Ionospheric TEC Variation: Importance to Scintillation Study and Radio Signal Propagation

Welcome. Thanks for visiting my blog. I will continue my discussion on the ionosphere and solar eclipse. However, today I will be writing on ionospheric TEC. I will also be discussing the irregularities its variations introduce to the propagation of radio signals through the ionosphere. For the research I am embarking on, I will be analyzing the effect of solar eclipse on TEC. Therefore I feel it is important we understand what TEC is, and why it is a parameter of interest. A lot of researches have been done with respect to the effects of both internal and external factors such as diurnal solar variation, solar wind/plasma, latitudinal difference, solar eclipse on ionospheric Total Electron Content (TEC). What is TEC? Why is there so much interest in the study of Ionospheric TEC? What is even the usefulness of it and how does it affect us? I will answer these questions below and give examples of the important uses of TEC data analysis.

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Total Electron Content (TEC)

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The Total Electron Content is a term used to describe the electrons that are available in the ionosphere that will provide a propagation path for radio signal from the sender to the receiver. It is the sum of all the electrons in a radio signal pathway. If you love mathematical terms, it simply means the integral of the electrons over the surface. Radio signals traveling through the ionosphere interacts with electrons. The more the electrons in its path, the more the interaction. The transmission of radio wave depends on its frequency, and the total electrons in its path. TEC allows the reflection of radio signals at some frequencies and also allow radio signal to pass through at other frequencies.

Essentially, a good understanding of the TEC in the ionosphere is desirable for space weather prediction, and propagation of radio signal. Ionospheric TEC is affected by a number of factors such as solar UV, geomagnetic storm, atmospheric waves traveling from the lower atmosphere etc. Therefore, ionospheric TEC depends on longitude, latitude, local time, solar activity, and solar cycle among other factor. The velocity of radio signal changes as they travel through the ionosphere, interacting with these electrons. So, there is a delay in the propagation of radio signal through the ionosphere.

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This change in velocity is particularly important to study, so as to enhance the accuracy of satellite navigation systems such as GPS/GNSS. If TEC is not considered, high values of error may be encountered when calculating positions.

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What can we understand from TEC data Analysis?
A number of useful information can be obtained when TEC data are analyzed. These information help radio engineers in designing and setting up radio communication systems. TEC, apart from helping us to know the phase delay introduced to radio signal, it is particularly useful in determining scintillation. Wait a minute, ionospheric scintillation is an important topic that has not been fully understood. Since TEC is now linked to scintillation, let us discuss that now.

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Ionospheric Scintillation
Ionospheric scintillation is an important factor when we talk of radio communication. Scintillation is a swift fluctuation in the amplitude and /or phase of the radio signal which is mainly due to the density of electron in its path. That is, once the TEC in the path of a transmitting radio signal fluctuates, scintillation is likely to occur. Scintillation will happen when the propagated signals passes through regions of irregularities such as low electron density. This causes the refractive index in the ionosphere to fluctuate, thereby leading to scattering of the radio signal. The resultant signal is hence the interference of refracted and diffracted radio signals.

When the scintillation is severe, a GPS Receiver might be unable to key into the transmitted signal, making it difficult and impossible for determination of position. Even when the scintillation is not severe but mild, it affects the accuracy of GPS positions. The power and phase of a radio signal are both affected by scintillation. It is essential that we are able to predict this occurrence and TEC data provides us with the means to do so. Scintillation mostly occurs in low latitude and high latitude. Although ionospheric scintillation has been extensively studied, it is still very difficult to predict. If by chance you are interested in ionospheric study, you may consider studying scintillation. Remember that, we can say to a large extent that the most important layer of the ionosphere is the F layer due to the fact that it is the layer that allows for trans-Atlantic communication. Right? but scintillation also predominantly occur in this layer. E layer irregularities such as Sporadic E can also lead to scintillation, but its effect on radio signal is very minimal.

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Impact of Scintillation
Of course, since scintillation mainly affects the F layer, it will have impact on trans-ionospheric propagation of signals. That is, communication over long range. It might also have damaging and irrevocable impact on satellite communication and GPS.
Particularly, amplitude scintillation could raise the signal to noise ratio allowing the transmitted signal to be significantly dominated by noise level. It could be so severe that the receiving satellite will be unable to lock the signal due to the fact that the signal’s threshold frequency has significantly dropped below the receiving capabilities which will lead to the need to re-transmit.
It also leads to data loss, cycle slip and limited accuracy of navigation systems.
On the other hand, phase scintillation, may lead to phase loss of signals. It also has effect on devices such as space based radars, and scientific equipments such as radio astronomy facilities.

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Summary
Drawing the link between cause and effect and understanding how to accurately predict the occurrence of various irregularities in the ionosphere is of utmost importance. Ionospheric TEC data is a rich source of information when analyzed. No wonder it is mostly used in researches that concerns the ionosphere. TEC available for signal propagation per time could either provide an effective propagation path or distort the transmitted signal. A signal whose noise level is very high due to irregularities is of little or no use to the receiver. Therefore, ionospheric TEC is an important data to study.


References
Total Electron Content
About Ionospheric Scintillation

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