Outline Of Ionospheric Effects

The ionosphere is a partially ionized region of the upper atmosphere loosely partitioned into three major regions termed D, E, and F. It extends from roughly 50 km to 2000 km in altitude as defined by its sensible effect upon radiowave propagation systems. The reader is referred to Figure 3-1 in the last chapter. Although only a cartoon, the figure is still instructive. It depicts the "layering" properties of the midlatitude ionosphere for both daytime and nighttime conditions, and these differences are important for system operations. A brief but rather thorough description of the ionosphere and its effects on radiowave propagation is contained within an ITU-R handbook [ITU-R, 1998]. The interaction of radiowaves with the ionosphere depends upon the radio frequency employed as well as the details of the ion and electron density distributions that may be encountered. The interactions are complex, especially at the lowest frequencies, and the governing relationships involving refractive index are embodied in the Appleton-Hartree formalism described in many texts, including Davies [1991]. The ionosphere is immersed in a magnetic field and exhibits the following properties in connection with radiowave propagation:

■ Dispersion: The index of refraction is a function of frequency, and the group velocity is not necessarily equal to the phase velocity.

■ Absorption: The ionospheric refractive index is complex, having real and imaginary parts. The absorption is always dissipative and represents a conversion of wave energy into heat through the collision process.

■ Birefringence: The index of refraction has two distinct values, owing to the presence of the uniform geomagnetic field and free electron mobility. This property suggests the possibility of two ray paths, each characterized by different phase and group velocities.

■ Anisotropy: Each of the two indices of refraction is a separate function of the orientation of the normal to the surface of constant wave phase with respect to the background (uniform) geomagnetic field.

Dispersion and absorption will exist even in the absence of the earth's magnetic field, but its presence leads to the last two properties, birefringence and anisotropy. The Faraday effect is the most prominent phenomenon that results from birefringence, and it has been long exploited as a scheme to deduce the total electron content (TEC) of the ionosphere. One obvious distinction between the ionosphere and the underlying troposphere is the manner in which radiowaves interact with the respective regions. The ionosphere exhibits a frequency-dependent index of refraction that is less than unity, whereas the troposphere possesses an index that is frequency-independent but greater than unity. Furthermore, we note that the absolute value of the atmospheric index is generally greatest at the surface where the gas density is greatest, and it exhibits a roughly exponential decay with altitude (see Bean et al. [1971]). The absolute value of the ionospheric component, on the other hand, is virtually zero below an altitude of 60-70 kilometers, and rises to a maximum at the peak of electron concentration in the ionosphere, which typically occurs in the range of 250-400 km. The departure of the atmospheric index from unity is quite limited in comparison with the ionospheric index, especially for frequencies at VHF and below. As a consequence, we may generally concentrate on ionospheric interactions in connection with radiowave propagation below, say, 300 MHz. Between 300 MHz and 1 GHz, the effects are competing, and the dominant radiowave interactions depend critically upon the system application and geometrical situation involved. This does not mean that ionospheric effects may be ignored in comparison with tropospheric effects if frequencies in the GHz regime or higher are employed. This is because a proper accounting must be taken of the path lengths involved. Indeed, ionospheric ray trajectories are typically much larger than their tropospheric counterparts. Furthermore, we must also consider the presence of refractive index inhomogeneities, which will give rise to a different class of effects broadly classified as scintillation. In the ionosphere these refractive index inhomogeneities are directly related to irregularities in the free electron number density.

The ionospheric effects on radiowave systems may be characterized in a number of ways, depending upon the focus of the treatment. Popular breakouts may organize the effects in terms of system type, medium properties, or frequency band. Our plan is to organize the discussion into two broad groups of ionospheric effects: terrestrial systems and earth-space systems. Frequency issues are considered within each group. Table 4-1 is a listing of the radio bands, the frequency range, the wavelength range, and the primary modes of propagation. This is followed by Table 4-2, which shows the primary uses of the specified bands.

Table 4-1 : Radio Bands and Primary Propagation Modes

Band

Frequency

Wavelength

Primary Modes

ELF

<3 kHz

>100 km

Waveguide Groundwave

VLF

3-30 kHz

100-10 km

Waveguide Groundwave

LF

30-300 kHz

10-1 km

Waveguide Groundwave

MF

300-3000 kHz

1000-100 m

Groundwave Skywave (E)

HF

3-30 MHz

100-10 m

Groundwave Skywave (E,Es,F1,F2)

VHF

30-300 MHz

10-1 m

LOS Meteor Scatter Es Scatter

UHF

300-3000 MHz

1000-100 mm

LOS

SHF

3-30 GHz

100-10 mm

LOS Troposcatter

EHF

30-300 GHz

10-1 mm

LOS

The full array of ionospheric effects may be organized in terms of specified radio frequency regimes, and these regimes, in turn, may be largely identified with certain propagation "modes." For example, at the low end of the spectrum (viz., ELF and VLF), we may associate the effects with propagation within an effective waveguide bounded by the earth below and the ionosphere above. There is some penetration of the ionosphere at ELF because of the enormous wavelengths involved, but details of ionospheric layering are effectively disguised. Just above the ELF band, at VLF, the penetration of the wave is reduced but lower ionospheric structures become more important. The ionospheric impact gradually increases as we proceed upward within the longwave part of the spectrum. On the other hand, at the high end of the considered radio spectrum (viz., SHF), the effects are associated with the so-called earth-space mode in which penetration of the sensible ionosphere is complete. For both ELF and SHF, the ionosphere has a definable but limited role. Between these two extremes, centered at HF, one encounters the most intense ionosphere effects; these effects are associated with skywave modes otherwise termed ionospheric-reflected or refracted modes. We now will sketch out the main effects in more detail beginning with longwaves.

Table 4-2: Utilization of the Radio Bands

Frequency Band

Uses

VLF

Navigation Standard Frequency Standard Time

LF

Broadcasting Navigation

MF

AM Broadcasting

HF

Communication WWV, WVVVH OTH Radar Direction Finding Systems Amateur Service Citizens

VHF

Television FM Broadcasting Aviation Communication

UHF

Satellite Communication Radar Surveillance Satellite Navigation and Timing Television

SHF

Satellite Communication Radar Navigation Television

0 0

Post a comment