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150
1009:52.6 UT
100
50
0
(a)
150
1010:12.6 UT
100
50
0
(b)
150
1010:32.6 UT
100
50
0
0
0.5
Electron concentration (10
6
el cm
23
)
1.0
1.5
(c)
Figure 1.5
Sequence of E-region electron concentration profiles obtained using the
Chatanika, Alaska, radar on September 27, 1971, near 1010 UT; azimuth
04
◦
,
=
209
.
58
◦
. [After Baron (1974). Reproduced with permission of the American
Geophysical Union.]
elevation
=
76
.
1.3 D-Region Fundamentals
The D region of the ionosphere was once thought to be of little plasma physics
interest. The level of ionization is so low that the plasma physics is quite different
from that of weakly collisional or collisionless plasmas, which captured the most
attention of theorists. Furthermore, the region is too high for balloons and too
low for satellites. Rockets often pass through it without even removing nosecones
or windows. But as is usual in geophysics, as soon as better instruments were
developed, interest rose steeply. In this section we discuss the basic source of the
D region, which is fairly similar at equatorial and midlatitudes. We postpone
discussion of its structure to future chapters.
A summary of rocket electron density measurements is presented in Fig. 1.7a
for quiet conditions and 1.7b for an active sun. The flights were all made for solar
zenith angles near 60
◦
, and all were flown from the NASA base at Wallops Island,
Virginia
9
◦
N, 284
5
◦
E
(
37
.
.
)
. The electron density increases by almost five orders
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