Ions in the atmosphere
Ionization by the process described above is only one way of producing gaseous ions. Another method is to allow high-energy radiation to pass into the gas. Various experiments have shown that the atmosphere contains ions which have been produced by radiation from radioactive minerals in the earth’s crust, by ultraviolet light from the sun and by the so-called cosmic radiation which enters the earth’s atmosphere from outer space.
The presence of these ions in the atmosphere explains why a charged conductor will slowly lose its charge over a period of time even when mounted on a good insulator. The charge on the conductor is gradually neutralized by ions of opposite sign which are attracted towards it out of the surrounding air.