From AMS Glossary
In reference to conductivity near the ground, the primary source of ions in the lowest 1 or 2 km of the atmosphere above land. (Cosmic radiation is dominant over the oceans and at higher altitudes.)
The ionizing radiation can take the form of 1) emission of α, β, or γ radiation by radioactive materials in the soil; or 2) emission from gaseous radioactive daughter products (radon) that emanate from the soil. The α radiation emitted by materials in the ground is absorbed in the lowest few centimeters of the atmosphere, β radiation penetrates a few meters, and γ radiation a few hundred meters. The ionization produced by radioactive gases in the atmosphere is highly variable and depends on the rate of emission from the soil and also on atmospheric dispersion. 222Rn, which has a half-life of 3.8 days, is produced by the uranium decay series. 220Rn (thoron) is produced by the thorium decay series and has a half-life of 54 seconds.