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Ionosphere and its Influence in Communication Systems

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Symmetry, An annual publication of Central Department of Physics Vol. X, 2016

Ionosphere and its Influence in Communication Systems

Narayan P. Chapagain, PhD

Associate Professor
Department of Physics, Patan M. Campus, Tribhuvan University
Patangate, Lalitpur

Abstract
Ionosphere is the region of the upper atmosphere with the sufficient amount of charged
particles also referred to as plasma. Even though ionosphere is a small part of the atmosphere, it has
a profound effect on the properties and behaviour of the medium. The model result of vertical profile
of electron density shows the different layers of the ionosphere, which depend on the day or night-
time period as well as solar conditions. When the radio waves propagate through the ionosphere, they
are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization and
scattering. So the ionosphere has practical importance as it influences radio wave propagation to
distant places on the earth, which directly impacts on radio wave communication systems.

What is ionosphere?
Ionosphere is the region of the Earth’s upper atmosphere extending from altitudes of
approximately 60 km from the surface of Earth to the altitudes beyond 1000 km, where the
sufficient amount of charged particles exists. In other words, this is the region of a shell of
electrons and electrically charged particles (atoms and molecules), also known as plasma that
surrounds the Earth. In plasma, the electrostatic force attracts the negative free electrons and
the positive ions to each other, but they are too energetic to stay fixed together in an
electrically neutral molecule. The plasma concentration may amount to only about 1% of the
neutral concentration and the total ionosphere represents only less than 0.1% of the total
mass of the Earth's atmosphere (Kelley, 1989). Even though it is such a small part, it is
tremendously important and it has a profound effect on the properties and behaviour of the
medium.

Scientifically, the ionosphere is not another atmospheric layer. During the daytime,
the ionosphere separates into several layers or regions depending upon the local time of day.
The main layers are D-region, E-region, F1-region, and F2-region as shown in Figure 1. The
layers are generally characterized by a density maximum at a certain altitude and a density
decreases with altitude on both sides of the maximum. The D-region ranges from about 60
km to 90 km and is controlled by ionization of neutrals by solar X-Rays and Lyman alpha
radiation, which cause two- and three-body recombination and electron attachment. The
dynamics of the D-region are
mostly dominated by the
neutral atmosphere. In this
region, the plasma density
range is 102-104 cm-3. The E-
region extends from ~90 km
to 150 km altitude with a
plasma density ~105 cm-3.
This region is chemically
dominated and contains Figure 1. Earth’s Ionospheric layers (source: https://www.
google.com.np/search ?q=ionospheric+layer).
molecular ions such as N2+,
O2+, NO+ as primary constituents (Shunk and Nagy, 2000). The F1-region ranges from ~150
km to 200 km altitude with plasma density range of ~105-106 cm-3. The F2-region extends
from an altitude of ~200 km to 500 km and the plasma density maximum varies around 300
km up to 106 cm-3. This is the region of peak electron density of the ionosphere, which is
usually over an order of magnitude greater than the E-region peak density. These F1- and F2-
regions are dominated by monoatomic oxygen and the ions transported through diffusion.
The D- and F1-regions vanish during the night and the E-region become much
weaker. The daytime plasma densities are greater than that of the night-time and also larger
during the solar maximum than in solar minimum. At solar maximum, the electron densities
are greater by a factor of two to four than at solar minimum, especially in the F-region. The
E-region and lower part of the F-region undergo relatively greater variations in electron
density between day and nighttime than does the upper F-region. The F2-layer is the
principle-reflecting layer for HF communications and is responsible for most sky wave
propagation of radio waves. Thus, this region is of the greatest interest for radio wave
propagation. Unfortunately, this layer is also the most anomalous, the most variable and the
most difficult to predict.

How is ionosphere formed?

The solar radiation of all wavelengths travels towards the Earth, first reaching the
outer regions of the atmosphere. When radiation of sufficient intensity strikes an atom or a
molecule, the photon transfers its energy to the electron as excess kinetic energy. When this
excess energy under some circumstances exceed the binding energy in the atom or molecule,
the electron splits the influence of the positive charge of the nucleus. This leaves a positively

2
charged nucleus or ions and a negatively charged electron, although as there are the same
number of positive ions and negative electrons, while the whole gas still remains with an
overall neutral charge. For example, Figure 2 shows helium atom is incident by the solar
radiation; electron is ejected from the atom and get ionised giving the positively charged ion
and free electrons. Similar phenomena happens to other gases to form ionisation and hence
to form ionosphere.

Most of the ionisation in the

ionosphere results from ultraviolet (UV)
radiation. At each time of the ionisation, an
atom or a certain amount of energy is used
thereby reducing the intensity of radiation.

Because of this reason that the UV

Figure 2: Ionisation of molecules by solar radiation.
radiation causes most of the ionisation in
the upper regions of the ionosphere, but
at lower altitudes the radiation that is able to penetrate further cause more of the ionisation.
Accordingly, extreme UV (ultraviolet) radiation and X-Rays give rise to most of the
ionisation at the lower altitudes.
The level of ionisation varies over the extent of the ionosphere with respect to
altitudes as the level of radiation reduces with decreasing altitude. So the density of the gases
also varies with altitudes. In addition, there is a variation in the proportions of monatomic
and molecular forms of the gases that is the monatomic forms of gases being far greater at
higher altitudes. Accordingly, the ionization constituent’s also vary with altitudes with
dominant of monoatomic charge in higher altitudes.

The level of ionisation in the ionosphere also varies with time of day, seasons and
many other external influences including the solar activities. While the radiation from the
Sun causes the atoms and molecules to split into free electrons and positive ions, on the other
hand, when a negative electron meets a positive ion, they may combine. Consequently, two
opposite effects of splitting and recombination of charges are taking place. This is known as
a state of dynamic equilibrium. Accordingly, the level of ionisation is dependent upon the
ionisation and recombination rate. This has a significant effect on radio wave propagations
and hence in communication systems.

3
 We have calculated the
model results of the vertical
structure of the electron density
(or plasma density) of the
ionosphere at the local noon
(solid lines) and local midnight
(dashed lines) for solar minimum
(blue lines) and solar maximum
(red lines) conditions as shown in
Figure 3. The data were obtained
by running the International
Reference Ionosphere (IRI)
model developed by Bilitza,
(2008) for solar minimum
Figure 3. Model calculation of electron density (or plasma
(September 22, 2006) and density) profile of the equatorial ionosphere at noon (solid
lines) and midnight (dashed lines) for solar minimum (blue
maximum conditions (on
lines) and solar maximum conditions (red lines).
September 22, 2001) at
Jicamarca, Peru, which lies close to the dip latitude. The dip latitude is the imaginary
horizontal line running east-west normal to the magnetic field lines and is an angular
measurement in degrees ranging from 0° at the magnetic equator to 90° at the magnetic poles
(Chapagain, 2011). The Figure clearly shows the plasma density is maximum with around
from 250 km to 400 km altitudes depending on the day- or night-time period, or with solar
conditions. Daytime electron density is high during solar maxima compared to night-time
period in solar minima conditions.

Radio wave communication system

Radio waves are affected by the phenomena of reflection, refraction, diffraction,
absorption, polarization and scattering when traveling through the ionosphere as similar to
the form of electromagnetic radiation, like lighting waves (Booker and Wells, 1938). Radio
propagation is the behaviour of radio waves when they are transmitted, or propagated from
one point on the earth to another, or into various parts of the atmosphere. When the radio
waves are transmitted from the surface of the Earth, they are reflected back from the
ionosphere and able to reach the transmitter (as shown in Figure 4). So the ionosphere has

4
practical importance because, among other
functions, it influences radio propagation to
distant places on the earth.

Radio wave was being used on a

daily basis for broadcasting as well as for
two-way radio communication systems.
Radio waves, microwaves, infrared and
visible light can be used for communication Figure 4: Radio wave propagation from Ionosphere
system. For instance, radio waves are used (source: https://www.google.com.np/search?espv=2)

to transmit television and radio programs, while the microwaves are used to transmit satellite
television and for mobile phones. Additionally, infrared wave can be used to transmit
information from remote controls.
The radio wave propagation is affected by many factors such as by the daily changes
of water vapour in the troposphere and ionization in the upper atmosphere, due to the Sun.
Understanding the effects of varying conditions on radio propagation has many practical
applications, from choosing frequencies for international shortwave broadcasters, to
designing reliable mobile telephones systems, or radio navigation, to operation of radar
systems. Moreover, radio wave propagation is also affected by several other factors
determined by its path from point to point. This path can be direct line of sight path or an
over-the-horizon path added by refraction in the ionosphere, which is a region between
approximately 50 and 600 km. In addition, factors influencing ionospheric radio signal
propagation can include inonospheric variability such as sporadic-E, Spread-F and
ionospheric layer tilts, and solar activities such as solar flares, geomagnetic storms, and solar
proton events and so on.

Radio waves of different frequencies propagate in different modes. At extra low

frequencies (ELF) and very low frequencies have very much larger wavelength than the
separation between the earth’s surface and the D-layer of the ionosphere. Consequently,
electromagnetic waves can propagate in this region as a waveguide. Actually, for frequencies
below 20 kHz, the wave propagates as a single waveguide mode with a horizontal magnetic
field and vertical electric field. The interaction of radio waves with the ionize regions of the
atmosphere makes radio propagation more complex to predict and analyse than in free space.

5
 Radio wave propagation through the ionosphere, especially during the disturbed
ionospheric conditions also referred to as ionospheric irregularities (Chapagain et al., 2009),
is more complex to predict and analyse than in free space. Ionospheric radio wave
propagation has a strong association to space weather. A sudden ionospheric disturbance or
shortwave fadeout is observed when the X-ray associated with a solar flare ionizes the D-
region ionosphere. Consequently, enhanced ionization in that region increases the absorption
of radio signals passing through it. During the active solar period, such as strongest solar X-
ray flares complete absorption of virtually all ions spherically propagated radio signals in the
sunlit hemisphere can occur. These solar flares can disrupt HF radio propagation, which
disturb GPS (Global Position System) accuracy. Since radio wave propagation is not fully
predictable, such services as emergency locator transmitter in flight communication for long
distant such as with ocean crossing aircraft, and television broadcasting have been moved to
communications satellites. A satellite link, through expensive, can offer highly predictable
and stable line of sight coverage of a given area.

References
Bilitza, D., and B. Reinisch (2008), International Reference Ionosphere 2007: Improvements and new
parameters, J. Adv. Space Res., 42 #4, 599-609, doi:10.10166/j.asr2007.07.048.
Booker, H. G., and H. W. Wells (1938), Scattering of radio waves by the F-region of the ionosphere,
J. Geophys. Res., 43, 249-256.
Chapagain, N. P., B. G. Fejer, and J.L. Chau (2009), Climatology of post-sunset equatorial spread F
over Jicamarca, J. Geophys. Res., 114, A07307 doi:10.1029/2008J A013911.
Chapagain, N. P. (2011), Dynamics of Equatorial Spread F Using Ground-Based Optical and Radar
Measurements, All Graduate Theses and Dissertations. Paper 897 http:// digital
commons.usu.edu/etd/ 897.
Kelley, M. C. (1989), The Earth's Ionosphere: Plasma physics and electrodynamics, vol. 43,
Academic Press, San Diego, California.
Schunk, R. W., and A. F. Nagy (2000), Ionospheric physics, plasma physics, and chemistry,
Cambridge University Press, New York.
http://www.radio-electronics. com/info/propagation/ionospheric/ionosphere.php) (obtained – January
2016).
https://www.google.com.np/search?q=ionospheric+layer&espv=2&source=lnms&tbm=isch&sa=X&
ved=0ahUKEwjhybiF0YrKAhXHQI4KHYOXBQMQ_AUIBygB&biw=1203&bih=631(obtained –
January 2016).
http://en.wikipedia.org/wiki/List_of_countries_by_number_of_mobiel_phones_in_use_ite_note-1
(Obtained - June 4, 2015).
https://www.google.com.np/search?espv=2&biw=1203&bih=631&tbm=isch&sa=1&q=radiowave+p
ropagation+from+ionosphere&oq=radiowave+propagation+from+ionosphere&gs_l=img.12...126651
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(obtained – January 2016).

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