Radio atmospheric In the case of the ionospheric D-layer, it depends, in addition, on time of day, season, latitude, and the geomagnetic field in a complicated manner. VLF propagation within the Earth–ionosphere waveguide can be described by ray theory and by wave theory. When distances are less than about 500 km (depending on frequency), then ray theory is appropriate. The ground wave and the first hop (or sky) wave reflected at the ionospheric D layer interfere with each other. At distances greater than about 500 km, sky waves reflected several times at the ionosphere must be added. Therefore, mode theory is here
International Union of Radio Science Geophysical Union) as well as the "Journal of Atmospheric and Solar-Terrestrial Physics". URSI's original objective (to encourage "scientific studies of radiotelegraphy, especially those which require international cooperation") has been broadened to include all radio science, from telecommunications to radio astronomy, acquisition of radar information about distant passive objects, studies of the radiation stimulated or spontaneously emitted by these objects, biological effects of electromagnetic radiation and active modification of objects by radio waves, within the spectrum from extremely low frequency to the optical domain. A few Commissions are engaged with international projects in cooperation with other international bodies, for example with