General Properties Of The Ionosphere 321 Basic Structure

The ionosphere is an ionized region in the upper atmosphere that, by generally accepted convention, lies between an altitude range of 60-1000 kilometers. Nevertheless the region above 1000 kilometers but below 2000 kilometers, called the protonosphere, is also ionized and may be an important region 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 (i.e., 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 kilometers. 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 that tends to be best organized by geomagnetic rather than geographic coordinates. Moreover, being a magnetoionic medium, the ionosphere has a profound impact upon radiowaves that interact with the medium.

The ionospheric electron density distribution is logically evaluated first in terms of its height profile, followed by its geographical and temporal variabilities. Even 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 some 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 3-1 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.

To Sun

Figure 3-1: Depiction of the Ionospheric Layers for middle latitudes, including the day to night variation. Solar cycle variations are suggested in the cartoon, but seasonal effects are suppressed. The daytime ionosphere is defined by three "refractive" layers: the E-region, the F1 -region (or ledge), and the F2-region. The ionosphere also has an "absorption" layer principally for shortwave propagation, termed the D-region. The D-region extends from ~ 50 km to the base of the E-region. The nominal altitude of the ionization peaks for the various layers are: D-region (60 km), E-region (100 km), Fl-region (200 km), and F2-region (300 km). However, large variations may occur, especially in the F2-region. During the nighttime, the E-and F1-regions vanish due to recombination. The F2-region diminishes slowly following ionospheric sunset, since electronic loss is governed by an attachment process that has a low cross section. The F2 layer exhibits significant variability that is not under solar control. Overall, the ionosphere exhibits higher ionization densities in the daytime. Cartoon derived from Goodman [1991] and a variety of public domain sources.

Figure 3-1: Depiction of the Ionospheric Layers for middle latitudes, including the day to night variation. Solar cycle variations are suggested in the cartoon, but seasonal effects are suppressed. The daytime ionosphere is defined by three "refractive" layers: the E-region, the F1 -region (or ledge), and the F2-region. The ionosphere also has an "absorption" layer principally for shortwave propagation, termed the D-region. The D-region extends from ~ 50 km to the base of the E-region. The nominal altitude of the ionization peaks for the various layers are: D-region (60 km), E-region (100 km), Fl-region (200 km), and F2-region (300 km). However, large variations may occur, especially in the F2-region. During the nighttime, the E-and F1-regions vanish due to recombination. The F2-region diminishes slowly following ionospheric sunset, since electronic loss is governed by an attachment process that has a low cross section. The F2 layer exhibits significant variability that is not under solar control. Overall, the ionosphere exhibits higher ionization densities in the daytime. Cartoon derived from Goodman [1991] and a variety of public domain sources.

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