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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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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.
The ground wave tends to follow the Earth's surface, while the sky wave is refracted back down towards the ground by the D-layer ionisation.
Mark calculates that this flare pushed the ionosphere D-layer downwards by about 4km.