General Properties of the Ionosphere

Basic Structure. The ionosphere is an ionized region in the upper atmosphere that, by generally ac­cepted convention, lies between an altitude range of 60 km to 1000 km. Nevertheless, the region above 1000 km but below 2000 km, called the protonosphere, is also ionized and may be important when considering the to­tality of ionization effects on radio systems. As a matter of convenience, some specialists have combined the ionosphere and protonosphere into a single region of ionization. For example, the integrated electron density from a ground station to a geosynchronous satellite (referenced to the vertical) is referred to as the total electron
content of the ionosphere (TEC), even though both ionospheric and protonospheric electrons contribute to the integral. For the purpose of this article, we shall use the more restricted definition for the ionosphere, generally placing the upper limit at approximately 1000 km. While there are equal numbers of free electrons and pos­itive ions within the ionosphere, it is the electron number density that characterizes the array of interesting phenomena associated with the region. The ionosphere is imbedded in the earth’s magnetic field, and this situation influences the distribution of the ionized constituents. A clear indication of this may be seen in the worldwide distribution of electron density in the upper ionosphere, which tends to be described by geomagnetic rather than geographic coordinates. Moreover, being a magnetoionic medium, the ionosphere has a profound effect upon radiowaves that interact with the medium.

To sun

10ОО I—

10a 104 106 N(h)

Fig. 1. Depiction of the ionospheric layers and the diurnal variation. [By permission of J. M. Goodman and Kluwer Academic Publishers, Norwell, MA (11).]

The ionospheric electron density distribution is logically evaluated first in terms of its height profile, followed by its geographical and temporal variabilities. Though there is abundant evidence suggesting a rather complex electron density profile comprised of several peaks and valleys, the basis for understanding fundamental properties of the ionosphere comes from a simple picture of an ionized medium dominated by a single region, or layer, having a distinct maximum in electron density. This is not without justification, since the highest and thickest component region, the so-called F layer, typically exhibits the greatest electron density. Moreover, in many radiowave applications, it is the F layer that exhibits the dominant interaction. Figure 1

Fig. 2. Various atmospheric and ionospheric layers, the depth of penetration of solar radiation, and the thermospheric temperature profile. [From a National Research Council report (62).]

depicts the various regions or layers of the ionosphere in terms of the electron number density. It has been observed that the height profile varies diurnally, seasonally, and as a function of solar activity.

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

In the lower atmosphere, species such as N2 and O2 dominate the constituent population, 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 of the atmosphere, so efficient below 100 km, ceases rather dramatically, and the region where this occurs is called the turbopause.

The process of dissociation is so efficient that we treat the distribution of neutral species in a vast segment of the upper atmosphere (i. e., above 200 km) as that of a monatomic gas. In the lower atmosphere (i. e., below

Fig. 3. (a) Profiles of ion concentrations, as a function of height, for daytime conditions. (b) Electron density distributions for day/night and solar maximum/minimum conditions. [From Jursa (8).]

roughly 200 km), the gas is largely polyatomic, although the transition between the two regimes is rather gradual between 100 km 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, so that mixing of the various species becomes unimportant in comparison with diffusive forces. As a consequence, diffusive separation occurs, with constituents of the neutral gas seeking their own height distributions dictated by their atomic masses, the gas temperature, and the acceleration of gravity. Figure 3(a) shows height profiles of ionic species in the upper atmosphere, and Fig. 3(b) shows typical distributions of midlatitude electron density for daytime and nighttime under solar maximum and minimum conditions.

It may be seen that ionized monatomic oxygen is the majority ion between roughly 180 km and 800 km, and is wholly dominant between about 200 km and 500 km. Atomic hydrogen ions become important above 500 km, and the region from about 800 km to 2000 km 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 those engaged in transionospheric TEC measurements.

Ionospheric Layering. Table 1 provides information about the various ionospheric layers, the altitude ranges of each, the principal ionic constituents, and the means of formation. A comment is appropriate here on the nature of ionospheric layering, with some emphasis on the historical distinctions made between the words layer and region as they pertain to the ionosphere. Often the terms are used interchangeably, and while neither is generally preferred, region is the more accurate description. This is because it does not convey the incorrect impression that sharp discontinuities in electron density exist at well-defined upper and lower boundaries. This is especially the case for the F region, and to a lesser extent for the D and E regions. From a historical perspective, the concept of layering derives from the appearance of the ionospheric regions on vertical — incidence ionospheric soundings, called ionograms (see the subsection “Sounder Measurement Method” below). Furthermore, the alphabetic designation of the ionospheric regions was also based upon the early sounding studies. On the other hand, there are certain situations for which the restrictive term layer is acceptable. For example, the normal E region may occasionally be characterized by an electron density profile displaying a degree of boundary sharpness. Aside from this, the most significant localized concentration of free electrons in the ionosphere, called sporadic E (or Es), exists as an isolated layer within the boundaries of the normal E region (see the section “Sporadic E” below). It is termed sporadic because it exhibits a generally unpredictable temporal and geographical distribution, and because of its limited geographical extent, it is sometimes referred to as a sporadic E patch.

As indicated above, the ionosphere is often described in terms of its component regions or layers. These were the so-called D, E, and F regions. These designations are largely based upon data obtained from crude sounder (i. e., ionogram) measurements undertaken in the 1920s and 1930s. These early measurements often exhibited evidence for an additional layer between regions E and F in the daytime ionosphere. This led to the notion that the F region is actually composed of two distinct regions (F1 and F2) having different properties.

The lowest region of the ionosphere, the D region, is important in the characterization of absorption losses for short-wave systems, and also as a reflecting layer for long-wave communication and navigation systems. There is also evidence for a bifurcation in the D region, with the upper portion (above 60 km) being produced by solar flux, and with the lower portion (below 60 km) being produced by galactic cosmic rays.

Ground-based vertical-incidence sounder measurements have provided the bulk of our current informa­tion about ionospheric structure (see the subsection “Sounder Measurement Method” below). Through applica­tion of ionogram inversion technology to allow for the radio-wave interaction effects, individual sounder stations provide information about the vertical distribution of ionization to the altitude of the F2 maximum (i. e., 300 km to 400 km). In addition, the worldwide distribution of these systems has allowed a good geographical picture to be developed using sophisticated mapping algorithms. These measurements are somewhat limited in the characterization of certain features such as the so-called E-F valley, and they cannot evaluate ionization above the F2 maximum. There is also a paucity of data over oceanic regions. Satellite measurements (viz., topside sounders and in situ probes) have been invaluable in the characterization of the F-region ionization density

Table 1: Properties of the Ionospheric Layers

Region Height Range

JW Range

ft

Major

Basis of

(km)

(m-3)

(MHz)

Ingredient(s)

Formation

D 70-90

10^-10”

N0+.0+

La X rays

E 90-130

~10n (day)

~0.3 (night)

0+,N0h

Lft X rays;

110

~1010 (night)

(smooth diurnal variation)

~3.0 (day)

Chapman layer

Es 90-130

-ID12 (highly variable)

Metallic ions

Wind Shear & meteoric debris;

equatorial

electrojet;

Auroral electrojet and precipitation

Fi 130-210

~2 X 10’1 (day)

~ 3-6 (day)

0+,N0+

Helium II line;

Kmi™ 180

~ 0 (night)

(merges with F2

UV radiation;

layer at night)

(smooth diurnal

Chapman layer

variation)

F2 200-1000

~1012 (day)

~ 5-15(day)

0+

Upward

hmax’v 300

~2 x 1011 (night)

~ 3-6 (night)

diffusion from

(asymmetric

(presunrise

the F1 layer;

diurnal variation)

minimum)

photoionization

over oceanic regions. Rocket probes and incoherent backscatter radar measurements, which provide a clearer representation of the true electron density profile, typically reveal a relatively featureless profile exhibiting a single F-region maximum with several underlying ledges or profile derivative discontinuities. Nevertheless, a valley of ionization may often be observed between the E and F regions. Ionization above the F2 maximum may be deduced from satellite probes and Thomson-scattering radars, but a large amount of information has been derived from total electron content measurements using Faraday rotation or group path measurements of signals from geostationary satellites or Global Positioning System (GPS) satellites. Hunsucker (10) describes various ionospheric measurement techniques.

Simple layering occurs as the result of two factors. First, the atmospheric neutral density decreases expo­nentially with altitude, while the solar ionizing flux density increases with height above sea level. This leads to the formation of single region for which the ionization rate is maximized, and ultimately results in a layer having the so-called Chapman shape. This shape is based upon a simple theory advanced by Sidney Chapman in 1931 (Ref. 17; see Fig. 4). We observe nonetheless a degree of structure in the ionosphere, which suggests more than one layer. One cause for multilayer formation is the existence of a multicomponent atmosphere, each component of which possesses a separate height distribution at ionospheric altitudes. But there are other factors. Solar radiation is not monochromatic as suggested in the simple Chapman theory, and it has an en­ergy density that is not evenly distributed in the wavelength domain. Furthermore, its penetration depth and ionization capability depend upon wavelength and atmospheric constitution. All of this results is a photoion­ization rate, and an associated electron density profile, that are structured functions of altitude. It has been shown that the Chapman model is valid for the D, E, and F1 regions but is not generally valid for the F2 region.

Chapman Layer Theory. One of the basic tenets of Chapman theory is that solar radiation will penetrate to an altitude for which the total number of atoms or molecules, P (populating a column of unit cross sectional area directed toward the sun) is equal to the reciprocal of the absorption (or interaction) cross

Fig. 4. An idealized representation of ionization production in the atmosphere as the solar radiation encounters a neutral gas with exponentially increasing density. [By permission of J. M. Goodman and Kluwer Academic Publishers, Norwell, MA (11).]

section a that is P = 1/a. The peak in ionization will be produced in the neighborhood of that altitude, and the concept is valid for oblique solar illumination as well as for the case in which the sun is directly overhead. It is convenient to look at the production rate in terms of its deviation from the peak (overhead) value at height h0. For this it is useful to define a reduced height z, corresponding to the normalized departure of an arbitrary value of ionospheric height h from h0.

where h0 is the peak height for vertically incident radiation from the sun, and H is the neutral scale height given by the following expression:

(2)

H — kT/mg

where k is Boltzmann’s constant, T is the absolute gas temperature, m is the atomic or molecular mass, and g is the acceleration of gravity. Within the thermosphere (with hfi 100 km), the gas temperature is monotonically increasing, reaching an asymptotic level near the base of the exosphere. The temperature rises from « 180 K at the mesopause (and incidentally near the turbopause) to levels approaching a diurnal range of 600 K to 1100 K at solar minimum and 800 K to 1400 K at solar maximum. The heat sources include solar radiation, the dissipation of atmospheric gravity waves, and particle precipitation. The asymptotic levels of T are due to limits on the thermal conductivity of the gas. The scale height H is a convenient parameter, since it may be used as a measure of layer thickness for an equivalent fixed-density slab. More importantly, it has a physical meaning. If the atmosphere is in diffusive equilibrium governed by the force of gravity and the gas pressure
gradient, and N is the atomic or molecular gas density (as appropriate), we have

H = (*

where N0 is the atomic or molecular density at some reference height.

In a diffusively separated atmospheric environment, each constituent has its own unique scale height governed by its own molecular (atomic) mass. In an ionized gas in which the electrons and ions are coupled by electrostatic forces, the effective value of the mean molecular mass is «1 the mass of the positive ion. This is because the mass of the electron is essentially zero in comparison with the ion mass.

Figure 5 depicts the production-rate curves associated with an ideal Chapman-like production profile and a range of solar zenith angles x. It is seen that there are a number of curves, parametrized in terms of x, for which production rate maxima qmax, may be observed. The largest qmax occurs for x = 0 (overhead case corresponding to q = q0), and we see that other values for qmax, corresponding to oblique geometries wherein x = 0, will decrease in magnitude and occur at increasing heights as x becomes larger (i. e., the sun moves toward the horizon). Chapman theory yields the following rate-of-production formula:

At altitudes well above the peak in q, the rate of electron production drops off in an exponential fashion imitating the exponential decrease in gas pressure with height. In order to relate Chapman production curves to actual electron density distributions, we must examine loss processes and certain dynamic factors.

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