PASADENA CA — Can you possibly imagine what it would have been like, thousands of years ago, to gaze up into a completely dark and calm winter sky and see the Northern Lights? Just think about how awe-inspiring, perhaps frightening, those moments must have felt to our ancestors; to look up into a night sky ablaze with swirling, changing, multi-colored apparitions of light. I don't personally know any specific myths or stories relating to the northern lights, but there must have been loads of them. How could there not be? I've only seen the Northern Lights once, on a plane to Germany as a five-year old child. I can still remember pressing my face up against the small window to stare at giant, luminous green pillows that floated in the air above the North Pole.
Of course, in this day and age we have some good solid scientific explanations for how and why the Northern and Southern lights (which we call auroras) occur. We know they're caused by charged particles from our Sun spiraling in along the Earth's magnetic field and smashing molecules far up in our atmosphere, making them glow. That much is pretty well understood.
But there's still a substantial amount of mystery to the auroras. No scientist has yet been able to accurately predict when and where an aurora will occur, or explain how, exactly, the Sun's particle get channeled up to the Earth's poles in the first place. And at the heart of this is a very basic and vital relationship that we are only beginning to explore: the true connection between the Sun and the Earth.
The Sun, at it's simplest, is a giant hydrogen bomb that has managed to keep exploding for five billions years (and should continue for another five billion), held together by the force of gravity. The mechanics and behavior of such a beast are far from simple. Inside the Sun are unimaginably vast, seething convection currents of ionized gas, and a mysteriously-generated magnetic field that twists and wraps around the internal layers of the Sun.
We don't really have any good idea where this magnetic field comes from, but it probably has something to do with the core of the Sun spinning faster than the outer layers, creating a sort of dynamo engine. One thing we do know is that this field is far from stable. Sometimes bits of the magnetic field bulge out of the Sun's surface, giving rise to cooler (and therefore slightly darker) areas called Sunspots. At other times, part of the magnetic field loses its grip entirely, flinging off huge streamers of gas called Solar Flares or Prominences. All these sorts of magnetic "storms" throw electrically charged particles (mainly electrons, proton, and helium nuclei) off the surface of the Sun in all directions, giving rise to what we call the Solar Wind.
A tiny amount of the solar wind ends up heading towards the Earth, and if it weren't for our own magnetic field, we'd be fried alive. Why does the Earth have a magnetic field? Motions in the liquid iron core of the Earth generate a magnetic field which emerges from the magnetic poles (quite close to the North and South poles) of our planet. It's not a coincidence that the magnetic poles are very close to the real North and South poles. The poles are the axis of the Earth's rotation the line the Earth spins around once a day. That's also the direction the liquid iron in our core sloshes around in, creating the field. Why the magnetic poles and the rotational poles are not perfectly lined up is also a bit mysterious and too complicated for us to figure out right now.
This field extends out into space, and encases our planet in a protective magnetic envelope. You see, a magnetic field has the power to change the path of charged particles, deflecting them. As solar wind particles strike our magnetic field, they are channeled around the planet and dumped back off into space behind the Earth. This protective layer is called the magnetosphere, and we owe our existence to it.
Now, the Earth's magnetosphere and the solar wind are constantly involved in a complex dance of give-and-take. When the solar wind is particularly strong, the front of the magnetosphere is buffeted by the strength of the wind and compressed down toward the Earth's surface. When the solar wind is mild, the magnetosphere can relax back out into space. On the night side of Earth (opposite the Sun), the solar wind sweeps the magnetosphere out into a long tail, ending (we're not sure where) millions of miles out in space. This long magnetic tail is where scientists think the auroras really begin. A yet-to-be-understood interaction takes place between the magnetic field of the solar wind and our own magnetosphere, accelerating particles back toward the earth along the magnetic field lines.
Since the Earth's magnetic field comes out of the poles of our planet, the solar wind particles are drawn along the field toward the poles. The high-energy particles from the Sun eventually run into the gases of our atmosphere, smacking into molecules of oxygen and nitrogen. It's actually a very similar to what happens inside a neon sign - the molecules of gas gain so much energy from the collision that they release light, producing the telltale glow of an aurora.
The color of the aurora depends both on what sort of molecule is being hit by the solar wind particles, as well as the altitude in our atmosphere where the action is taking place. And auroras are going on constantly. Sometimes the auroras are too dim for the human eye to see, sometimes they glow in invisible colors of light, like ultraviolet. But they're always there. Depending on how active the Sun is, they may be clustered right up over the magnetic pole, or spread clear across the middle latitudes, like a recent aurora that reached all the way down to northern Florida. And the Earth isn't the only planet we've seen auroras on; both Jupiter and Saturn's are bright enough to be seen by the Hubble Space Telescope.
But there's a bigger issue here besides the fascinating and lovely auroras. For years now, scientists have tried to figure how the magnetic phenomena of the Sun (like sunspots, flare, and the solar wind) truly effect the environment of the Earth. At first you wouldn't think that far-away dark spots on the Sun, or lights in the polar skies would have any noticeable effect on our climate or weather. In truth, scientists are beginning to get a little nervous about that.
There is some evidence, still preliminary, that the magnetic behavior of the Sun may have dramatic consequences for the Earth. A famous example in our history is the "mini-ice age" and the "winter without a summer," events which took place during the seventeenth century. Even with the primitive scientific apparatus of the day, scientists noted that the extreme climate changes coincided exactly with the complete disappearance of sunspots. Scientists are still trying to link climate changes on Earth to solar activity. Sometime tantalizing correlations are seen, but other times the opposite seems to be true. The complex magnetic interactions of the Sun and Earth, and the energy they impart to our environment, are still beyond our understanding.
But one thing seems certain: a connection is there. It may end up being insignificant, or it may end up helping up predict dramatic changes in our weather and climate. Regardless, the Sun and Earth are a single unit, to some degree. So, if you have the chance on a cold winter night, go outside and watch the sky as far-flung bits of the Sun dance away in our upper atmosphere. And the Sun-Earth connection can be a personal one too.