Sporadic-E

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Normally, radio signals above the normal cut-off frequency of the E-layer ( red ) pass through it. During a sporadic E-event, the signals ( blue ) are either completely or partially broken in the E-layer, which worsens long-range connections, but leads to better reception within the first jump zone or dead zone .

Cloud- like areas of high ionization in the ionosphere at the level of the E-layer are referred to as sporadic E-layer (Sporadic-E, E s ) . The sporadic E-layer occurs irregularly and unpredictably, but preferably in the months May to July (in the northern hemisphere) and during the day, i.e. at times of generally relatively high ionization. Wind shear at the corresponding altitude and metal ingress from meteors are discussed as causes for the formation of E s .

The effects on shortwave radio traffic are as follows:

  • At high frequencies, on which no (or only weak) signals can be heard under normal conditions, suddenly very strong signals appear, preferably from a distance of a few hundred km up to 2300 km (first hop),
  • At medium frequencies, Sporadic-E leads to a reduction or disappearance of the so-called dead zone (inaccessible area between ground and sky waves).

An increase in ionization caused by Sporadic-E means that at high HF frequencies, transmitters can also be received at a shorter distance. This reduction in the dead zone is equivalent to an increase in the maximum usable frequency ( MUF ). In the frequency ranges above 30 MHz one speaks of overreach. However, the higher the frequency, the less likely there are useful propagation conditions. A few times a year z. B. the VHF radio band at 100 MHz, or the 2 m amateur radio band at 144 MHz, affected.

While Sporadic-E brings a welcome increase in range for radio amateurs at 50 MHz and 144 MHz, it is an annoying natural phenomenon from the point of view of professional radio services and broadcasters. The radio networks, which are usually dimensioned for a defined coverage area, interfere with each other when Sporadic-E occurs.

This type of propagation should not be confused with tropospheric overreaches in inversion weather conditions in the range above 50 MHz.

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Individual evidence

  1. ↑ https://www.researchgate.net/publication/227207773_A_Tutorial_Review_on_Sporadic_E_Layers