Over The decades there have been auroras in many different forms. The most common forms of the aurora are: a curtain, an arc, rays, a band, a corona, patches or veils.

*One of the forms of the aurora has the appearance of a curtain and is usually greenish white in colour. The lower border is located at a height of 100km, the upper border extends to heights of several hundred kilometers.

*When this form is seen at a certain distance, it appears as an arc rising from the horizon (picture m) and is called an arc, multiple arcs are often seen.

*When the aurora becomes active, the arc develops five pleats that are called rays.

*In a more active form than rays, the arc becomes wavy or folded. An active band with ray structure is called a rayed band.

*When a rayed band is seen from relatively nearby, the rays appear to combine in a small region, resulting  in an apparent fan-shaped form called a corona.

*After an intense auroral display, rayed bands often appear to break up into a number of isolated rays scattered over the whole sky. When the rays are not clearly seen they look like a cloud, this form is referred to as auroral patches.

*Another important form of the aurora is the veil, an extensive glow over a large region of the sky.

Auroras occur up in the earth's atmosphere. Since each atmospheric gas glows with particular colours, we see different colours in the auroras.

High-altitude oxygen (200 miles up) is the source of the rare all-red auroras. Oxygen at lower altitudes (60 miles up) produce a brilliant yellow-green, which is the brightest and most common auroral colour. Since ionized nitrogen produces blue light, and neutral nitrogen produces red, these nitrogens create the purplish-red colours in the auroras. Auroras were first measured by spectroscope in 1867 (picture E.) The colours in the auroras are not a continuous spectrum,
but a few separate colours.

Auroras occur when the solar winds (picture J; the solar winds are made from the particles the sun's surface throws into space;

J) Sunspots and solar flare activity on the sun's surface.

picture I) strengthen and the magnetic tail (picture B)  becomes unstable.

B) i] The earth's magnetic "tail".
ii] During an outbreak of auroras, the cylinder becomes constricted.

Charged particles dive inwards towards the center of the tail and cause it to increase in length and to thin out. The particles draw the magnetic field lines towards the center where they meet causing a magnetic "short circuit" about 15 times the earth's radius above the earth on the night side (picture H.)

Field lines from both sides now act as conductors in the outer circuits. The circuit closes where the particle reaches the ionosphere (picture L)

L)Aurora shown above the earth's atmosphere.

here the thin gasses are composed of ionized particles. This is also where the energy is converted to light  picture c).

The mechanism by which their kinetic energy is converted to visible light is called a quantum  leap (picture D.)

Imagine a hydrogen atom consisting of a single positive proton nucleus around which spins a single electron at a set distance. 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 these quantum leaps occurring simultaneously create the aurora,

Three Norwegian pioneers contributed to the breakthrough in understanding the auroras. Their names were Lars Vergard, Kristian Birkeland and Carl Stxrmer.

Lars Vergard was the first scientist to map the colours of the aurora. He used a spectrograph to record the wavelengths and hence colours of the lights and discovered the main colour (green) to be 558*10E-9.

Kristian Birkeland tried to find a theory explaining the cause of the lights. In 1897 he was able to reconstruct the aurora in his lab by bombarding a metal ball containing an electromagnet (which
represented the earth) with electrons (representing  the solar wind.) The electrical currents in the upper atmosphere are called Birkeland-currents after him.

Carl Stxrmer continued where Birkeland left off with his theoretical calculations. He calculated that there was a belt-like area around the earth in which particles were reflected to and fro between the poles (picture F;

now known as the Van Allen belt after it was measured by James Van Allan.) Stxrmer also calculated  the height of the aurora to be 80-130kms by comparing photos of their position with those of the stars.

 The auroras have no major impact on the way we live. In the past there have been many beliefs about the lights. In the middle-ages Europeans thought the lights were reflections of heavenly warriors. Others thought the lights were an omen.  The auroras have amazed people for centuries and evoked ambitions in scientists to understand how they work and to recreate
nature's light show.