Northern Dawn: The Aurora Borealis

© 1999 by Robert C. Moler

Astronomy provides many wondrous sights. There's the totally eclipsed sun whose time and location are known with high accuracy. Only the weather is an unknown. The same is true of a lunar eclipse, though the location from which it can be viewed is the entire night side of the earth. Returning comets can be predicted, but the most spectacular, like Hale-Bopp appear at random, though usually without the great advance warning we had with it.

One event that is absolutely capricious in its appearance is the Aurora Borealis, better known as the Northern Lights. It gets its name from Aurora, the Roman goddess of dawn, hence my title.

A few days before writing this article I spotted a perfect example of that appellation when I went out at quarter to 6 in the morning for the newspaper. Twilight was just beginning to appear in the east, and another dawn was peeking over some distant clouds in the northwest. The was the low glowing band that is the signature of most low grade auroras. The next morning I was greeted with an active display of northern nights as waves of light rippled upwards. It still wasn't very bright or extensive. The good news is that there will be more of these displays over the next few years.

The photos with this article were from an autumn 1997 display which I witnessed, thanks to a phone call from Bill Hathaway who spotted it first it from Suttons Bay. Thanks again Bill. The display that night had at least two peaks that filled the sky with light.

The cause of these displays was a mystery for a long time. Some folks still believe they are the result of sunlight reflecting off the polar ice cap. I can think of at least one reason why that can't happen. Can you?

Aurora Appearance OverheadThe auroral displays are caused by the interaction of the sun and earth, specifically the solar wind of charges particles and the earth's magnetic field. The intensity of the solar wind is highly variable in space as well as time. Huge explosions in the sun's photosphere near sunspots called solar flares spew out charged particles, mostly electrons and protons, that enhance the solar wind. It takes about two days for the solar particles to reach the earth's distance. Then the earth has to be in the path of these particles for anything to happen.

Actually the solar wind is always blowing and continually interacts with the earth's magnetic field. That interaction creates a comet shaped magnetic bottle around the earth called the magnetosphere, The solar wind flows around the magnetosphere, whose interface with it is called the magnetopause. The flow of charged particles past a magnet like the earth generates currents inside the magnetosphere. The normal manifestation of that is the Van Allen radiation belts discovered by the Unites States first artificial satellite Explorer I. In these belts protons and electrons circulate back and forth from the north to south magnetic pole. The belt dips lowest to the earth near the magnetic poles.

When a great enhancement in the solar wind occurs it generates great currents in the Van Allen radiation belts and they begin to leak earthward near the north and south magnetic poles. The generated current flows through the rarefied atmosphere up to 600 miles up causing the gasses to glow. This is exactly the same thing as sending electricity through a neon tube. The light of an aurora is not constrained by the walls of a tube and glow in striking patterns all over the sky. There are straight rays, undulating curtains that seem blown by an invisible wind. White, yellow and on rare occasions red can be seen. In the black and white image above the background glow on the right is pure red.

Remember to keep checking the night clear skies for these fantastic displays.

 

Recent Aurorae


Questions? Comments? Send Email to me at bob@bjmoler.org

Updated: 08/01/02