Page 101 - Intro to Space Sciences Spacecraft Applications

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Introduction to Space Sciences and Spacecraft Applications
the atmosphere at high altitudes. This process is called photoionization and
results in an electrically charged (ionized) region known as the ionosphere.
This region begins somewhere between 50 and 70 km altitude and extends
far into space. The ionosphere is characterized by the changes in the free
electron concentration levels with altitude as shown in Figure 4-5.
Figure 4-5 also shows the variability of the ionosphere. Solid lines indi-
cate levels during times of high solar activity and dashed lines are associ-
ated with low solar activity levels. Notice also the considerable difference
between day and night characteristics. This is due to the cessation of the
photoionization process when out of direct exposure to the sun. During
these times the electron concentration is reduced due to the recombination
of ions and electrons. This process proceeds slowly enough to allow a
concentration of free ions and electrons to persist throughout the night.
Some easily identifiable regions within the ionosphere can be seen in Fig-
ure 4-5 associated with changes in the electron concentration with altitude.
These regions (layers) are identified with letters (D, E, Fl, F2, or just F).
This layering of the ionosphere is associated with the changes in atmos-
pheric constituents with altitude which have differing ionization properties.
We will return to our discussion of the ionosphere later when we discuss
communications, remote sensing, and navigation satellites.

HIGH
ACTIVITY
800
800
400

W
n 200
2
!i 150
a
100
80
60 I 1 I
IO lo2 lo3 io4 io5 lo6

ELECTRON CONCENTRATION (cm3)
Figure 4-5. The ionosphere. Electron concentration in the ionosphere
changes with altitude, time of day, and solar activity.