The Northern Lights

The Northern lights are poetry, they are nature's light show, and they are

quantum leaps in the oxygen atom. They are elementary particle physics,

superstition, mythology and fairy tales. The northern lights have filled

people with wonder and inspired artists; they have frightened people to

think that the end is at hand. More exact explanations of the phenomenon

could not be given until modern particle physics were developed, and

knowledge about details in the earth's magneto sphere has been based on

measurements from satellites.

When the northern lights are seen over Tromsш, it happens in a set pattern,

although this pattern varies considerably. The outburst starts with a

phosphorocent glow over the horizon in northwest. The glow dies out and

comes back, and then an arch is lit. It drifts up over in the sky. And new

arches are lit and follow the first one. Small waves and curls move along

the arches.

Then within a few minutes a dramatic change is seen in the sky. A hailstorm

of particles hit the upper atmosphere in what is called an aurora sub-storm.

Rays of light shoot down from space, forming draperies, which spread, all

over the sky. And they really remind us of draperies or curtains, which are

flickering in the wind. And you can see a violet and a red trimming at the

lower and upper ends. Or the colors are mixed all together, woven into each

other. The curtains are disappearing and forming all over again by new rays

of light shooting down from space. Above our head we cans see rays going out

in all directions forming what is called an aurora corona. After 10 to 20

minutes the storm is over and the activity decreases. The bands are spread

out, disintegrating in a diffuse light all over the sky. We can not see

individual pockets of light, but the total effect is bright enough to enable

us to make out details of the countryside around us. If we look very

carefully, we can see the remains of the northern lights display as faint,

pulsating flames. Clouds of light which is turned on and off regularly every

5 - 10 seconds as though by an electric light-switch. The natures own

gigantic light show is over.

What causes the northern lights? To answer this, we start with the sun whose

energy production is far from even and fluctuates on an 11-year cycle.

Maximum production coincides with high sunspot activity when processes on

the sun's surface throw particles far out in space. These particles are

called the solar wind and cause the northern lights.

The sun's surface temperature is approximately 6,000o C, much cooler than

the interior, which is several million degrees. In the sun's atmosphere or

corona, the temperature rises again to several million degrees. At such

temperatures, collisions between gas particles can be so violent that atoms

disintegrate into electrons and nuclei. What was once hydrogen becomes a gas

of free electrons and protons called plasma. This plasma escapes from the

sun's corona through a hole in the sun's magnetic field. As they escape,

they are thrown out by the rotation of the sun in an ever-widening spiral -

the so-called garden-hose effect. The name originates from the pattern of

water droplets formed if we swing a garden hose around and around above out

heads.

After 2-5 days' travel trough space, the plasma reaches the earth's magnetic

field compressing it on the daylight side of the earth, and stretches it

into a "tail" on the night side. A few of the particles penetrate down to

the earth along the lines of magnetic field in the polar areas. Most,

however, are forced around the earth by the magnetic field and enter the

"tail" which stretches out into a long cylinder. Its diameter is equivalent

to 30-60 times the earth's radius, an its length up to 1000 times the same

radius. It is, in effect, as if the earth's magnetic field creates a tunnel

in the plasma current from the solar wind. Inside one end is the earth, and

around its surface the earth's magnetism and the solar wind interact.

The magnetic tail is divided into two by a sheet of plasma. The magnetic

field lines from the earth's north and South Pole stretch out in their

respective halves such that the fields are in opposition. The electrons and

protons in each half of the plasma rotate in opposite direction forming a

huge "dynamo" with the positive pole on the side of the plasma sheet facing

dawn and the negative pole facing evening. The current of charged particles

drives