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would be the dipole position and SP the south pole, we have Y
× SP .
Finally, the X-axis of the system completes an orthogonal right-handed set. The rela-
tion between the geomagnetic coordinates (
DP
=
λ m m ) and the geographic coordinates
(
λ g g ) follows:
sin
ϕ m =
sin
ϕ g sin
ϕ 0 +
cos
ϕ g cos
ϕ 0 cos
g λ 0 )
(54)
ϕ g sin
g λ 0 )
cos
cos
sin
λ m =
(55)
ϕ m
where
λ 0 are the geographical coordinates of the geomagnetic north pole.
According to the International Geomagnetic Reference Field (IGRF), the coordinates
of the north magnetic pole for the year 2010 are
ϕ 0 and
2 (W)
(IGRF 2011 ). According to Schaer ( 1999 ) the geomagnetic north pole is moving
about
0 (N) and
ϕ 0 =
80
.
λ 0 =
72
.
03
07
+
0
.
in the south-north direction and
0
.
in the west-east direction
per year.
4.3.1 Height Dependent Spatial Variations
The ionosphere is subdivided into different height-dependent layers, based on the
solar radiation wavelength which is most absorbed in that layer or the level of radi-
ation which is required to photo dissociate the molecules within these individual
regions. The main layers are known as D, E, F1 and F2.
The D layer is the part from approximately 60-90km, which absorbs the most
energetic part of solar radiation. This layer reflects long wavelength radio waves
transmitted from Earth's surface back to the Earth. This phenomenon makes long
distance radio communication possible. The D region is the most complex part of
the ionosphere from the chemical point of view. Several different sources cause the
ion production within this layer. The most important are: Lyman-
(Rhoads et al.
2000 ) that ionizes the NO molecule, ultraviolet radiation that ionizes O 2 and N 2 , hard
X-rays that depend strongly on the solar activity and are not significant at sunspot
minima, and galactic cosmic rays that affect mainly the lower parts of the D region.
The D region shows sudden changes in the electron concentration near sunrise and
sunset and remains almost constant during the day. During the nighttime at mid
latitudes this layer vanishes.
The E Layer is the part from about 85-140km, although the production peak is at
110-115km. Since at this height different molecular gases are present, molecular ions
are produced directly and the loss rate is dominated by dissociative recombination.
As this is not height-dependent, the resulting electron concentration in this region
closely follows the production profile. The E layer absorbs soft X-rays, and it reflects
standard AM radio waves, which are transmitted from the Earth, back to its surface.
The variations are regular and are mainly controlled by the Sun and may be described
by a Chapman law (see (Alizadeh et al. 2013 ) in this topic). The E layer does not
vanish at night, but a weakly ionized layer remains with an electron density of
approximately 2
α
10 9 electrons/m 3 .
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