Long-range communications using high frequency (HF) radio waves (3 - 30 MHz) depend on reflection of the signals in the ionosphere. Radio waves are typically reflected near the peak of the F2 layer (~300 km altitude), but along the path to the F2 peak and back the radio wave signal suffers attenuation due to absorption by the intervening ionosphere.
Absorption is the process by which the energy of radio waves is converted into heat and electromagnetic (EM) noise through interactions between the radio wave, ionospheric electrons, and the neutral atmosphere (for a more extensive description of the absorption process see Davies, 1990). Most of the absorption occurs in the ionospheric D region (50–90 km altitude) where the product of the electron density and the electron-neutral collision frequency attains a maximum. Within this region the neutral density is relatively constant over time, so variations in the local electron density drive the total amount of absorption. The electron density is a function of many parameters and normally varies with local time, latitude, season, and over the solar cycle. These "natural" changes are predictable, and affect absorption only moderately at the lowest HF frequencies. Much more significant changes to the absorption strength are seen as a result of sudden increases of electron density in the D region (the classic short wave fade) due to, for example, solar X-ray flares on the dayside or solar proton precipitation in the polar regions.
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