Ionospheric Layers: Description, Constituents, anomalies and Effect on Radio Wave Propagation

It is quite clear by now that solar eclipse will have effect on ionospheric dynamics and consequently our communication system. Vastly responsible for the formation of plasma in the ionosphere is solar UV and X-ray radiation, although other factors contribute too, so at any particular time, there are variation in the plasma formation and dynamics. Many internal and external processes contribute to the variation observed in the ionosphere. Such internal processes include ionization, buoyancy forces, electron capture, electrodynamic drift etc., and the major external process is the influence of solar radiation. It is however very complicated and extremely difficult to isolated and study the perturbation by either the external or internal processes without some ambiguity, as they occur simultaneously.

However during solar eclipse, since the sun has been partially or completely blocked by the moon, it is possible to study what happens in the ionosphere at the absent of solar radiation.
Ionization process in the ionosphere is vastly affected during solar eclipse for obvious reasons. Hence, the study of the ionosphere during solar eclipse is of great importance. For obvious reason, and since I will be working with TEC data in for my research, I will be discussing TEC variation with respect to solar eclipse in my subsequent post.
But before that, I will like to explain the layers of the ionosphere and why a particular layer is often mentioned. This is so that we can have an idea of what goes on in each layer.

IONOSPHERIC LAYERS

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The ionosphere is sub-divided into four layers, vis: E layer, D layer and the F layer, which is further divided into F1 and F2 layer. Then we have the topside, coupled with the magnetosphere.
Different processes go on according to the stratification of the ionosphere. Each layer is both unique in plasma formation and content, and thereby affects radio signals in different ways.

D LAYER

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The D layer is the layer closest to the ground (50km-90km). Ionization in this region is mainly due to cosmic rays emanating from different sources within and outside our galaxies. The prominent ion found in this region is NO+ (nitric oxide), which is the major positive charge carrier and O2-, electrons, and some other negative ions are the negative charge carriers. Neutral components of this layer include N2, O2, Ar, CO2, He, with traces of O3 and H2O. When radio signals are sent from Earth, its first destination is the D layer which acts as an attenuator especially for low frequency signals. The attenuation is inversely proportional to the square of the frequency. Therefore, if you increase the propagation frequency by say two, the level of attenuation will be reduced by a factor of four. This explains why low frequency signals are prevented from reaching higher layers except at night when the D layer has died out. When signals get to the D layer, they make free electron to vibrate and collide with other molecules, consuming a small amount of energy and consequently dissipating a small amount of the radio signal. Other factors such as the number of collision and the number of gas molecules present contribute to the degree of attenuation. The more the molecules of gases present, the more the collision and consequently the more the degree of attenuation.

The frequency of the radio signal is also an important factor. As the frequency increases, the wavelength decreases (frequency is inversely proportional to wavelength) so does collision. We might be tempted to assume that high frequency signals are immune to the attenuation effect in the D layer, but we will be totally wrong. Because recombination is high in this region, the level of free electrons present for radio propagation is low and therefore ghigh frequency signal are damped and suffer reduction in strength. This explains why HF radio waves, particularly at 10Hz below are absorbed. The attenuation effect is highest at noon, and it reduces progressively into the night until the D layer’s becomes thin.

During rare event such as Polar Cap Absorption, the level of ionization in the D layer can reach an unprecedented high level in the high latitude and the polar latitude, such that there is a significant increase in the level of Absorption of radio signal.
As a matter of fact, the level of absorption can increase to a level that almost all transmitted transpolar radio signal can be absorbed. Events like this can last between 24hrs to 48hrs.
This layer plays a crucial role in the observed appearance and disappearance of A.M radio waves with the transition from day to night and vice versa.

E LAYER

The E layer is also called Kennelly - Heaviside layer. This layer lie just above the D layer and span from about 90km to 150km. the name Kennelly- Heaviside was coined out form the names of the two scientists who predicted the existence of this layer in 1902. They are Edwin Kennelly and physicist Oliver Heaviside. However, its existence was later detected in 1924 by Edward V. Appleton and Miles Barnett.
Largely responsible for ionization in this layer is soft X-rays (1-10nm) and far UV radiation, which ionizes molecular oxygen. Radio frequencies within the range of 10MHz to 50MHz are reflected by this layer. The amount of ions present in this region during the day is very high, and therefore there is an increase in the frequency of radio wave that can be reflected. During the night, the strength of the E layer to reflect radio wave diminishes due to the absent of the primary ionization source that is the Sun.

ES layer

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There is a thin layer within the E layer, which sometimes can be independent of the normal E layer called the Sporadic E layer. The ES is characterized by intense ionization which can allow the propagation of VHF of frequencies between 30MHz to 300MHz over long distances called sporadic event. This sporadic event is not a permanent residence in the E layer has it can last for just few minutes and sometimes hours. ES occur at different times and it follows a seasonal pattern at high, mid, and low latitude. In the low and mid-latitudes, an ES occur mainly during the day and mostly during summer. At high latitude, it is most likely to occur during the night, and is often associated with aurora. ES layer is very difficult to predict due to its instability and random occurrence. It can however have electron density as high as the F layer.
Several theories have been proposed in bid to describe the ES layer, but the most accepted one is the wind shear theory. This theory explains the reason for the intense volume of ions present. There is a reversal of the east-west neutral wind which combines with the Earth’s magnetic field producing an upward and downward motion of heavy ions, consequently compressing them into this layer. The sporadic E opens up a wider radio propagation path which ordinarily would have not been possible. Scientists are still working on other possible causes of the Sporadic E (You too can join the league, and possibly come up with a more plausible explanation).

F LAYER
This is the topside of the ionosphere which splits into two layers during the day. Ionization in this layer is mainly due to UV rays from the Sun. it stretches from about 150km to way above 500km in altitude. This layer is also known as Appleton-Barnett layer. The F layer enables the transmission of high frequency radio signal. It has the highest density of all the layers, which as a result allows signals to escape into outer space. The dominant ions in this region are the lighter ions of hydrogen and helium. The F layer is responsible for most skywave propagation of radio signal enabling high frequency radio communication over long distances. The F layer is the most important of all the layers especially when it comes to radio communication and navigation systems.

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F1 layer

The F1 layer has altitude between 150km to 210km. the height of the F1 region fluctuates, and is dependent on solar activity, season, and geomagnetic activity. The primary ions present in this region are NO+ and O2+, while the secondary charge carriers are O+ and N+.
The main source ionization is the extreme ultraviolet (EUV) radiation from the Sun. with wavelength between 58.4nm and 30.4nm.
The F1 layer also fall under the Chapman layer alongside the E layer with peak electron density of about 2×1011 el/m3 during midday. Chapman layer is characterized by the existence of equilibrium between ionization and recombination. The F1 layer depend on the solar zenith angle χ and sunspot number R. Consequently, this layer is more active during summer than winter and it disappears at night. When this happen, the remaining single layer is referred to as F layer.

F2 layer

The F2 layer is always there under any solar-terrestrial condition. This layer is the most important of the entire ionospheric layers. Its nature and dynamics is unique, and somewhat different from the lower layers. The F2 layer exhibit high variability with time,varying with the 11 year solar cycle, interactions with the plasmasphere above it (altitude >1,000km) and solar-terrestrial conditions. The plasmaphere help the F2 region to remain even at night by providing free electrons for this region. The primary charge carriers are O+ ion while the secondary charge carriers are H+ and He+.
Ionization is mainly by EUV radiation with wavelength range between 5nm to 102.7nm form the Sun. the complexities and dynamics of the F2 region can be studies in terms of chemical changes, diurnal heating and cooling, winds in neutral air, electric field etc. peculiar to the F2 layer, the rate at which free electron are recapture depends on the concentration of ions auch as O2 and N2 while the rate of production depends on the concentration of oxygen atom.

Spread F

Spread F is a phenomenal of the F rwgion which occur as a result of irregularities leading to the scattering of radio wave. The received signal at the receiving end will then be a superposition of numerous waves which has been reflected at different heights and at slightly varyibng time in the ionosphere.
Spread F occur majorly at low latitudes, high latitudes and around the equinoxes. It rarely occur at mid-latitude. Spread F is mainly due to a reduction in electron density at F, which is usually observed during ionospheric storms.

SUMMARY

The study of the ionosphere is, and will continue to attract interest from many researchers for obvious reasons.
Attenuation or loss of strength of radio signal transmitted through the ionosphere due to irregularities could lead to world war III if such communication message is altered.
Various changes are going on in the ionosphere whose cause has not been fully understood e.g. sudden ionospheric disturbances (SID), plasma instability, ionospheric storm, sporadic E etc. these events occur without any prior warning and can have a damaging effect on our communication systems. It is therefore necessary that full understanding and isolation of cause and effect be carried out. This will require an interdisciplinary approach.

References

The polar cap absortion effect
Ionospheric facts
Sporadic E layer at mid-latitudes: average properties and influence
of atmospheric tides
Wikipedia: Ionosphere
Zolesi .B. Cander L.R. (2014): Ionospheric Prediction and Forecasting (Springer Geophysics,DOI:10.1007/978-3-642-38430-1-2)
Australian government in space and radio service report. Introduction to HF Radio Propagation (accessed online on 20/03/2015)

Thank you for reading my post. Stay stunned as I keep discussing the ionosphere vs. solar eclipse.

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