The ionosphere is part of a planet's upper atmosphere that is ionized by radiation from space and consists of a plasma. Ionospheres are characteristic of high electron density and hence high electrical conductivity. The ionosphere affects the radio waves.
The Gamma Site has an atmosphere that has a unique radioactive ionosphere, thus making it difficult to lock on to targets on the surface from orbit. The ionosphere made it problematic for Odyssey to lock onto the SG-1 and IOA delegations' signs of life on the surface after they were attacked by a large number of R-75. (SG1: "The Scourge")