The Ionosphere and Atmosphere - Ultra and Extremely Low Frequency Electromagnetic Fields - page 46

layer often survives the night at concentration about .3-30/ 10 3 cm 3 . Unusually
narrow layers with enhanced ionization have been occasionally observed in the E
and D regions. This short-lived formation, the so-called sporadic E s layer, is shown
in Fig. 2.1 with a sharp peak at the nighttime profile of the electron number density.
This layer often contains the ions of metals such as Mg C ,Fe C , and Ca C .Itis
assumed that the sporadic E s layers can arise in those regions that have a zero wind
velocity since these regions can accumulate the heavy ions of metals.
In the so-called D layer of the ionosphere, the height range below about 90 km,
the plasma number density is below 10 3 cm 3 . Owing to high values of the electron-
neutral and ion-neutral collision frequency the dynamics of plasma in this region are
mostly controlled by the neutrals.
The basic features of the ionosphere varies with latitude. It is customary to
recognize equatorial, mid-latitude, auroral, and polar ionospheres. The structure of
the ionosphere permanently undergoes the diurnal, seasonal variations and even the
11-years solar cycle.

2.1.2 Neutral Atmosphere

The atmosphere resembles a gas cocoon which surrounds the Earth. The horizontal
stratification of the lower atmosphere is mostly due to the gravity. The atmospheric
pressure P and the number density n m of neutral gas decrease with height
approximately exponentially

P D P 0 exp . z =H a /; and n m D n 0 exp . z =H a /; (2.3)

where the scale height of the atmosphere, H a , is about several kilometers. In the
isothermal atmosphere in hydrostatic equilibrium H a D RT=.Mg/ 8 km, where
R is the universal gas constant, T is temperature, and M is mass of air mole.
In contrast to the pressure and number density the temperature is a nonmonotonic
function of height as shown in Fig. 2.3 . The atmosphere is more sensibly organized
by a representative temperature profile as compared with the ionosphere. In the
so-called troposphere the temperature falls off with height in such a way that the
temperature gradient is about 6 K/km. The temperature trend changes abruptly at
altitude about 10 km within the transition layer, which is termed the tropopause.
In the overlying layer, termed the stratosphere, the temperature increases with
height. A maximum about 270 K is accomplished at the stratopause altitude
55 km, where the temperature trend reverses again. In the so-called mesosphere,
the height range 55-80 km, the temperature decreases to a minimum about 180 K.
In the exosphere, the height range above about 10 3 km, the temperature increases
drastically to values as high as 10 3 K and even more. This region is terminated by
the mesopause from the thermosphere.