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Noun1.D-layer - the lowest region of the ionosphere (35 to 50 miles up) that reflects low-frequency radio waves
ionosphere - the outer region of the Earth's atmosphere; contains a high concentration of free electrons
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On removal of D-layer (water table depth layer) a visual increase in the vulnerability index of the central part of the study area has been observed and is shown in Fig.
Equation (b) has been used, as a profile of the E region, to separate D-layer and E-layer absorptions.
At very low frequencies (VLF, generally below 30kHz) it is the ionospheric D-layer, between 50 to 90km in altitude, which is responsible for long distance radio propagation, with the X-ray and high energy UV from the Sun providing the ionising energy.
Mark calculates that this flare pushed the ionosphere D-layer downwards by about 4km.