According to our definition, a planetary ionosphere represents the thermal plasma (electrons and ions) produced by the interaction of ionizing radiations with the neutral atmosphere, which is controlled by the force field (gravity, magnetic field) of the planet. Thus, a planetary ionosphere may be thought to extend as far as the actions of these force fields allow the plasma to be confined in the planet’s vicinity. Further, various processes exist where plasma can be lost from a planetary atmosphere of a non- or weakly magnetized planetary body or from a planet whose magnetosphere is highly compressed due to a strong solar/stellar wind.