TRIBUNAL ORAL EN LO CRIMINAL FEDERAL DE LA PROVINCIA DE SAN JUAN

Vertically the ionosphere presents a generalised structure however, when looking at an electron density vertical profile it is possible to identify a series of layers. Each layer presents its own characteristics and has been given an internationally recognised letter. Starting from a height of 50-60 km to 90 km above sea level there is the D region. The D region is characterized by a weak electron density (10⁸-10¹⁰ m^-3) and it is absent at night. 𝑁₂, 𝑂₂, and 𝑁𝑂 are the most abundant neutral particles. Photoionization of 𝑁𝑂 by solar X-rays is the most important reaction of ion production for this layer. A special feature of this layer is the presence of negative ions produced by electron attachment reactions (as described in Reaction (2.23)). They are then destroyed by photo-detachment, associative detachment and mutual neutralization reactions. Above the D

14 region, between 90 and 120 km, there is the E region. Here the 𝑁2, 𝑂2, and 𝑁𝑂 are still the major neutral species, 𝑂₂⁺, and 𝑁𝑂⁺ are the most common ionized particles (several 10¹¹ m^-3). 𝑂₂⁺ is easily generated by the photoionization (2.3) by X-ray and UV, 𝑁𝑂⁺ is instead the product of a fast charge rearrangement (2.14), (2.16), (2.17). Dissociative recombination (2.20) (2.22) is the major factor of loss of charged particles. The transport element is not really effective; in fact the E layer dynamics can be described with a simplified continuity equation where the loss term is proportional to the squared ion/electron density:

𝜕𝑁_𝑒

𝜕𝑡 → 𝑄 − 𝑘_𝐿(𝑁_𝑒)² (2.29)

Where 𝑘𝐿 indicates a generic constant. This condition is called photochemical equilibrium and can be fairly accurately reproduced using the Chapman model. The E layer diminishes but persists during the night. In this region it is also possible to notice a sporadic E-layer, also known as Es. Es

is an irregular layer formed by localized clouds of plasma; they mostly appear during the day time with little seasonal variation. The Es-layer develops around 100-120 km as a thin ionized stratus of height extent about 1 km [Barclay, 2003]. It influences radio communication greatly because sporadic E enables long distance signal propagations that are otherwise not possible to occur.

Above the E region, the F layer goes from 170 km up to over 600 km. It can be divided approximately at 200 km in two different layers, F1 and F2. F1, which tends to disappear during night, can be considered as a transition region between the E-layer and the upper part of the ionosphere. The major neutral species are 𝑁2, 𝑂2, and 𝑂, the ion particles are 𝑂2+, 𝑁𝑂⁺, and 𝑂⁺ with densities of several 10¹¹ - 10¹² m^-3. The photoionization of 𝑂₂ and 𝑁₂ (reactions (2.4) (2.6)) are the most important sources of ions. One of the differences between this region and the E layer is its high level of 𝑂⁺. Its direct recombination (radiative recombination (2.19)) is very slow hence it mostly does not take place. 𝑂⁺ is lost by a chain of reactions that start with atom-ion interchange with 𝑂2 and N2 ((2.14)(2.15)) followed by dissociative recombination (reactions (2.20)(2.22)).

Although the F1-layer is characterized by these 𝑂⁺ dynamics, it is still ruled by the photochemical equilibrium. In order to have a non-Chapman type layer, it is necessary to move up to 200 km.

Here the F2-layer begins. It extends to 600 km of altitude, with a peak of electron density at 250-300 km. This region represents the highest concentration of charged particles; it persists overnight, and therefore has, a very important role in space communication. The ion density reaches several 10¹² m^-3 and essentially consists of 𝑂⁺. The ionization mechanisms are the same as those for the F1-layer except that they are magnified in the F2-layer. In fact, although the neutral species 𝑁2, 𝑂₂, and 𝑂, are still present, their densities rapidly diminish along the profile. This affects the rate of recombination which can be now assumed only proportional to the charged particle density.

The deviation from the photochemical equilibrium is also due to the activity of ionospheric

15 transport processes which become more important due to particular physics conditions. Above the F2 peak the electron density decreases with altitude. At 400 km there is a significant concentration of 𝐻⁺ and 𝐻𝑒⁺ which require the explanation of further chemical processes that will not be discussed in this report. The presence of these light ions and the dominance of transport processes are the most significant features of the topside region of the ionosphere (600-1000 km).When the concentration of 𝐻⁺ and 𝐻𝑒⁺ becomes greater than the atomic oxygen ion one, a fully ionised region called the plasmasphere or protonsphere begins.

Figure 2, taken from the work of [Hargreaves, 1992], shows typical vertical profiles of electron density at a generic mid-latitude location. It summarizes very well both the aforementioned ionospheric structures and their variability.

Figure 2. Electron density vertical profiles from a mid-latitude location in different diurnal and solar activity circumstances [Hargreaves, 1992].

The graph enables us to appreciate diurnal changes and changes related to the solar activity. The solid lines are the electron density profiles related to high solar activity and the dashed lines to low solar activity. Note that the changes are not simply related to the density; the layer’s shape is also very much affected.

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Summary

The Sun-Earth system and the terrestrial upper atmosphere were briefly described in this chapter.

The latter was structured in order to explain, at the same time, concepts that have been useful during the project: the definition of the principal processes, the meaning of different parameters and in general the characterization of the ionospheric medium. For this reason, some of the topics will be propose again in the following chapters. In particular, Chapter 5 reports how the principal ionospheric dynamics have been implemented in the model ANIMo. For example, photo-ionization and ambipolar diffusion transportation will be further expanded.

The next chapter continues the background review by introducing some of the techniques used for measuring the ionosphere and its features. It focusses on the sources of observations that were adopted during the project.

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Chapter 3 Observational techniques and data