The ionosphere is the ionized component of the Earth's upper atmosphere. Two different ionization processes are involved in its creation: photoionization, principally by solar extreme ultraviolet (EUV) and x-ray photons, and impact ionization by charged particles (including solar and galactic cosmic rays). During the daytime and at subauroral latitudes, photoionization is the dominant process, while at high latitudes and at night impact ionization by precipitating auroral electrons plays an important role in the production of ionospheric plasma. The low-altitude ionosphere occupies approximately the same altitude range as the neutral mesosphere and thermosphere and, between 60 and 800 km, is vertically structured in three layers or regions that differ from one another in composition, density, ionization sources, degree of variablity, chemistry, and dynamics--the D (60-90 km), E (90- 150 km) and F (150-800 km) regions. The dominant ions in the D and E regions are NO+ and O2+; in the F region, where the bulk of the ionospheric plasma resides, O+ dominates. Above the F region is a region of exponentially decreasing density known as the "topside ionosphere." This region extends to an altitude of a few thousand kilometers and, at mid-latitudes, feeds into the plasmasphere, the region of cold, dense ionospheric plasma in the inner magnetosphere that is controlled by the Earth's co-rotating electric field.

How the ionosphere varies

Plasma densities in the ionosphere are characterized by strong day-night variability. The maximum ionospheric plasma density (approximately one million electrons per cubic centimeter) occurs in the noon F region, at an altitude of 250-300 km. At night, ionospheric densities can drop by as much as two orders of magnitude, depending upon the region and altitude. The largest and most rapid decay occurs in the E and D regions, whose molecular ion constituents recombine with the ionospheric electrons much more rapidly than the F-region O+ does. Ionospheric plasma densities also vary markedly with season, solar cycle, and level of geomagnetic activity.

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