Ionosphere

The ionosphere is a region of the upper atmosphere containing ions and free electrons. Its changing electrical properties influence radio waves and arise mainly from solar radiation.

Photoionization: The production of ions when atoms or molecules absorb photons and lose electrons. Solar ultraviolet and X-ray photons create much of the ionosphere’s free-electron population.
Skywave: Radio propagation in which waves refract in the ionosphere and return toward Earth beyond the horizon. This propagation mode lets high-frequency signals cover distances beyond line of sight.
Solar ultraviolet radiation: Ultraviolet electromagnetic radiation emitted by the Sun across a range of wavelengths. Its absorption in the upper atmosphere supplies much of the energy that ionizes atmospheric gases.
Neutral atmosphere: The atmospheric component made primarily of electrically neutral atoms and molecules. Below and within the ionosphere, neutral air dominates mass even where a small fraction is ionized.
F region: The highest major ionospheric region, where electron density commonly peaks and long-distance radio waves can refract. Its dense daytime and nighttime layers often enable high-frequency skywave communication.
High-frequency communication: Radio communication using frequencies from 3 to 30 megahertz, often over long distances by skywave. Operators exploit ionospheric refraction to communicate across continents without satellites.
Solar X-rays: High-energy electromagnetic radiation emitted by the Sun, including bursts associated with solar flares. Intense solar X-rays can rapidly increase ionization in the lower ionosphere.
Magnetosphere: The region around Earth in which the planet’s magnetic field controls the motion of charged particles. It is a larger space environment coupled to the ionosphere, not an atmospheric layer itself.
D region: The lowest major ionospheric region, which becomes strongly ionized during daylight and absorbs radio waves. Daytime absorption here can weaken high-frequency signals before they reach higher layers.
Maximum usable frequency: The highest radio frequency expected to support a specified ionospheric path between two locations. Its value varies with ionospheric conditions and guides frequency selection for skywave links.

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