Ionospheric Layering

Table 3-1 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 in the D and E regions. From an historical perspective, the concept of layering derives from the appearance of the ionospheric regions on vertical incidence ionospheric soundings, called ionograms (see Section 3.4.1). 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 is called sporadic E (or Es) that exists as an isolated layer within the boundaries of the normal E region (see Section 3.7). Since the Es layer exhibits a generally unpredictable temporal and geographical distribution, it is termed sporadic, 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 comprised of two distinct regions (i.e., 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, but is important as a reflecting layer for longwave communication and navigation systems. There is also evidence for a bifurcation in the D region, with the upper portion (i.e., above 60 Km) being produced by solar flux, and with the lower portion (i.e., below 60 Km) being produced by galactic cosmic rays.

Table 3-1: Properties of the Ionospheric Layers

Region

Height Range (km)

Nmax Range (m-3)

(M'Hz)

Major Ingredients(s)

Basis of Formation

D

70 to 90

108 to 109

NO+, 0+

La x-rays

E

90 to 130 ''max-110

~10n (day) ~1010 (night) (smooth diurnal variation)

-0.3 (night) -3.0 (day)

0+, NO+ 2

Lß x-rays; Chapman Layer

Es

90 to 130

~1012 (highly variable)

Metallic Ions

Wind shear and meteoric derbis;

equatorial electrojet; Auroral electrojet and precipitation

130 to 210

(smooth diurnal variation)

-3 to 6 (day) (merges with Fj layer at night)

0+, NO+

Helium II Line; UV Radiation;

Chapman Layer

Fl

200 to 1000 >W -300

(asymmetric diurnal variation)

-5 to 15 (day) ~3 to 6 (night) (presunrise minimum)

o+

Upward diffusion from the /''| Layer; photoionization

Ground-based vertical incidence sounder measurements have provided the bulk of our current information about ionospheric structure (see Section 3.4.1). Through application of ionogram inversion technology to account for the radiowave interaction effects, individual sounder stations provide information about the vertical distribution of ionization to the altitude of the F2 maximum (i.e., 300-400 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 F-region ionization density 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 scatter 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 [1991] 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 [1931] (see Figure 3-5). 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 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 depends upon wavelength and atmospheric constitution. All of this results in 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.

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