Absorption

Absorption will arise as a radiowave interacts with electrons in the D and lower E-region of the ionosphere where the collision frequency is relatively large. The electron collisions have the effect of robbing energy from the radiowave and converting it to heat in the interaction region. It may be shown that the absorption in dB can be related to the product of the electron density N and the collision rate v. The formula for absorption follows:

Figure 4-17 shows the variation of electron density and electron collision rate as a function of altitude. During solar flares and PCA events, there is an enhancement in the value of N in the D region. As indicated in Section 4.3.5, absorption can be a significant problem at lower frequencies like HF. Indeed, absorption is the primary control for the lowest propagation band supported by the ionosphere, and an x-ray flare can cause complete disruption of communication for the duration of the flare. In the polar cap region, energetic particle events (i.e., solar cosmic rays or protons) can cause absorption of 100 dB or more. On the other hand, since D-region absorption is inversely proportional to the radio frequency squared, most satellite frequencies (i.e.,/> 250 MHz) are operationally unaffected even during PCA events.

Figure 4-16: Time delay dispersion. The group time delay difference is a measure of the pulse distortion. (From ITU-R, 1996]).

Group Time Delay Difference (us)

Figure 4-16: Time delay dispersion. The group time delay difference is a measure of the pulse distortion. (From ITU-R, 1996]).

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