Sporadic E

This presentation was given online to the Mid Sussex Amateur Radio Society on the 23rd September 2022. It gives an overview of Sporadic E propagation. The texts and pages below elaborate. Click the titles or links for more.

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 ionospheric phenomena. Sporadic E supports anything from upper HF signals to upper VHF/lower UHF signals. Read more.

Ablated meteors and plasma

The Earth is constantly being bombarded by chunks of rock of various sizes from grains to fist sized rocks. The number of meteors incoming per day averages 8,000. Rock atoms are ablated or stripped off from the meteors and join the normal atoms to make an atmospheric soup or plasma. The sun beats down on the plasma. This energy input to the plasma causes ionisation. Useful ionisation occurs at around 125km up, with more stable ionisation at about 100km. This whole process is key to Es. Read more.

Tides, winds and atmospheric gravity waves

The basic process of meteor descent is insufficient to support VHF propagation – and certainly insufficient to cause Es. Something special is needed to densify the ionosphere at about 100km. Something else is needed to explain how Es comes about. And it’s all about nodes. Densified nodes of plasma enable E region openings at VHF in the early morning, afternoon from mid-day to about 9pm, and early morning around 3am. But several phenomena from time to time insert chaos into the stable structure turning plain old E into Es. Read more.

Earth’s magnetic field disturbance

If there was only node creation going on, propagation would be more E and less Es. There are several mechanisms that break up the higher electron density nodes. One mechanism is disturbance in the Earth’s magnetic field. So just when you were looking forward to a great day working Es, along comes a Sun-induced storm that wrecks everything! Here’s how it happens.

Patches and timings

Sporadic E propagation results from nodes of plasma (ionised gas) set up in the E region of the ionosphere. There, existing ions of helium and hydrogen are augmented by metal ions from ablated atoms from meteors. These E region high-density nodes are stable in the short term and support refraction of waves in the VHF bands. Occurrence of nodes of sufficient electron density is sporadic. Read more.

Probability statements and prediction

Sporadic E propagation is termed ‘sporadic’ for a reason! The very word means erratic, patchy or random. So, I’d argue that anyone claiming that they can predict when this erratic propagation mode will occur is misguided. My discussion here illustrates how one must talk about Es – in terms of probability.