Atmospheric electricity

          Atmospheric Electricity:

The earth's surface has a surface charge density of about 1nCm^-2.There is a corresponding electric field of about 100V/m in the atmosphere above the earth's surface (in the downward direction).So,the electric potential increases by about 100V as one move up by 1m.But the magnitude of the electric field gradually decreases as one move up.

At about 50km above the earth's surface,the field is negligible.Thus, the potential difference between the earth's surface and the top of the atmosphere is about 400kV. 

The atmosphere contains a large number of positive and negative ions. Their main sources are cosmic rays which come from the solar system and the interstellar bodies like stars and galaxies. The cosmic rays contains lots of protons,neutrons, electrons, neutrino and other particles and radiation as well.

Dust particles also become charged due to friction when they move through the air.

Because of the electric field in the atmosphere, postive ions come down and negative ions go up in the atmosphere.Thus,there is an electric current in the atmosphere having magnitude of about 3.5×10^-12 A/m^2.

So, the total electric current over the entire surface of the earth is about 1800 A.

At about 50km above the earth's surface, the conductivity of the air is very high due to increased density of ions.If a current of 1800A flows towards the earth through the atmosphere, all the negative charge of the earth should get neutralized in about half an hour and so the electric field in the atmosphere should reduce to zero.But actually this never becomes zero.The two natural phenomena; thunderstorm and lightning prevent it to become zero and so a potential difference of 400kV is always maintained.

At a height of about 3-7 km,there is a very low temperature of about -10°C to -20°C. Due to this lower temperature,water gets condensed to form a number of thunderstorms.Its upper part contains excess of positive charge and its lower part contains excess of negative charge.These negative charge of the cloud create a potential difference of about 20 to 100 M Volts between these clouds and the earth. 

The high electric field between the lower part of the cloud and the earth is often sufficient for the dielectric breakdown of the air and so the air becomes conducting.

Negative charges thus jump from the cloud to the earth's surface.This phenomenon is called"lightning". The postive charge in the upper part of the cloud (storm) gradually moves up and enter the high altitude (≈50km) where there is highly conductivity.

In one lightning stroke, about 20C of negative charge is deposited on the earth.

It takes about 5 seconds for the clouds to regain the charge for the next lightning stroke. There are a number of thunderstorms occuring everyday throughout the globe.They constantly supply negative charge to the earth and postive charge to the upper part of the atmosphere.In this way,the charge, electric field and the potential difference in the atmosphere are maintained!

Author: Scientificirfan