Geoscience Reference
In-Depth Information
ion production rate decreases due to the ionization of the high atmospheric layers
and the reduction of photons. The potential of the increased quantity of photons at
higher latitudes, however, is limited by the low molecular density. Therefore, the
altitude of maximum ion production is found at heights of about 200-700km.
In spite of the simplifying assumptions under which the Chapman theory is
derived, it is able to explain the main characteristics of the ionosphere and pro-
vides a reliable reference for the basics in the ionosphere modeling (Kelly
1989
).
As shown by the altitude profiles, the maximum electron density is concentrated in
a relatively thin layer, typically located at a height between 300 and 500km above
the Earth's surface. Based on that result, the Single Layer Model (
SLM
) has been
introduced (see Alizadeh et al. (
2013
) for more details). In
SLM
it is assumed that
all free electrons are concentrated in an infinitesimally thin layer at a fixed height.
Usually, this height slightly exceeds the altitude of maximum electron density in
order to balance the effect of the more extended topside ionosphere.
4.3 Variations in the Ionosphere
The ionosphere is a complicated medium. It varies with a number of different para-
meters, which cause variations in spatial and temporal structure of electron density
and so in the ionospheric layers. The main parameters driving the ionosphere are
the solar activity and the behavior of the geomagnetic field, so when studying the
electron density it will be beneficial to introduce two additional coordinate systems
besides the geographical coordinate system, which take the characteristics of the
medium into account.
The Sun-fixed coordinate system is used to keep the change in the Sun position
minimal. The coordinate system should co-rotate with the Sun, so the temporal
variation of the electron content is slow and can be averaged for a short period, e.g.
1-2h. The origin of this Sun-fixed coordinate system is set at the center of mass of the
Earth, the terrestrial rotation axis is chosen for Z-axis and the X-axis is defined by the
mean solar meridian; the Y-axis completes the coordinate system to a right-handed
one. In that way the geographical longitude and latitude (
λ
g
,ϕ
g
) can be transformed
into sun-fixed latitude and longitude (
λ
s
,ϕ
s
) as follows:
ϕ
s
=
ϕ
g
,
(52)
hours
λ
s
=
s
=
λ
g
+
UT
−
π
=
λ
g
+
(
UT
−
12
)
,
(53)
where
λ
s
are in degrees and UT is the Universal Time in hours. In order
to unify the units in Eq. (
53
),
ϕ
s
and
should be multiplied by 15
◦
/h. After the
transformation, the latitude remains unchanged (Eq.
52
) and the sun-fixed longitude
Eq. (
53
) matches the hour angle of the Sun.
The geomagnetic coordinate system is defined by its Z-axis parallel to the axis
of magnetic dipole, and its Y-axis perpendicular to the geographic poles. So if
−
DP
(
UT
−
12
)
