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 Tromsø, 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 6,000° 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