D-layer

The Ionosphere's D-layer also affects 80 meters significantly by absorbing signals.

The lowest usable frequency depends on the absorption in the lower layer of the ionosphere (the D-layer).

The effect of this is that for a short time the radiation builds up an absorbing layer from the bottom of the D-layer upwards.

This signal strength assessment was made according to the reflection model with a reflection (one hop) on the ionospheric D-layer.

In reality, the electron density of the D-layer increases with altitude, and the wave is bounded as shown in Figure 2.

AM transmissions cannot be ionospherically propagated during the day due to strong absorption in the D-layer of the ionosphere.

In the case of the ionospheric D-layer, it depends, in addition, on time of day, season, latitude, and the geomagnetic field in a complicated manner.

At low altitudes (such as the D-layer) the pressure is such that when the main ionising source is removed (sunset) the ions rapidly recombine.

This means that a vertically polarized incident wave after reflection at the ionospheric D-layer converses to a vertically and a horizontally polarized wave.

It turns out that the radiation reaching the D-layer is mainly near-UV and it only has enough energy to ionise oxides of nitrogen, but not air molecules themselves.

The region between Earth's surface and the ionospheric D-layer behaves thus like a waveguide for VLF- and ELF-waves.

The daytime enhancement due to ions (electrons) injected into the D-layer during geomagnetic storms can provide a stable path enhanced by up to 10dB during the middle of the day.

The sodium emissions come from a thin sodium layer approximately 10 km thick at an altitude of 90 - 100 km, above the mesopause and in the D-layer of the ionosphere.

In the VLF range, the transfer function is the sum of a ground wave which arrives directly at the receiver and multihop sky waves reflected at the ionospheric D-layer (Figure 1).

At night, the ionisation of the D-layer normally decays as the layer moves into shadow, this leaves a reflection height at 90 to 100kms altitude with little absorbing material for the signal to pass through.

This is associated with a significant decrease in the signal strength and depends on many factors, e.g., the daytime and season, the angle of incidence of the skywave on the D-layer and the solar activity.

The D-layer, when present during sunlight periods, causes significant amount of signal loss, as does the E-layer whose maximum usable frequency can rise to 4 MHz and above and thus block higher frequency signals from reaching the F2-layer.

At mid-latitudes, HF communications are disrupted by solar radio bursts, by X-rays from solar flares (which enhance and disturb the ionospheric D-layer) and by TEC enhancements and irregularities during major geomagnetic storms.

As the maximum usable frequency for long distance communication seldom dips below 3.5 MHz anywhere on the planet, the main propagation barrier to long distance communication is heavy D-layer absorption during the daytime, ensuring that DX paths must be largely, although not entirely, in darkness.