Medium frequency travel longer distances via skywave propagation, in which radio waves radiated at an angle into the sky are reflected (actually refracted) back to Earth by layers of charged particles (ions) in the ionosphere, the E and F layers. However at certain times the D layer (at a lower altitude than the refractive E and F layers) can be electronically noisy and absorb MF radio waves, interfering with skywave propagation. This happens when the ionosphere is heavily ionised, such as during the day, in summer and especially at times of high solar activity, At night, especially in winter months and at
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