The D region is responsible for most of the absorption encountered by HF signals, which exploit the skywave mode. In most instances, D region absorption is a primary factor in the determination of the lowest frequency that is useful for communication over a fixed skywave circuit. In addition, the D region supports longwave propagation at VLF and LF, and the medium is exploited in certain legacy navigation systems and strategic low-rate communication systems. The sounder method as described in the previous section is not useful for measurement of the D region since the electron densities are relatively low. Details of D region electron concentration are sketchy in comparison with information available about the E and F regions, principally because of the difficulty in making diagnostic measurements. Moreover, analysis is hampered because many photochemical processes with poorly defined reaction rates take place in the D region. Over 100 reactions have been compiled.
Table 3-1 shows that the D region lies between 70 and 90 Km. In fact, the upper and lower levels are not precisely defined. It is evident that more than one source of ionization gives rise to the D region electron density distribution. Sources include solar radiation at the upper levels and galactic cosmic rays at lower levels. In addition, relatively rare polar-cap absorption events (i.e., PCA) are characterized by highly energetic solar protons that provide an additional source for ionization of the lower D region within the polar cap. Some investigators place the lower boundary of the D region at 50 Km to account for the contribution of galactic cosmic rays in the neighborhood of 50-70 Km. This altitude regime, termed the C region, is not produced by solar radiation. It exhibits different characteristics from the region between 70 and 90 Km. Specifically, a minimum in electron concentration is observed during solar maximum for the lower portion (viz., region C) while the reverse is true in the upper portion (viz., region D). This can be explained if we assume that the galactic cosmic ray source is partially diverted from the earth by an increase in the interplanetary magnetic field (IMF), which generally occurs during solar maximum conditions.
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