Introductory and Background Material - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 5

of the solar radiation. This effect maximizes at 50 km, where the temperature
trend again reverses at the stratopause. Radiative cooling creates a very sharp
temperature decrease to a minimum in the range 130-190K at about 90 km.
For heights above the altitude of the temperature minimum (the mesopause),
the temperature increases dramatically due to absorption of even higher energy
solar photons to values that are quite variable but are often well above 1000K.
Not surprisingly, this region is termed the thermosphere. The atmosphere is rela-
tively uniform in composition below about 100 km due to a variety of turbulent
mixing phenomena. Above the “turbopause” the constituents begin to separate
according to their various masses.
The temperature increase in the thermosphere is explained by absorption of
UV and EUV radiation from the sun. The EUV radiation is also responsible for
the production of plasma in the sunlit hemisphere, since these solar photons have
sufficient energy to ionize the neutral atmosphere. Equal numbers of positive
ions and electrons are produced in this ionization process. One requirement
for a gas to be termed a plasma is that it very nearly satisfies the requirement
of charge neutrality, which in turn implies that the number density of ions,
n i , must be nearly equal to the number density of electrons, n e . Experimenters
who try to measure n i or n e usually label their results with the corresponding
title, ion, or electron density. In this text, we shall usually refer to the plasma
density n , where n = n i =

n e is tacitly assumed to hold. Of course, if n i exactly
equaled n e everywhere, there would be no electrostatic fields at all, which is not
the case.
A note on units is in order here. Rationalized mks units will be used except in
cases where tradition is too firmly entrenched. For example, measuring number
density per cubic centimeter is so common that we shall often use it rather than
per cubic meter. Likewise, the mho and the mho per meter will be used for con-
ductivity. Some further discussion of units and of various parameters of interest
to ionospheric physics is included in Appendix B.
Returning to Fig. 1.1, two plasma density profiles are given in the right-hand
part of the figure, one typical of daytime midlatitude conditions and one typical
of nighttime. In daytime, the solar spectrum is incident on a neutral atmosphere
that is increasing exponentially in density with decreasing altitude. Since the pho-
tons are absorbed in the process of photoionization, the beam itself decreases in
intensity as it penetrates. The combination of decreasing solar flux, increasing
neutral density, and diffusion provides a simple explanation for the basic large-
scale vertical layer of ionization shown in Fig. 1.1. The peak plasma density
occurs in the so-called F layer and attains values as high as 10 6 cm − 3 near noon-
time. The factor that limits the peak density value is the recombination rate, the
rate at which ions and electrons combine to form a neutral molecule or atom.
This in turn very much depends on the type of ion that exists in the plasma and
its corresponding interaction with the neutral gas.
Some experimental data on the ion and neutral composition above 100 km are
reproduced in Fig. 1.2. Below and near that height, N 2 and O 2 have the same