roman goddess of the dawn, and boreas, the Greek name for the north wind, was coined by Galileo in the early 17th Century. In the southern hemisphere, the lights are called aurora australis, and generically they’re called the aurora (or aurorae, when the plural form is used).
To see the aurora, you need three things: activity on the Sun that generates a powerful solar wind; clear skies on Earth, so that you can see what’s happening in the upper atmosphere; and darkness. Aurorae are quite striking and can be very bright, but they don’t compete well with other sources of light. Streetlights and other city lights make them hard to see and conceal much of their beauty. In the daytime, there’s really nothing to see at all.
Because the magnetic north pole doesn’t quite coincide with true north, aurorae are slightly more likely to be seen in North America than in Asia; but in most of the United States, only truly intense solar storms produce visible northern lights.
So far in my stay in Iceland, solar and terrestrial weather have coincided on a regular basis to produce visible aurorae every few days. When I see the lights, I usually send out a text message to my classmates in the master’s program. I’ve gotten pretty good at spotting them – so much so that I acquired the nickname “the aurora app” sometime during the winter. But the process of aurora hunting requires a fair amount of luck, and luck was something we didn’t have when Kevin was visiting. His last night here, there was a very faint band of light over the city, but that was all. Literally the next night, though, the northern lights came out in all their glory.
The more intense the solar activity, the more likely the aurora will take on a visible “structure,” shaped by the Earth’s magnetic field. You watch the lights ripple and sway, changing colors and flickering overhead in total and eerie silence. Sometimes, a group of especially-energetic particles will produce a burst of light directly overhead, and it’s as if the gods were pouring out the light of the heavens.
My wife Lisa thinks that the Bifrost, the bridge between the realms of heaven of Norse mythology, may have been inspired by aurorae. Although that’s not the theory generally accepted by scholars you have to wonder: what did the ancients think produced the bashful lights?
All images by author except as otherwise specified in caption.