Geoscience Reference
In-Depth Information
In addition to the regular E layer of the ionosphere, there exist other irregular layers
known as the sporadic E layers (Es). Depending on themechanismof formation, there
are different sporadic E layers. In the equatorial zone, the sporadic E layer may be
interpreted in terms of the two-stream ion wave instability in the plasma (Feltens
et al.
2009
). In the polar ionosphere, the same mechanism may sometimes apply as
well as ionization due to penetration of charged particles. In other cases, turbulence
may also play a role in the formation of the sporadic E layer. Auroral sporadic E is
produced by energetic electrons from the magnetosphere. Furthermore, in the auroral
zone, nighttime ionization is produced by particles from the magnetosphere (Mayer
and Jakowski
2009
). Sporadic E layer can occur over a range of heights from about
90-120km or even more. In some cases the Es is a relatively thick layer with a well-
defined maximum of electron density whereas in other cases it is extremely thin. In
some cases the Es layer is opaque and blankets the upper layers; in other cases the
upper layers can be seen through the Es, which suggests that the radio waves are
penetrating through the gaps.
In the F layer the peak production height is at 150-160km. But the electron
concentration peak is well above this height at around 250-300km. Among the solar
radiation the F layer absorbs EUV radiation. The F region reflects radio waves with
shorter lengths transmitted from Earth's surface. Visible light, radar, television and
FMwavelengths are all too short to be reflected by the ionosphere and they penetrate
through this medium. So these wavelengths are suitable for satellite communications.
Through the F layer, by increase in height, the loss rate gradually becomes dominated
by the ion-atom exchange rate and starts to decrease. In fact the loss rate drops
down faster than the rate of production, resulting in an actual increase in electron
concentration. As the plasma diffusion takes over, the electron concentration becomes
distributed similar to the neutral gas concentration, and a maximum is reached.
The height where the loss rate transits from dissociative to ion-atom exchange can
vary. If this height is above the peak production, a reduction in the actual electron
concentration happens. This results in a secondary peak in the electron concentration
profile at the peak of production called the F1 peak. Therefore the F layer is subdivided
into two layers, the F1 layer extending from about 140-200km and the F2 layer from
200-1000km. Table
5
designates the four principal layers of the ionosphere.
From altitudes of about 1000km the density of
O
+
ions starts to fade and the
H
+
ion turns into the dominate particle. This height is known as the transition height.
This layer of the upper atmosphere is called the plasmasphere. The plasmasphere is
bounded on the upper side by the plasmapause where plasma density drops by one
or two orders of magnitude. Due to the fact that neutral densities are very low in this
region, the plasma profiles are determined by transport of electrons and ions. The
Ta b l e 5
Characteristics of the main ionospheric layers
Layer
D
E
F1
F2
Height domain (km)
60-90
85-140
140-200
200-1000
Electron density (elec/m
3
) ay
10
8
-10
10
10
11
5
·
10
11
10
12
2
·
10
9
10
9
3
·
10
11
Night
-
