the ionosphere  is a region of the upper atmosphere, from about  to  altitude, and includes the thermosphere and parts of the mesosphere and exosphere. it is distinguished because it is ionized by solar radiation. it plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. it has practical importance because, among other functions, it influences radio propagation to distant places on the earth.k. rawer. wave propagation in the ionosphere. kluwer acad.publ., dordrecht 1993. isbn 0-7923-0775-5 thumb|400px|relationship of the atmosphere and ionosphere ==geophysics== the ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the earth, stretching from a height of about  to more than . it owes its existence primarily to ultraviolet radiation from the sun. the lowest part of the earth's atmosphere, the troposphere extends from the surface to about . above  is the stratosphere, followed by the mesosphere. in the stratosphere incoming solar radiation creates the ozone layer. at heights of above , in the thermosphere, the atmosphere is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. the number of these free electrons is sufficient to affect radio propagation. this portion of the atmosphere is ionized and contains a plasma which is referred to as the ionosphere. in a plasma, the negative free electrons and the positive ions are attracted to each other by the electrostatic force, but they are too energetic to stay fixed together in an electrically neutral molecule. ultraviolet (uv), x-ray and shorter wavelengths of solar radiation are ionizing, since photons at these frequencies contain sufficient energy to dislodge an electron from a neutral gas atom or molecule upon absorption. in this process the light electron obtains a high velocity so that the temperature of the created electronic gas is much higher (of the order of thousand k) than the one of ions and neutrals. the reverse process to ionization is recombination, in which a free electron is "captured" by a positive ion. recombination occurs spontaneously, and causes the emission of a photon carrying away the energy produced upon recombination. as gas density increases at lower altitudes, the recombination process prevails, since the gas molecules and ions are closer together. the balance between these two processes determines the quantity of ionization present. i
Ionosphere Radio_frequency_propagation Radio_terminology