The ionosphere and radio propagation

The ionosphere and radio propagation

1. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 11, November (2014), pp. 09-16 © IAEME
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THE IONOSPHERE AND RADIO PROPAGATION


Geeta Rana
1
, M K Yadav
2


1
Department of Physics, K L Mehta Dayanand College for woman, Faridabad, Haryana
2
Department of Humanities and Applied sciences, YMCA University of Science and Technology,
Faridabad, Haryana


ABSTRACT


Ionosphere is an important component of the upper atmosphere and of the solar terrestrial. It
plays a unique role in earth’s environment because of its strong coupling quality to the regions below
and above it. Ionosphere plays a significant role in radio communication and has great intrinsic
scientific interest which makes it of extensive study during past 50 years.
So various topics concerning the propagation, generation and detection of high frequency
radio waves in the ionosphere has been discussed here. It has been practiced to know, what happens
when radio waves interact with ionosphere, how their refraction takes place, and what are the
conditions necessary for radio communication etc.

Keywords: Ionosphere, Radio Communication, Solar Terrestrial.

INTRODUCTION

THE IONOSPHERE

Our Earth is surrounded by a beautiful and protecting layer called atmosphere. Scientists
divided this atmosphere into seven layers or regions which are different from each other in terms of
temperature, pressure, humidity level & the natural events that occur in them. All these layers which
cover a distance of about 3000 km are named as troposphere, stratosphere, ozonosphere, mesosphere,
thermosphere, ionosphere and exosphere, on going from earth towards sky. Each layer plays an
important role i.e. formation of rain clouds, prevention from harmful radiations reaching on the
Earth, reflecting radio waves and inactivating meteors.
Out of these layers one of the important layers is ionosphere which plays a significant role in
the completion of many activities embedded in our daily life. The ionosphere is a layer which
strongly couples with the thermosphere and magnetosphere. Several photochemical, chemical,
dynamical and electro dynamical processes take place in this layer. As a result, exchange of mass,
momentum and energy occurs here in a complex manner [13].
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The ionosphere extends from about 50km altitude to about 600km where it merges with near
Earth’s space environment. The ionosphere is a charged environment where the air is sufficiently
ionized and the ionization is done by both, electromagnetic radiations and particles coming from the
Sun. Due to this ionization, the Earth's upper atmosphere (ionosphere) gets consist of free electrons
and ions [7]. The reason for the presence of free electrons in the ionosphere is that at these altitudes
the density of the Earth's neutral atmosphere is sufficiently low that collisions between particles
happen far less frequently than in the lower atmosphere. So, here the free electrons live much longer
before they get recombined.
The ionosphere possess varying rate of absorption of EUV (extreme ultra violet) radiations
by different atoms / molecules. As a result, different ionization peaks or layers are formed, which are
called as D, E, F1 and F2 layers [1].

Figure (1): Ionospheric layers

LAYERS: VARIATION IN TEMPERATURE AND ELECTRON DEN SITIES


Figure (2): variation in electrons, ions and neutral particles with temperature.

D-layer :-
The D-layer is located, lowest among all the layers and it does not have any exact starting
point. It is characterized by the small ion densities and collision frequencies of electrons and ions
with the neutral particles. Hence, it has low values of neutral temperature (T
n), ion temperature (Ti)
and electron temperature (T
e) (9).
As in D-layer, the recombination rate is higher and ionization is very low hence the high peak
value of ionization density reaches about 90 km, which may drop to 60 km, due to enhanced solar x-
ray flux. This layer exists mainly during the day and disappears at night which allows radio waves
to penetrate into a higher level of ionosphere. [3]

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E-layer :-
Above the D-layer, there is E-layer (also called Kennelly-Heaviside layer). In the E-layer,
neutral temperature (T
n) and ion temperature (Ti) are nearly the same while the electron temperature
(T
e) starts to deviate to higher values during the day time (9).
Here, after sunset the electron density falls by a factor of 10 or more within a very short time.
E-layer also disappears like D-layer after Sunset.

F-layer :-

The F-layer is also known as Appleton layer which has the highest concentration of charged
particles. In the F-layer, up to the height of 250km, thermal coupling between the ions and neutrals is
so good that ions temperature (T
i) is equal to the neutrals temperature (Tn). At greater heights this
coupling becomes worse, but coupling between ions and electrons improves. Hence T
i >Tn . Ti
increase with increasing altitude, while T
n becomes constant with altitude and at very high altitudes
T
i and Te values are nearly equal (9).
Although changes happen in the F-layer during day time but still it remains constant relative
to other layers and extreme ultra violet (EUV) radiations are absorbed here. It has two parts, the
lower F1layer and the higher F2 layer. The F2 layer is more electron dense than F1 layer. The peak
density of electrons in F1 layer is at about 200 km, which is more pronounced in summer and during
high Sun spot numbers. The F2 layer exists both during day and night. In F2 layer the peak density
of electrons is at about 300 km during day time and at higher altitudes. The F layers are responsible
for most of the sky wave propagation of radio waves.

IONOSPHERIC PLASMA

The Ionosphere consists of charged ions and electrons. The Ionosphere do not behaves like a
pure gas because the number of ionized particles is very large there, hence it is reasonable to
consider it as fourth state of matter i.e. plasma. Ionospheric plasma is one of the closest naturally
occurring plasma. The Ionospheric plasma is defined in terms of four main parameters like electron
density, electron temperature, ion temperature and ionic composition [2].The ionosphere is mainly
affected by certain natural phenomena’s like solar flare, sunspot, magnetic storms and sudden
Ionospheric disturbances. The solar flares can be classified as X-class flares, C-class and M-class
flares, according to their intensity. Out of these flares the X-class flares can easily penetrate the
ionosphere.
The solar flares take place at the surface of Sun where a huge amount of energy (about
6×10
25
J) is ejected. In this ejection a large amount of electrons, ions, and atoms reaches to the
surface of earth which can affect the ionosphere [12]. Actually, these varying solar activities control
Ionospheric parameter. The variations in solar activity produce variations in neutral density,
temperature and densities of electrons and ions, neutral winds, and electric fields in the ionosphere
[13].
The X-ray produced due to the solar flare increases the ionization in the lower region of
ionosphere (D layer). This change in the electron profile of ionosphere is known as Sudden
Ionospheric Disturbances (SID) [12]. This disturbance phenomenon (SID) includes sudden cosmic
noise absorption induced by sudden electron density enhancement in the D layer, short wave
fadeouts, sudden phase anomalies, sudden increase in TEC (total electron count), sudden frequency
disturbances.[13].
As the solar activity rises and falls in a cycle over a span of around 11years. So when the Sun
is at its most active phase it is called the solar maxima. During this period dark spots arise on Sun’s
surface and these dark spots erupt tons of hot plasma into the space. If this erupted plasma hits
(ionosphere) or earth, it can harm astronauts and can disrupt radio wave communication, electrical

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grids and short out satellites. During Solar minima, Sun calms down and both Sun spots and
eruptions happen rarely. The sun spot numbers are therefore used to predict Ionospheric parameters
[4].

RADIO COMMUNICATION AND IONOSPHERE

Maxwell was the first person who developed the “THEORY OF ELECTROMAGNETIC
INDUCTION” in 1871. Maxwell claimed that the electromagnetic waves can undergo reflection,
refraction, & absorption like light. Further, Marconi conducted experiments with wireless telegraphy
and in 1896 Marconi succeeded in sending signals through a wireless telegraph up to a distance of
few kilometers.
In 1902 Oliver Heaviside of England and Arthur E Kennelly of United States explained the
existence of an electrically conducting layer, known as ionosphere.
The ionosphere can effect radio propagation from extremely low frequencies (<3 kHz) to super high
frequencies (>30GHz) (5). For frequencies which are below 30 MHz , the ionosphere acts as a
helpful aid for the radio propagation but for the frequencies which are above 30MHz , it acts as a
source of band pollution.

REFRACTION IN THE IONOSPHERE

A radio wave is an electromagnetic wave. When a radio wave reaches in the ionosphere then
its refraction, reflection or absorption may take place. Refraction is caused due to the abrupt
changes in the velocity of upper part of radio wave as it enters in a new medium .The refraction of
radio wave depends upon the following factors.(1) The ionization density of Ionospheric layer. (2)
The frequency of radio wave. (3) The angle at which the wave enters the Ionospheric layer.

DENSITY OF IONOSPHERE


Figure (2): Effect of ionization density on radio waves

The ionosphere has variable density layers as shown in fig (2) .When a radio wave enters into
the region of increasing ionization density then there occurs an increase in the velocity of upper part
of wave due to which it tends to bend back towards low density region that means towards earth.

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When the wave is in the highly dense or maximum ionized portion of the layer then refraction occurs
more slowly because the density of ionization is almost uniform. Now as the wave reaches to the
layer of decreasing ionization density, the wave bends away from earth because of decrement in the
velocity of upper part of wave and hence, it goes into space.

FREQUENCY OF RADIOWAVE


Figure (3): “Reflection” of radio signals by the various layers of the ionosphere

Each Ionospheric layer has a maximum limit of frequency, at or below which the radio waves
can be transmitted and refracted back to earth .This maximum frequency is known as critical
frequency of that particular layer. When a radio wave of higher frequency as compared to the critical
frequency of a particular layer, is transmitted then it will get pass out through that layer. While, if the
same wave enters in an upper layer of higher critical frequency, it will be refracted back to earth [8].

ANGLE OF INCIDENCE

The refraction of radio wave also depends upon the angle with which it enters the Ionospheric layer.


Figure (4): Refraction of radio waves with different incident angle

The wave A enters the ionosphere with a very vertical incident angle where it bends slightly
and then goes into space. For the same wave if the incident angle is reduced than earlier, the wave
(B) strikes the layer and gets refracted back to earth. Wave C strikes the layer at the smallest angle at

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which it gets refracted and still returns back to earth. Hence it shows that the angle of incidence of
any transmitted wave should not be greater than the critical angle otherwise, it will lost into space.
Moreover, there is also a relationship between the frequency of radio wave and it’s critical angle.
With the increase of frequency of radio wave the critical angle must be reduced for refraction to
occur .As shown in fig (5), 2MHz wave gets refracted back to earth but a wave of 5MHz (of higher
frequency and lesser incident angle, with dotted line) do not gets refracted. However the same wave
of 5MHz (with solid line) gets refracted, when the angle is still reduced.


Figure (5): Combined effect of critical angle and frequency on radio waves.

SEVERAL HOPS



When the signals undergo multiple reflections between the ionosphere and the earth’s
surface, to reach at a larger distance on earth then it is referred to as multiple hops.
If a radio signal is to be transmitted from one station to the other side of the globe then
considerably large distance will have to be spanned by the signal. This world-spanning propagation
requires several reflections in between the earth’s surface and the ionosphere. When a radio signal
returns to Earth from the ionosphere, the Earth’s surface acts as a reflector and returns the signal
back to the ionosphere, where it is reflected back to Earth yet again. In this way signals can travel
around the globe (sometimes in several directions!).[11]
The wave can travel greater distance if the hoping takes place in the higher region of
ionosphere. For example F2 skip can travel up to 2500 miles while E skip can travel up to 1200
miles.

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If the two signals reach the receiver in-phase (both signals are at the same point in the wave
cycle when they reach the receiver), then the signal is amplified. This is known as an “up fade.” If
the two waves reach the receiver out-of-phase then the strength of the final signal is weakened. If
the two waves are 180º out of phase when they reach the receiver, then they can completely cancel
out each other out so that a station does not receive a signal at all. A location where a signal is
canceled out by multipath is called a “null” or “down fade.”The nature of the Earth’s surface also has
an effect on hoping. Desert areas are poor reflectors, but oceans are quite effective. This means that
the signals bounced off from the Atlantic Ocean, for example, will be stronger than those reflected
by areas such as the Sahara desert.

ABSORPTION

The ionosphere is usually considered as an area where radio waves of the short wave bands
are refracted or reflected back to Earth. However, it is also found that signals get reduced in strength
or attenuated as they pass through this area. The major factor that has the greatest adverse effect on
radio waves is ABSORPTION.
Absorption results in the loss of strength of received signals and in their ability to
communicate over long distances. Most of the Ionospheric absorption occurs in the lower regions of
ionosphere (D layer) where ionization density is greater. There is some absorption in the E and F
layers also, but the level of absorption is very much less than that experienced in the D layer and it
can generally be ignored.
When a radio wave enters the D layer, it loses some of its energy to the free electrons. If
these free electrons do not collide with gas molecules of low energy then most of the energy lost by
radio wave is reconverted into electromagnetic energy and the wave continues to propagate with
little change in its intensity. However, if these high-energy free electrons collide with other particles
(molecules, ions or electrons) then most of the energy is lost, resulting in absorption of energy from
the wave and this is manifested as a loss in the strength of the signal [7].
The absorption of energy or the amount of lost energy depends upon the number of collisions
between the particles. In turn, number of collisions also depends upon other factors. The first one is
presence of other molecules, electrons and ions. As in D layer greater is the ionization density,
greater is the probability of collisions and as a result greater will be the absorption of radio wave.
The second factor is frequency of wave .The amount of Ionospheric absorption varies inversely as
square of the frequency of transmitted wave, that means if frequency of wave is doubled then the
absorption will reduce by a factor of four. And that is why the lower frequency waves are more
attenuated than the higher ones, while the HF signals get succeed to propagate beyond D layer.

CONCLUSION

Although nowadays most of the long distance radio communications or communication of
radio waves having frequency more than 30MHZ, is done with the help of satellites, but the
Ionospheric radio communication still provides cost effective means of radio communication. In fact,
the radio communication through the ionosphere plays an important role in the study of structure of
the ionosphere.
Artificial satellites imitate the Ionospheric layer and acts as a reflector for the radio waves.
These waves are reflected only when they are within their coverage area. But these artificial satellites
also have some limitations, as they have limited coverage area, their cost is very high and they last
for about 25 years. Unlike artificial satellites, the ionosphere (that covers whole of our planet) acts as
a natural satellite for all the radio waves having frequency less than 30MHZ. Because of this
characteristic of the ionosphere, we do not require to focus at certain point. Moreover, maintenance

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and energy supplements are not needed as these are provided by the nature itself. The atmosphere or
the ionosphere is available for us as long as the Earth exists. Exploring new facts about the universe,
makes us realize that not a single matter is meaningless or useless in this universe. Therefore, we
must understand that the universe is packed with wonderful favors and blessings which are addressed
directly to humanity.

REFERENCES

[1] http://en.wikipedia.org/wikiionosphere.
[2] http://en.wikipedia.org/plasma(physics).
[3] Prof. kjell Ronnmark (2003) lecture notes on space physics from, the sun to
the aurora.
[4] http://en.wikipedia.org/solar flare /sunspots.
[5] General article on ionosphere and its influence on radio communications by R S Dabas.
[6] www.maisonthenezay.fr.pdf radio wave propagation (by M H BARRINGER).
[7] www.radio-electronics.com Antennas and propagation, Ionospheric absorption of radio
signals.
[8] Mukumu. E.pdf (2006) space physics 5p project, Radio waves in the ionosphere.
[9] Utd500.utdallas.edu/ionosphere.htm, William B. Hanson centre for space sciences.
[10] MICHAEL.C.KELLEY (The earth’s ionosphere) (Plasma Physics & Electrodynamics).
[11] Antenna and wave propagation by SISIR K. DAS, ANNAPURNA DAS.
[12] Ionospheric disturbances due to solar activity detection using SDR by DIVYA HARIDAS,
K.P SOMAN & SHANMUGHA SUNDARAM.
[13] Space physics and Space weather geophysics, Solar activity effects of the ionosphere: A brief
review by, LIU LIBO, WAN WEI XING, CHEH YI DING & LE HUI JUN.
[14] Srijibendu Bagchi, “Cognitive Radio: Spectrum Sensing and Performance Evaluation of
Energy Detector Under Consideration of Rayleigh Distribution of the Received Signal”,
International Journal of Electronics and Communication Engineering & Technology
(IJECET), Volume 2, Issue 1, 2011, pp. 17 - 23, ISSN Print: 0976- 6464, ISSN Online:
0976 –6472.