Walking down to the lake one last time before heading to the tent, I couldn’t resist turning off my headlamp and looking up at the stars one last time, too. Here, in the Boundary Waters Canoe Area Wilderness of northern Minnesota, we are many miles from any significant light pollution. The dry, breezy September day had cleared humidity from the sky, too, resulting in a brilliant celestial show. Even familiar constellations were almost lost in the lavish abundance of light.
As I watched, I began to notice something else obscuring the stars. Just above the tree line, a gauzy haze had materialized. It swept around to the north, and dimmed a swath of the sky. I was puzzling at such a peculiar intrusion of clouds, when suddenly the haze developed two vertical bars. These weren’t clouds—they were northern lights!
Snapping my headlamp back on for expediency, I hurried back up the toe of granite toward our cluster of tents. John was already on his way down. “Northern lights!” I exclaimed as we passed. I shook Brenda and JoAnn’s tent gently, “The northern lights are out!” Ed was still reclining next to a few glowing embers in the fire grate. I could hear my dad rustling around in our tent, and see Jane’s light bobbing in hers. “Northern lights!”
One by one, our crew on the Natural History Paddle in the Boundary Waters picked their way down the campsite’s classic lakeside rock outcrop, and came to sit and stand at the water’s edge. Silently, we watched as the arc of greenish-white light rose and lowered, then split into two layers like a double rainbow. Dim vertical bars came and went almost imperceptibly. There were no wild dances, no flashing colors, just a demure display of physics above a gathering of new friends.
Before scientists discovered that solar winds cause the Aurora, many cultures created beautiful and fascinating stories to explain this mysterious phenomenon. Norse mythology tells of mounted warriors with light reflecting off their armor, and the massive ice of glaciers releasing stored energy. Chinese mythology imagines dragons’ fiery breath flashing across the sky. Others believe in spirits of ancestors dancing overhead, and reflections off ice at the North Pole.
In just the past 50 years, science has told us that the northern lights, or Aurora Borealis, are produced through an interaction between the Sun, the Earth’s magnetic field, and gasses in our atmosphere.
It begins with a coronal mass ejection (CME). The Sun ejects a stream of material, called solar wind. If it is aimed toward Earth, then the solar takes between one and four days to arrive at Earth. Once it arrives, the Earth’s magnetic field channels most of the electrically charged particles in the solar wind safely around the Earth.
Only where the magnetic field dives downward – near the North and South Poles – do the particles get close. Here, 50-200 miles up in our atmosphere, the electrons and other charged particles in the solar wind bump into oxygen and nitrogen molecules. Oxygen and nitrogen have their own electrons, which can gain or lose energy. So, when electrons from a solar wind bump into and transfer energy to oxygen and nitrogen, it boosts their electrons into a higher energy state. These “excited” electrons are unstable.
As an excited electron relaxes back to a more stable energy level, it releases energy in the form of a photon of light. The size of the energy change determines the color of the light. As we relaxed back into a more stable energy level after the excitement of the day, perhaps we glowed a little bit, too.
Of course, none of us were thinking about the physics of our experience as we stretched our stiff legs and wiggled cold fingers. We were admiring the stars (still reflected in the rippling lake), anticipating the warmth of fluffy sleeping bags, and exhaling gratitude. Watching northern lights from a slab of bedrock in the Boundary Waters, canoes tucked safely into the trees nearby…is there any better way to spend an evening?
“I wondered…if anything—even knowing the physical truth—could ever change the beauty of what I had seen, the sense of unreality. Indian warriors, exploding atoms, beds of radium--what difference did it make? What counted was the sense of the north they gave me, the fact that they typified the loneliness, the stark beauty of frozen muskegs, lakes, and forests. Those northern lights were part of me and I of them.”
-- Sigurd Olson
|The Aurora-watching rock was great for sunrise fishing, too. Photo by Larry Stone|