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 earths 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 current of charged particles
drives the “dynamo” between the two poles.
When the northern lights break out the following happens. The solar wind
strengthens and the magnetic tail becomes unstable. Charged particles dive
inwards towards the center of the tail and cause it to increase in length
and to taper. The particles draw the magnetic field lines toward the center
where they meet causing a magnetic “short-circuit” approximately 15 times
the earth’s radius above the earth on the night side. This occurs especially
at the “dynamo’s” two poles where a large amount of energy becomes stored.
The magnetic field lines from both sides of the plasma layer now act as
conductors in the “dynamo’s” outer circuit
The circuit closes when the particles reach the ionosphere, the outer layer
of the earth’s atmosphere. Here the thin gases are composed of ionized
particles and consequently act as electrical conductors. It is here that the
“dynamo’s” energy is converted to light
Most of the northern lights we see originate in the electrons accelerate
into the ionosphere. The mechanism by which their kinetic energy is
converted to visible light is called the quantum leap. To explain this
mechanism, let us first imagine a hydrogen atom consisting of a single
positive proton nucleus around which spins single electrons at a set
distance. Normally the electron is in an orbit as close to the proton as
possible. In such a state the hydrogen atom a minimum of energy. There are
however other possible orbits further away from the nucleus in which the
electron can spin. When a free electron collides with the hydrogen electron
at high speed, it releases energy. This results in the spinning electron
moving into another, higher energy orbit further out from the nucleus. It
now contains more potential energy, but is unstable and unable to retain
this energy. It returns to its original orbit, releasing the extra energy as
a photon of light. Billions of such quantum leaps occurring simultaneously
create the northern lights. Only a bare minimum of the aurora is a result of
quantum leaps in the hydrogen atom. The green color, which dominates the
northern lights over North Norway, is a result of such leaps in oxygen while
red is usually formed in nitrogen.
Lars Vegard was the first scientist to map the colors of the aurora, and his
work contributed to the building of the Aurora Observatory in tromsø. He
used a spectrograph to record the wavelengths, and hence colors of the
lights and determined the main green color to be 558 x 10E-9 m.
The particles which stream down from the magnetic tail reach the earth in a
belt called the northern lights oval. This belt is wider on the night side
of the earth than on the dayside and is centered around the magnetic pole
while the earth revolves around the geographic poles. This means that the
belt covers Tromsø from early evening until early the next morning. The
width of the belt on the night side is up to 600 km. A common misconception
is that the frequency of the northern lights increases with latitude.
This is not so. In fact, when tromsø is directly under the belt, the chances
of seeing the aurora from Svalbard are less. The magnetic poles are not
stationary such that when the saga was written, the northern lights passed
to the north of Norway. That is why they were only seen on Greenland. The
northern lights outburst, the way it is described in the beginning of this
article, has to do with both how tromsø moves in and out under the northern
lights oval, and with gusts in the solar wind.
In the days of old, the weather forecast was sometimes based on the northern
lights. They were however, often contradictory. In Labrador, colored lights
forecasted fine weather, whereas on Greenland they were a sign of southerly
winds and storms. Even at the turn of last century, one could read in the
Encyclopedia Brittanica that the northern lights and thundery weather were
the result of the same phenomenon, but with different forms of electrical
discharge. In North Norway, the northern lights were often associated with
cold weather.
Between 1645 and 1715, there was little sunspot activity and therefore
little northern lights activity. This period is called the Maunder minimum,
after the leader of the Greenwich Observatory in England who was the first
to document this low activity. Petter Dass, a famous Norwegian priest an
author of the same period, has described much of the North Norwegian way of
life, but never mentions the northern lights. The northern lights oval was
then in such a position that the northern lights should have been visible,
but the sun was less active and the northern lights failed to appear. During
such periods, the climate on earth has generally been colder and the Maunder
minimum coincides with what is now known with as the Scandinavian “little
iceage”. Since then, sunspot activity has increased and reached a maximum in
1991. This was the largest maximum in 300 years with more solar energy
release, greater sunspot activiy and more northern lights. How much today’s
global warming is a result of increased solar activity is difficult to say,
but we do know that when sunspots and northern lights were lacking, the
climate was colder in the north.
Overview
1) The activity on the Sun produces particles that are thrown out into
space. This stream of particles, called the solar wind, consists primarily
of protons and electrons.
2) The particles in the solar wind are captured by the earths magnetic
field. Gathered in the night side of the earths magnetosphere and then
accelerated along the open magnetic field lines down to the Polar Regions.
They hit the earth in a narrow belt called the northern lights oval.
3) Seen from space, the northern lights can hang down like a carpet or like
draperies. Upper edge red, lower violet and blue, and green in the middle.
4) The aurora activity occurs both around the North Pole and the South Pole
at the same time. Around the South Pole it is called the southern lights.
Photographs taken from airplanes verify that the patterns are symmetrical.
5) The first indication of a northern lights display is faint glow low on
the horizon.
6) The faint glow dies out, but after a while an arch of light is lit. It
can stretch all over the sky.
7) Bands of northern lights one above the other, raising towards the zenith
indicates that the sub storm is starting.
8) Rays of light shoot down from space tells about higher activity.
9) Draperies are formed with waves at the lower end.
10) Curls and waves wave along the draperies.
11) The draperies look as if they are flickering in the wind. Maximum
activities close.
12) The interaction between the moving charged particles and the earths
magnetic field creates a charging magnetic field. The particles stream down
along the magnetic field lines.
13) Rays and draperies can die out in one place of the sky, and form at
another.
14) During the maximum of a sub storm, the whole sky can be full of light.
15) An all-sky (fish eye) picture showing that the draperies are stretched
from east to west, through zenith.
16) When the activity reaches Zenith, by an optical illusion, it seems like
the rays stretch out in all directions above our heads. This form of the
northern lights is called aurora corona.