It all starts on the sun. The sun sends out a
continous stream of positively charged particles from sun spots that
explodes. These particles are sent into space in all directions, some of
it is destined for earth and is named solar wind.
When the solar winds strikes the magnetic field on
the illuminated side of earth it gets deflected around the earth to the
dark side of earth and then towards the poles along the lines of the
magnetic field. The solar winds arrive over the poles with a speed of
over 1 million kilometers per hour (over half a million mph). The
particles collides with molecules in the upper atmosphere and creates a
fantastic light in different colors. This is the phenomenon that we call
the northern lights and southern lights in Antarctica.
The northern lights never appears in altitudes lower than 80 kilometers
and seldomly higher than 500 kilometers. Nothern lights with maximum
intensity is often found in heights of 110-200 kilometers above the
earths surface. The colors depends on the altitude, blue and violet
lights are mostly in altitudes lower than 120 kilometers. Most often,
the northern lights will be green, which means it operates in altitudes
of 120-180 kilometers. Above this, we see the red and yellow northern
is the international name
for the northern lights.
The Northern Lights Oval
The center of the the northern lights oval in the northern hemisphere is
at 65 degrees north and is around 10 degrees wide, but will expand
with high northern lights activity. Tromsø is in the middle of
the oval. The northern lights oval extends through huge areas, but
a lot of these areas are sparsely populated or unpopulated and not
accessible for most people.
With high northern lights activity and strong solar winds the
northern lights oval expands and the lights will be visible
further south. With strong solar winds the northern lights can be
visible over the central and southern Europe and also parts of the
Geomagnetical solar storms (solar winds) are measured by a
KP-index from 0 to 9.
KP-index numbers of 0 to 3 is little activity, numbers of 5 and
higher are classified as solar storms. In Tromsø, we can see the
lights even if the KP-index shows 0.
Aurora: Inside the Sun to Earth's Poles