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1000
N
1000
e
O
He
He
500
500
N 2
H
O
300
200
300
200
250
NO
250
O 2
O 2
N
He
N 2
O
150
100
N 2
150
100
Ar
e
NO
O 2
O
10 2
10 3
10 4
10 5
10 6
10 7
10 8
10 9
10 10
10 11
10 12
Number (cm 3 )
Figure 1.2 International Quiet Solar Year (IQSY) daytime atmospheric composition,
based on mass spectrometer measurements above White Sands, New Mexico
32 N,
(
106 W
. The helium distribution is from a nighttime measurement. Distributions above
250 km are from the Elektron 11 satellite results of Istomin (1966) and Explorer XVII
results of Reber and Nicolet (1965). [C. Y. Johnson, U.S. Naval Research Laboratory,
Washington, D.C. Reprinted from Johnson (1969) by permission of the MIT Press,
Cambridge, Massachusetts. Copyright 1969 by MIT.]
)
:
ratio as in the lower atmospheric regions—about 4
1—and dominate the gas.
Near 120 km the amount of atomic oxygen reaches that of O 2 , and above about
250 km the atomic oxygen density also exceeds that of N 2 . This trend is due to the
photodissociation of O 2 by solar UV radiation coupled with molecular diffusion
and the absence of turbulent mixing above the turbopause. The dominance of
atomic oxygen in the neutrals is mirrored by the plasma composition. The curve
labeled e is similar to the right-hand side of Fig. 1.1 and represents the electron
density (thus labeled with e )
. Near the peak in the plasma density, the ions
are nearly all O + , corresponding to the high concentration of atomic oxygen in
the neutral gas. The altitude range 150-500 km is termed the F region, and the
maximum density there is termed the F peak. (The F region is often separated
into F1 and F2 during daytime due to the role of molecular ions.) Below the
peak, NO + and O 2 become more important, dominating the plasma below
about 150 km. The altitude range 90-150 km is called the E region, and the
ionization below 90 km is, not surprisingly, termed the D region. These rather
pedantic names have a curious history. The E region received its name from the
electric field in the radio wave reflected by the “Heavyside” layer (the first name
for the ionosphere). The other layers were simply alphabetical extensions. It was
assumed initially that a plasma is absent between the layers. Unfortunately, many
phenomena have been named for the instrument used to measure them or some
other obscure parameter.
At the highest altitudes shown in Fig. 1.2, hydrogen becomes the dominant ion
in a height regime referred to as the protonosphere. Helium ions are quite variable
but sometimes reach 50% of the total ions at the base of the protonosphere. The
 
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