Electrodynamic Properties of Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 28

These are D -, E -, F 1-, and F -layers (arranged according to altitudes). The
D -region occupies heights between 60 and 90 km. It is remarkable that the
plasma in this region is a mixture of electrons and negative and positive ions.
Concentration of negative ions is comparable to that of positive ions at around
80 km height. At night there is no noticeable ionization at the D -layer. In the
E -region, located between 90 and 140 km, the electron concentration is higher
by about 2 orders of magnitude than in the D -region, reaching 10 5 cm − 3 .
Above the E -layer, F 1- and F 2-layers are located with maximal electron con-
centration in F 1of N e ≈ 2 . 5 × 10 5 cm − 3 during years of minimal solar activity

10 5 cm − 3 during years of maximum solar activity. The F 2-region
is very sensitive to variations of solar activity. The altitude for the maximal
electron concentration in F 2, which coincides with the maximal electron con-
centration in the ionosphere, fluctuates between 200 and 400 km and is on
average 5

and ≈ 4 . 5 ×

10 6 cm − 3 .
Figure 2.2 demonstrates the height distribution of electrons and ions, ac-
cording to the IRI-2000 model [9] for the 50 ◦ geomagnetic latitude. Height
profiles corresponding to winter noon and winter midnight conditions for
sunspot numbers R = 15 are shown in the left top and bottom frames of
Fig. 2.2, respectively. The same dependencies for the summer noon and sum-
mer midnight for the disturbed solar activity ( R = 150) are shown in the right
top and bottom frames of Fig. 2.2.

10 5

× −

Collision Frequencies

To calculate the transmission and reflection characteristics of hydromagnetic
waves, it is necessary to know the electrical conductivities. Consequently it
is necessary to know the collision processes between ionospheric particles.
Charge particles gain directed velocity under the action of the wave electric
field. At the same time, electrons and ions collide and exchange energy with
each other as well as with neutral particles. The energy of a directed particle
movement is partially converted into chaotic particle motions at collisions.
The energy of hydromagnetic waves is converted into Joule heating of the
ionospheric plasma.

Electron-Neutrals

To define the collision frequency ( ν en ) of electrons ( e ) with neutral ( n )par-
ticles ν en it is necessary to know the effective cross-section of an electron
scattered on the colliding particles. Using these cross-sections and integrat-
ing by velocities, it is possibly to define an effective collision frequency of
an electron with molecules ([6], [22]). Approximated expressions for the col-
lision frequencies of an electron with nitrogen N 2 and oxygen O 2 mole-
cules as well as atomic oxygen O, hydrogen H and helium He are given
in [22]