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Ionosphere refers to the layer of the Earth's atmosphere that has a significant number of ionized atoms and electrons, due largely to photoionization by ionizing radiation (ultraviolet and X-rays) from the Sun. Given the defining characteristics of the ionosphere, the thermosphere, and the exosphere, they overlap. The altitudes of various sublayers of the ionosphere vary over time, due to atmospheric conditions as well as when and how much ionizing radiation Earth is receiving (it arrives during the day, and the resulting ions dwindle a bit through the course of the night). Additionally, when space weather from solar activity constitutes extra solar particles reaching Earth atmosphere, they also create (and contribute) ions: when these arrive at night, the resulting ionization can be visible to us, i.e., auroras. The ionosphere reflects some frequencies of radio, contributing to Earth atmosphere's pattern of blocked and non-blocked EMR frequency-ranges (atmospheric windows). Such reflection is used for communication, allowing reception of communication radio waves sent from beyond the horizon. The amount and type of reflection are affected by the ionosphere's variation.
The term ionosphere is also used for such layers of ions on other planets and moons, e.g., Titan.