Electrodynamic Properties of Space - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 39

90

60

Summer
Σ yy

30

12

18 27

0

0 60 120 180 240 300 3

LOCAL TIME

Fig. 2.8. Same as Fig. 2.6 but for Σ yy component

Earth's crust and in the atmosphere as well as cosmic rays. Solar UV rays
reaching the lower atmospheric layers (with wavelength λ> 285 nm) have no
part in their ionization.
Under conditions of normal atmospheric pressure, electrons are created
almost instantly through ionization. Less than 10 − 6 s later the electrons attach
to neutral atoms and negative ions are created. Such molecular ions exist
for fractions of a second, since under the action of polarization forces they
are joined by 10

15 molecules from the ambient air. The emerging clusters
(light particles) are stable enough, but they can join bigger dust particles
and give up their charge to the latter (larger particles). The mobility of the
light components exceeds that of the larger particles by at least 3 orders of
magnitude. Therefore the atmospheric conductivity is predominantly (90%)
determined by the light component.
Henceforth for estimations, we will put the value of the near-ground at-
mospheric conductivity to be σ a =10 − 4 s − 1 . Turbulent mixing of the air
causes inessential altitude dependency of the conductivity up to

−

≈ 3 km height.

Above this, the conductivity increases rapidly.
In most known ULF-problems, the near-ground atmospheric conductiv-
ity is not essential for obtaining ground distributions of the waves. One can
present the near-ground atmospheric layer as a perfect insulator. There is one
exception, though. The vertical electric pulsation field associated with verti-
cal electric currents, the value of which directly depends on the atmospheric
conductivity. But the electromagnetic wave mode with vertical electric field
in the ULF-range usually has an extremely small intensity. Any attempts
to observe it experimentally were unsuccessful. σ a ( z ) may be roughly ap-
proximated by an exponent with a scale of the order of the atmospheric
scale-height, i.e. σ a ∝ exp( a/H σ ), where the scale-height of the atmospheric

conductivity is H σ ≈ 8 km. The altitude profile σ atm ( z )ispresentedin

Fig. 2.9.