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Atmospheric layersIonized particles, which make up the ionosphere, are produced from the neutral particles, which comprise the neutral atmosphere. The density of the neutral atmosphere, highest near the surface of the Earth, decreases exponentially with increasing altitude. As different physical and chemical processes dominate at different altitude regions due to vastly different neutral density, for convenience scientists separate the neutral atmosphere into different regions: troposphere (~0-15km), stratosphere (~10-50km), mesosphere (~50-85km), and thermosphere (~85km and up).

The total mass of the neutral atmosphere is 1012-1013 times larger than the total mass of the ionosphere, and at all ionospheric altitudes charged particles can be considered a minor constituent in the particle mix. Clearly, the composition and dynamics of the thermosphere (i.e. neutral particles) are imperative for the behavior of ionosphere (i.e charged particles). However, ionospheric processes, in their turn, can significantly effect thermospheric behavior, especially during geomagnetic disturbances. Although behavior of the thermosphere and ionosphere is governed by different physical processes, these processes are tightly coupled and mutually dependent, with a complicated chain of cause-and-effect phenomena.

The state of the thermosphere affects the ionosphere in many ways. The thermosphere absorbs ionizing solar radiation, thus affecting the intensity of the radiation at lower altitudes. It also serves as a supplier of particles which can be ionized. Hence, the density of neutral particles determines the speed of ionization and regulates the recombination processes. The charge exchange during collisions between neutral and charged particles affects diffusion and ion drag processes. Heating on the dayside and cooling on the nightside sets regular thermospheric circulation, which affects ionospheric density when it pushes electrons up or down the magnetic field lines. During periods of geomagnetic disturbances, when large amounts of energy are deposited at high latitudes, regular thermospheric circulation becomes severely disrupted, further affecting ionospheric processes. Untangling the coupling processes is a complicated task, and studies of thermosphere-ionosphere interactions are currently an active research area.



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