The Scientific Explanation about what causes the Aurora Borealis
After the magnificent display of the northern lights, seen across much of Europe on 6th March, 1716, the seats of learning in the Old World began to turn their attention to its causes. During the 19th century it was discovered that the northern lights were an atmospheric phenomenon that was connected with the sun and partially influenced by the earths magnetic field.
The earth is continuously immersed in a stream of charged particles flowing outward from sun. This stream is called the solar wind. It was soon realised that this solar wind plays an important role in aurorae, by supplying the necessary electrons and by perturbing the magnetosphere so that the particles trapped in the radiation belts are dumped into the atmosphere near the mirror points.
Our planet's magnetic field usually does a good
job protecting us from solar wind storms. Magnetic lines of force deflect
charged particles from the Sun so that they don't hit our atmosphere head
on. However the magnetosphere (an area of space controlled by Earth's
magnetic field) is not invincible.
If the solar wind is strong enough or if the magnetic field inside the wind
partially cancels the magnetic field of the Earth, some plasma (ionized gas) can
get through and reacts with the atmosphere.
Every 11 years or so, the Sun boils over with violent, dark storms-sunspots and solar flares-that spray Earth with shotgun blasts of subatomic pellets. The solar invaders are not electrically neutral atoms but broken fragments bristling with charge. They inflame our planet's protective magnetic field.
When charged particles are thrown across a magnetic field in this manner large amounts of electricity are generated. As a result the presence of, the Sun's detritus in the upper atmosphere produces a hundred trillion watts. That is an incredible million amps.
Within this charged layer the trapped particle move in tight cylindrical helices along magnetic lines of force. The electron move from west to east and the protons move from east to west.
Some of the imprisoned ions eventually work their way out of the magnetic trap. The weakest portion of it occurs along the night side of the planet at the place where the Van Allens Belts come nearest to the earth. The particles that escape in this way are dumped into the ionosphere where they interact to form complete atoms and liberate large amounts of energy.
Polewards the weakest portions of the Van Allens Belts is a region where the magnetic field of the earth does not trap or deflect the particles of the solar wind at all.
They spiral directly towards the poles along the line of magnetic force (see dig.) until they strike the ionosphere. Here they also interact energetically with the thin gases. These invasions of the ionosphere raise its temperature to very high levels. At altitudes of 63 miles temperatures of 870 oC have been measured over the equator, and 1427oC over the North Pole. The energy pouring into the ionosphere also generates brilliant displays of aurora borealis of aurora australis, the 'northern and southern dawn'.
Auroral light is thus similar to light from colour television. In the picture tube, a beam of electrons controlled by electric and magnetic fields strikes the screen, making it glow in different colours, according to the type of chemicals (phosphors) that coat the screen. Auroral light is the light from the air glowing as charged particles, particularly electrons, rain down along the Earth's magnetic field lines. The colour of the aurora depends on the type of atom or molecule struck