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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. |