The Ionosphere and Atmosphere - Ultra and Extremely Low Frequency Electromagnetic Fields - page 45

Fig. 2.2 A schematic
representation of vertical
structure of the ionosphere

Height, km

400

F

300

F 2

200

F 1

E E

100

D

Day Night

equilibrium between the rate of production and recombination of the ions and
electrons. The ion number density has approximately the same profile because of
plasma quasineutrality. This means that the total number density of all the ions,
n i , must be nearly equal to the number density of electrons, n e , that is the plasma
density n ' n e ' n i . As is seen from Fig. 2.1 , the electron density exhibits a
number of peaks, which occur in several large-scale horizontal layers of ionization.
It is widely accepted to divide the ionosphere into several layers shown in Fig. 2.2 .
The main peak of the electron number density occurs in the so-called F layer. The
altitude for which the electron density reaches a maximum is termed the F peak.
At night the F layer occupies the altitude range 300-400 km. In the daytime this
layer lowers and is split (typically in summer) into two regions. The first density
maximum is situated within the F 2 layer in the altitude range 220-320 km and the
next maximum lies in the F 1 layer in the range of 160-200 km.
The ion and neutral composition varies with altitude. The atomic oxygen is the
dominant neutral gas above about 250 km due to the photodissociation of O 2 by solar
UV radiation, that in turn results in the dominance of ions O C in the plasma. Not
surprisingly, at these altitudes the profiles of the electron and O C number densities
are similar due to the quasineutrality of plasma. The peak value of the number
density of both plasma constituents attain values as high as 10 6 cm 3 near noontime.
The O C plasma at higher altitudes, on the other hand, is often sustained through the
night at concentration between 10 4 -10 5 cm 3 .
The ionizing shortwave radiation of the Sun is mostly absorbed at the altitude
range about 100-200 km, which includes the F 1 layer and the E region that is
usually located in the interval of 90-150 km. At this region the ionospheric param-
eters undergo the most regular diurnal and seasonal changes. The peak electron
density about .1-5/ 10 5 cm 3 occurs in this layer near noontime. At nighttime the
plasma number density decreases dramatically in the region of altitude 125-160 km
due to the absence of ionization source. Interestingly enough the plasma in the E