Thursday, January 13, 2011

Green Ghosts - The Aurora from Tromsø, Norway

      It's 11 p.m., and Marit's son Hronn bursts into the living room where we are all sitting around talking and drinking, with "have you decided not to watch the aurora tonight?"
      Within seconds everyone is in the entry hall throwing on jackets, slipping stocking feet into boots that are only partly laced and tied off so that they can be put on with firefighter speed. We hurry out into the dry, cold night air, fumbling with gloves, stocking caps, and hoods. The thermometer reads -11 degrees, but that's Celcius. The "real" temperature is 12 degrees Fahrenheit, and you can feel the outer edges of your nostrils getting crinkly.
      The sky is a very dark blue-black except over the island of Tromsø where a few low-lying clouds are reflecting back the orange glow of the lights of the town. Extending from behind the sharp ridge of Storstein that rises sharply behind the last row of houses and across the entire span of sky, is a ghostly, faintly glowing greenish band that looks at first like the disappearing remnants of a contrail left in the wake of a long-gone jet plane. Perhaps it is a high, thin, ice crystal cirrus cloud that marks the outer edge of an approaching low pressure system, glowing the light of the nearby first quarter moon, but as we lift our chins high the cloud begins to glow a bit brighter along the middle. It seems to be gathering itself inward, a gradual metamorphosis into a long, sinuous, ropey looking worm of a shape that begins to writhe, leisurely developing bends as if it were trying to slither across the sky.
      Farther north, up the sound, another greenish cloud fades into visibility, this one like green grassy filaments spread out and stretched off into the distance in the southwest, more or less parallel to the now slowly wavering line.
      We keep looking back and forth across the bowl of the sky, for the horizon to horizon display is too wide to take in all at once. The green ribbon brightens some more over the town of Tromsø while its other end fades almost to invisibility over the mountain ridge. Vertical streaks gradually appear beneath it until it looks like a diaphanous curtain trailing down from the sky, and it begins to move, visibly rippling along its length as if it were being moved by an ethereal breeze.
      In truth, this is exactly what is happening. The Sun continuously ejects tons of ripped apart atoms, protons and electrons away from its surface at more than a million miles per hour. At that rate it takes this electrically charged plasma about three and a half days to get to the Earth. Encountering the powerful magnetic field surrounding our planet, most of it is bent right around the Earth and passes harmlessly on into interplanetary space. Some of those electrically charged particles, caught in the Earth's magnetic field, move rapidly toward the north or south magnetic poles. Colliding with the tenuous upper atmosphere, they make the air molecules glow, somewhat similar to the process inside a fluorescent light.
      Proximity to the Sun has no particular effect on whether or not the solar wind generates aurora. The Earth reached perihelion, only about 91,400,000 miles from the Sun just a few days ago. The main factor is solar activity. When the Sun belches out large amounts of ionized gas or there is an ejection of plasma from the corona of the Sun toward the Earth, it results in spectacular auroral displays about three days later.
      Now the vertical streaks, oscillating gently in the solar wind begin to take on subtle pastel shades of yellow and red, scintillating just a bit along their lower edges. Darker vertical sections travel along the softly glowing wall of aurora, looking vaguely like the shadows of people moving behind a back-lit curtain. It doesn't take a vivid imagination to understand why some Inuit people believed that they could see the spirits of their ancestors moving just behind the northern lights.
      A few minutes later the curtains of light fade about the same time that the long green tendril of light across the sky starts to brighten. As we watch, it begins to move sinuously, writhing itself into bends, then whorls, in places fading then brightening again, the motion of the curves somehow snake-like.
      So entranced and excited we can barely breathe except for guttural gasps and utterances, we stand in the middle of the street, boots squeaking and crunching in the snow as we turn this way and that, trying to take it all in.
      The magic light show begins to fade after about twenty minutes, although we are told that sometimes it continues for hours on end. As we troop back into the house for warm drinks we know that we have seen one of the most amazing wonders of the world.

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