why a atmospher is electrically neutral
Answers
An important part of the reason for electric fields in the atmosphere is the ionosphere. It is a part of Earth’s atmosphere that is electrically conductive. The reason it is conductive is that ultraviolet radiation from the Sun ionizes the gas at that altitude, i.e. separates some electrons from the atoms in which they normally reside, leaving a free electron, with negative charge, and an ion, i.e. an atom with net charge, in this case positive.
The dashed line in the upper right shows the density of free electrons, which is a measure of the electrical conductivity of the atmosphere.
Below the ionosphere, the atmosphere is electrically neutral = uncharged, but it has a fairly strong electric field, around 100 volts per metre. This field does not cause a 180-volt shock in a person 1.8 metres tall, because the atmosphere is an insulator. There are two reasons why the atmosphere is not conductive below the ionosphere:
Only highly energetic, i.e. short-wavelength, ultraviolet light has enough energy in each photon to ionize air molecules. That light is absorbed in doing so, and therefore extremely little of that light reaches the ground. (This is important for terrestrial life, because that light would damage most organic material, such as people.)
The free electrons and the positively ionized atoms diffuse randomly until an electron and a positive ion come close together. Then, they recombine. At lower altitudes, such recombination will occur rapidly because of the relatively high density of the atmosphere.
Most of the surface of Earth is a moderately-to-very good conductor. Salt water is an excellent conductor because of the dissolved ions (mostly sodium+ and chlorine- of dissolved common salt), and the soil is mostly a fairly good conductor because of contained water with assorted ions dissolved from the soil.
Consequently, Earth is a capacitor, with the surface as one plate and the ionosphere as the other. That means that it can store an electric charge between those two conductors. Its capacitance is a rather modest 710 microfarads.
The above-stated electric field occurs because Earth indeed holds a charge. The ionosphere has an electrostatic potential of around +250,000 volts above the ground potential. That means that there is a deficiency of electrons versus positive ions in the ionosphere, and an excess of electrons in the ground. The stored energy works out to 22 million joules.
The reason this charge exists, and is maintained, is that thunderstorms carry excess electrons to the ground, basically dissolved in raindrops. For a more detailed explanation of why they do that, see Ron Davis’s answer to Where does the energy in lightning come from? How is it converted to electrical energy?.
The tops of thunderclouds thus become positively charged. This positive charge is carried up to the ionosphere in electrical discharges called sprites, that occur at high altitudes above thunderclouds. Sprites are elusive, but may be visible to observers on the ground as brief flashes of light on a clear night, above thunderstorms that are otherwise out of sight beyond the horizon.
Sprites differ from lightning in that their temperatures are close to that of the surrounding air. They are similar to the glow in a neon light, which occurs at relatively low temperatures and energies because the gas is at very low density. Thus, even in the modest electric field that occurs in the atmosphere, a free electron can be accelerated to high enough energy to cause ionization before it collides with a gas molecule.