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The Sun's electromagnetic radiation is a continuum that spans radio wavelengths through the infrared, visible, ultraviolet, x-ray, and beyond. Ultraviolet radiation, through a process termed photo ionization, interacts with upper atmospheric constituents to form an ionized layer called the ionosphere. The paper describes how the ionosphere affects radio signals in different ways, depending on their frequencies, which range from extremely low (ELF) to extremely high (EHF). On frequencies below about 30 MHz, the ionosphere may act as an efficient reflector, allowing radio communication to distances of many thousands of kilometers. Radio signals on frequencies above 30 MHz usually penetrate the ionosphere and, therefore, are useful for ground-to-space communications. The existence of the ionosphere allows the use of high frequency (HF) radio in, for example, ground-to-air or ship-to-shore communication over long distances. The effect of different types of solar activity on the ionosphere and consequently on communications is described.