Formation of the Ionosphere

The sun exerts a number of influences ever the upper atmosphere, but the interactions of most importance for our discussion are photodissociation and photoionization. Figure 3-2 depicts the neutral atmosphere, its various regions and the depth of penetration of the various components of solar flux.

Figure 3-2: Atmospheric and Ionospheric Layers. The primary atmospheric regions are nominally: troposphere (0-10 km), stratosphere (10-45 km), the mesosphere (45-95 km), the thermosphere (95-500 km), and the exosphere (> 500 km). The bulk of the D-region is within the mesosphere. The majority of the ionospheric electron content resides within the thermosphere. The protective o/one layer lies within the stratosphere. The depth of penetration of the solar radiation is also depicted. Large variations in penetration depth exist for specific bands. Below 200 km, the ionosphere is dominated by polyatomic species, a fact that favors recombination processes. Between 200 and 600 km, the F-region is predominantly monatomic oxygen. Eventually an altitude is reached where atomic hydrogen dominates and helium is in evidence, and this region is called the protonosphere. From National Research Council report [NRC, 1981].

Figure 3-2: Atmospheric and Ionospheric Layers. The primary atmospheric regions are nominally: troposphere (0-10 km), stratosphere (10-45 km), the mesosphere (45-95 km), the thermosphere (95-500 km), and the exosphere (> 500 km). The bulk of the D-region is within the mesosphere. The majority of the ionospheric electron content resides within the thermosphere. The protective o/one layer lies within the stratosphere. The depth of penetration of the solar radiation is also depicted. Large variations in penetration depth exist for specific bands. Below 200 km, the ionosphere is dominated by polyatomic species, a fact that favors recombination processes. Between 200 and 600 km, the F-region is predominantly monatomic oxygen. Eventually an altitude is reached where atomic hydrogen dominates and helium is in evidence, and this region is called the protonosphere. From National Research Council report [NRC, 1981].

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Figure 3-3: Profiles of ion concentration, as a function of height, for midlatitude daytime conditions. Note that even where the ionization density is greatest (i.e., the F2 peak), the neutral density is still larger by several orders of magnitude. There is a distinct E-F valley of ionization during nocturnal hours, which is deeper at solar minimum. This region can be a ducting vehicle for trapping of shortwave signals. Figure from Jursa [1985].

Figure 3-3: Profiles of ion concentration, as a function of height, for midlatitude daytime conditions. Note that even where the ionization density is greatest (i.e., the F2 peak), the neutral density is still larger by several orders of magnitude. There is a distinct E-F valley of ionization during nocturnal hours, which is deeper at solar minimum. This region can be a ducting vehicle for trapping of shortwave signals. Figure from Jursa [1985].

In the lower atmosphere, species such as N2 and 02 dominate the constituent population even though other species such as water vapor, carbon dioxide, nitric oxide, and trace element gases are influential in specific contexts. In the upper atmosphere, however, molecular forms are dissociated by incoming solar flux into separate atomic components. Formally the lowest portion of the ionosphere is the so-called D-layer at an altitude of ~ 60 Km ± 20 Km, but the free electron and ion population rises dramatically at an altitude of - 100 Km, which is the median altitude of the E-layer. Two things occur at this altitude. First, oxygen becomes dissociated as a result of solar UV radiation. Secondly, the mixing process of the atmosphere, so efficient below 100 Km, ceases rather dramatically, and the region where this occurs is called the turbopause.. This process of dissociation is so efficient that we refer to the distribution of neutral species in a vast segment of the upper atmosphere (i.e., above 200 Km) as a monatomic gas. In the lower atmosphere (i.e., below roughly 200 Km), the gas is largely polyatomic, although the transition between the two regimes is rather gradual between 100 and 200 Km. This has implications for the lifetime of ion-electron pairs created through photoionization. Also, in the altitude regime above about 200 Km and well above the turbopause, collisions become a rarity with mixing of the various species becoming unimportant in comparison with diffusive forces. As a consequence, diffusive separation occurs, with constituents of the neutral gas seeking their own unique height distributions dictated by their atomic masses, the gas temperature, and the acceleration of gravity. Figure 33 shows height profiles of ionic species in the upper atmosphere, and Figure 3-4 contains typical distributions of midlatitude electron density for daytime and nighttime under solar maximum and minimum conditions.

Midlatitude Density Profiles

Midlatitude Density Profiles

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Figure 3-4: Electron Density Distributions for day/night and solar maximum/minimum conditions. Note the dominance of atomic oxygen within the F2-region, where the molecular species have suffered considerable dissociation. Nitric oxide (singly-ionized) is dominant between 100 and 200 km. From Jursa [1985],

Density (cm-3)

Figure 3-4: Electron Density Distributions for day/night and solar maximum/minimum conditions. Note the dominance of atomic oxygen within the F2-region, where the molecular species have suffered considerable dissociation. Nitric oxide (singly-ionized) is dominant between 100 and 200 km. From Jursa [1985],

It may be seen that ionized monatomic oxygen is the majority ion between roughly 180 and 800 kilometers, and is wholly dominant between about 200 and 500 Km. Atomic hydrogen ions become important above 500 Km, and the region from about 800 to 2000 kilometers is called the protonosphere. It should also be noted that above 500 Km (i.e., the base of the exosphere), the neutral atmosphere is virtually collisionless and particles tend to move about freely. On the other hand, electrons and ions in the exosphere are still influenced by the earth's magnetic field and electrodynamic forces. The electron density distributions in the ionosphere and protonosphere are variable. Because of this, the boundary between the ionosphere and the protonosphere is not sharply defined, being dependent upon a number of factors including time-of-day, season, and solar activity. The protonosphere is often referred to as the plasmasphere, especially by magnetospheric scientists and propagation specialists engaged in measurements of the total electron content (TEC) of the ionosphere using GEOs.

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