Background to Sporadic E

Sporadic E, or perhaps “occasional VHF propagation via the E region of the ionosphere as a result of extra ionisation from ablated metal atoms heated by the Sun”, is one of many phenomena. Ablated here just means ‘stripped off’.

There are three latitudes of effect worth noting and differentiating. The following describes what happens in the northern hemisphere. The same but obverse applies in the southern.

My focus here – the mid-latitudes

Generally, the Earth rotates and scoops up incoming meteors. The linear velocity is greater toward the equator and more meteors couple with the Earth’s atmosphere when compared to the north.

First, from the equator to about 23 degrees north, the Sun’s radiation beats down on the ionosphere. Both E and F region are ionised, and both will, as a result, support HF and sometimes VHF propagation. And atoms are ablated from those meteors, which when ionised, add greatly to the already high equatorial electron density further supporting VHF propagation. Propagation here is described as via Equatorial Es.

Second, at higher latitudes (north of 66N, say), any contribution from the Sun toward ionisation on the E and F regions is muted. Likewise, any contribution to polar electron density from ablated metal ions from meteors is muted. But from time-to-time plasma in the solar wind transits from the poles southward following the geomagnetic field. This plasma may enhance E region ionisation. Enhanced ionisation from ablated ions is muted. If the solar wind is active enough, it may on occasion go on to set up a radio aurora.  Propagation here is described as being by Auroral Es.

Finally, and more relevantly to our discussion here, we have the mid-way effect between about 23˚ north and 66˚ north. Here, atoms are ablated, just as in other latitudes and the resulting atoms are ionised by the Sun. By a series of mechanisms described elsewhere in this topic, and unique to those mid-latitudes, the E region electron density is occasionally enhanced. This promotes an otherwise marginal electron density to a density sufficient to support VHF propagation. Propagation here is described as being by plain Sporadic E, or Es.

In essence, there’s a time sequence with three parts to Sporadic E.

  • Meteors enter the Earth’s atmosphere. About 8,000 meteors per day enter – more at peak times.
  • High up in the ionosphere, atoms from those meteors are ablated or stripped off and then ionised.
  • And, if several mechanisms coalesce, enhanced electron density occurs further down in the E region of the ionosphere.

This electron density is sufficient to support VHF propagation. For reference the E region, or layer since it’s thin, is at about 100km up.

Now, just to note. Whilst I talk here of VHF propagation, Sporadic E supports anything from upper HF signals to upper VHF/lower UHF signals. The following describes the incidence of Sporadic E with frequency and time.

Es more frequently supports mid-VHF frequencies

A normal frequency distribution is shown. This is used just to illustrate that mostly Es supports mid-VHF frequencies. Higher VHF and higher HF frequencies are supported less often. The distribution may or may not be normal or bell-shaped.