This column was first published in The Tablet in February 2011
The aurora borealis as seen from Michigan’s upper peninsula. Alas, I did not take this photo!Earlier this month [2011], NASA satellites observed a set of flares on the surface of the Sun and predicted that glorious aurorae would soon be visible – a rare sight anywhere in Europe south of Scandinavia. But that was the week I was visiting my brother in northern Michigan, so I resolved to take a moment to look for them.
The Northern Lights are a marvelous sight. The easiest to see are sheets of light in the northern sky, moving like curtains in the wind. If they’re bright enough, you can see colors, mostly green, swirling like a light show at a 60’s rock concert.
The science behind them is equally fascinating. Electrons and protons burst from the Sun in explosions of an expanding plasma as large as planet Earth. Hot enough to overcome the Sun’s gravity, as they move from the Sun they feel even less gravity; they move further, move faster, feel less gravity, move even faster, even further, until they’re part of a supersonic Solar Wind filling interplanetary space. Because it is electrically charged, it drags the Sun’s magnetic field with it, warping the familiar bar-magnet dipole into a twisted spiral.
And then this plasma and magnetic field hits the Earth’s magnetic field, pushing and dragging it about, and dumping many of those charged particles into Earth’s Van Allen belts.
All these charged particles follow the dipole magnetic field of the Earth, pouring into the north and south magnetic poles. When they hit, they rip the electrons off the atoms in Earth’s upper atmosphere. Then the electrons recombine with those atoms they just left, emitting light as electrons jump from one energy state to another; different colors for different energy states. Most of the emission is in a range beyond what the human eye can see, but one of those colors is a pale green: the main light of the auroras, borealis and australis, visible if you’re close enough to the magnetic poles where all these ions are hitting.
The green light comes from a transition that, according to the rules of physics, shouldn’t take place. (It violates spin parity.) But it happens anyway; just not too often. What’s the use of a rule that doesn’t work? Actually, it’s a very useful rule; it’s just that nature’s more complicated sometimes. Ultimately, all the rules of physics are like that. They’re useful, but they don’t quite explain everything. You have to understand where they apply, and know how to use them intelligently. Being a physicist (like being a Catholic) is more than just knowing a lot of rules.
Look at a map and you’ll see that even the northernmost part of Michigan is at a far more southern latitude than the UK. But it’s much closer to the Earth’s magnetic north pole, which is located in northern Canada. Even though London is five degrees further north, Marquette is 500 km closer to the magnetic pole. As a result, the auroras are more commonly seen there.
It’s also thousands of kilometers from any ocean, so the weather is significantly colder… as I noticed when I went out looking for the aurora. I also noticed what the news reports forgot to mention, that the nights of the maximum aurora were also the nights around the full Moon.
Shivering in the cold, I watched the stars of Orion play tag with low-lying clouds, swept about by the west wind. The moonlight illuminated the clouds from above, and from below they were lit by the nearby bonfires set to guide the annual midnight dogsled races, being held that night. If there had been a glow of green between the stars I would never have seen it.
Still, the nighttime sky is a busy place, and a marvelous inducement to wonder… about light and dark and forbidden transitions.
