Dust From a Distant Sun

Aurora Borealis by Heather HinamAutumn has flown by, marked by brilliant leaves and skies filled with birds winging their way to warmer climes. The bustle of the season swept me up with back to school (I haven’t taught a fall course in over 7 years) and my regular work as a naturalist/guide/illustrator, leaving this blog sitting on the shelf for a while.

However, now, as the nights turn truly cold and the days become darker, I finally have a chance to settle and get back to sharing those things that fascinate me the most. I thank you for sticking with me.

The colder temperatures remind me of the many reasons I love living in the more northerly reaches of the planet. Not the least of those is the chance we get, now and then, to witness one of the most amazing natural phenomena on earth: the auroras. Here, in the northern hemisphere, they are the aurora borealis or northern lights. They’re not actually more common in the colder months; but many tend to associate them with winter, probably because the longer nights give us more opportunity to see them.  The picture above was actually taken in August.

For people who have never seen them, aurora are kind of hard to describe. They appear with no warning, beginning usually with a barely noticeable glow just above the horizon. You stare, transfixed, wondering if you’re seeing things. Suddenly, the silent flames grow, licking out across the sky, a rippling curtain of light that is ceaseless in its movements. The shifting colours hold you in their thrall until, just as quickly as they had appeared, the lights dissolve into the ether, leaving you feeling a little bereft for their loss.

Just what are these silent, shimmering waves of light? Though they are best seen on the darkest of nights, aurora are a product of the sun. Being a giant ball of hot plasma (ionized gas particles), the sun is a tempestuous place to be. Protons and electrons are being flung about the atmosphere, creating ‘solar winds’, which are streams of plasma that escape the star’s gravity and sail across the universe at truly mind-boggling speeds of millions of kilometres per hour. On occasion, fountains of particles will spew out of the sun’s atmosphere in a coronal mass ejection, sending a wave of protons and electrons on a collision course for earth.

When they reach our magnetic field, most are deflected, riding the field lines to the poles, where they start to swirl around, like atomic tornadoes, in the ionosphere (the height at which the International Space Station orbits). Whirling faster and faster, the ions become unstable, colliding with nearby gas atoms, releasing so much energy that they glow. The colour of the light depends on the gas they interact with and how far above the earth they are. The green and yellow we are most familiar with is created by an interaction with oxygen, while blue and violet are caused by nitrogen.

So, what you’re seeing is millions of chemical reactions playing out several hundred kilometres above the earth. The unearthly flame is concentrated in a halo around each pole, an auroral ring that shifts ever so slowly with the movement of our magnetic poles.

For the layperson, the appearance of these ghostly fire dances are impossible to predict. However, scientists in Canada have spent over a hundred years studying the phenomenon and have teased out some trends. Some years are better than others. It turns out that solar activity (solar flares, mass ejections and other radiation) goes through a relatively predictable 11 year cycle that should be hitting its peak sometime over the next few months.  Besides being a treat for aurora watchers, this intensified light show will be invaluable for researchers looking for ways to protect our satellite and communications networks from this increased radiation. While they may be beautiful, the ions spiralling through space can, and have, wreaked havoc on our electrical grids.

This year’s maximum has turned out to be the weakest in over a century, but there are still lights to be seen.  So, look up, look waaay up and hopefully you will have the chance to experience a true natural wonder.

P.S. to find out when and where your best chances for aurora spotting are, visit: www.gi.alaska.edu/AuroraForecast

Double Rainbow All the Way

Double rainbow over Lake WinnipegThere’s just something about rainbows. They’ve been immortalized in endless songs, myths, stories, movies and even cellphone commercials based on the painfully hilarious mushroom-induced exaltations of an overly-enthusiastic youtube star.

Rainbows just capture the imagination. For the vikings of old, they were the Bifrost Bridge between Asgard, the home of the gods and our world of Midgard. In ancient Rome, they were the path of a messenger between Earth and the heavens and of course we all know they’re where leprechauns store their pots of gold.

But what are they really? Why can you never find the rainbow’s end? I think Kermit the Frog had it right when he labelled them ‘only illusions’.

Rainbows are the product of an observer standing in the right place at just the right time. What is that right place? It’s about 42 degrees from the direction opposite the sun. You can never get to the end of the rainbow because it will keep moving with you as you walk towards it. Stray off the bearing and the image will vanish into the mist.

What you’re seeing is sunlight being refracted, dispersed and reflected back at you through millions of water droplets suspended in the air. You usually only get enough water hanging around after a storm has passed, hence the name ‘rainbow’. Of course, if you’re standing next to a waterfall, fountain or someone’s sprinkler, you can often get the same effect if you’re in that magical optical sweet spot.

I’ll try and keep the physics simple, but here’s how it works. The white sunlight enters the water droplet and is dispersed into the full spectrum of colours. Then, it’s reflected off the back of the raindrop, just like the inside of a camera. On the way back out of the drop, each wavelength is refracted (their direction of travel is changed) as it passes from the water back out into the air.  How much each wave is refracted depends on the wavelength (colour) of the light. Red light (short wavelengths) are refracted less than blue (long wavelengths). The result is what was once a beam of ‘white’ light is now spread out into an arc of continuous colour. Double rainbows appear when the light is reflected off the back of the droplet lens twice. This second fan of light comes out at a slightly different angle and the spectrum of colour is inverted.

To us, the viewer, we see that colour in bands of red, orange, yellow, green, blue, indigo and violet because of the way photopigments in our eyes receive the light that is then interpreted by our brains. Take a black and white photo of a rainbow and you won’t see any bands, just a continuous gradation in intensity.  Animals whose brains can interpret wavelengths we can’t, like ultraviolet or infrared, would see a completely different rainbow than we do.

I think that’s what I find so fascinating about rainbows. Their beauty is truly in the eye of the beholder.