You will see several mentions of Sporadic-E on this site, so I’ve taken a shot here at explaining the phenomenon in my own words. I hope you enjoy the tangent from the usual posts, but if you are already versed in solar physics feel free to skip along- wait maybe you won’t get that reference unless you read this bit… 🤔
Sporadic E events- a phenomenon frequently observed in the early 90s as the increased frequency of the Sun’s coronal mass ejections created observable disturbances in the Earth’s ionosphere.
The phenomena occurs naturally every 11-year cycle, waxing and waning between the peaks- known as the solar maximum– during these periods, the most sunspots are produced.
Sunspots are a product of the complex interactions between the suns powerful magnetic fields and the fusion reactions occurring deep within the core of the sun. Magnetic fields twist and contort through the plasma as the sun rotates on its axis, eventually some of these powerful magnetic fields can no longer hold on to their couplings and break apart in violent and destructive solar flares, scattering the sun’s plasma deep into space. Sometimes, directly at Earth.
Powerful magnetic interactions during solar flare events also cause magnetic disturbances that travel at the speed of light towards Earth. These are known as geomagnetic storms and can in the most extreme cases cause severe problems for the electrical grids that deliver power to our industry and homes, as well as damage satellites that operate out beyond the protection of our magnetic field.
Approximately three days after a geomagnetic storm, the sub-light-speed particles ejected from the sun arrive at Earth and hit the magnetic field, and if powerful enough can sometimes reach the upper ionosphere. These particles are extremely high-energy, if they were to reach the surface of Earth they would have a devastating and deadly effect on biological organisms.
The magnetic field of Earth is generated by the planet’s liquid core- much like a bicycle dynamo, the magnetic interaction though rotation of the different densities of liquid metal in the core produce strong magnetic fields that protrude out from the north and south poles, enveloping the Earth with a magnetic shield.
As the sun shines, it also emits solar radiation in the form of a constant stream of charged particles- this ambient bombardment is known as the solar wind.
The solar wind speed increases dramatically during sunspot activity, putting greater pressure on Earth’s magnetic envelope. As the magnetic field interacts with the solar wind, particles billow around the envelope towards the poles- the weakest points of the magnetic field- where the charged particles interact with the oxygen and nitrogen gas in the upper atmosphere- this can be observed visually as the Aurora Borealis and Aurora Australis.
Sporadic-E is the name designated for the unpredictable property of the ionosphere when the solar wind travels deep and at high enough energy through the ionosphere that it causes a plasma to occur. This plasma is conductive, and therefore capable of reflecting radio frequency back to the ground.
This is how high frequency (HF) radio waves that would otherwise be absorbed by the ambient ionosphere propagate great distances beyond line of sight. Although the radio waves always travel in a straight line from the source, they can bounce off the sporadic plasma and back towards the ground. In combination with the ocean, this can sometimes happen multiple times before the radio wave is intercepted by a radio receiver. This is colloquially known as radio skip.
Other forms of radio propagation such as Tropospheric Ducting affect VHF and higher frequencies.
For the most part the F1 and F2 layers form from the separation of the radio-absorbing D layer at night when the solar wind reduces pressure on the magnetic field facing away from the sun, as Earth rotates from day to night- this allows shortwave to propagate further which is why distant radio stations can only be heard coming through in the evening hours.
Sporadic-E layers of the ionosphere are responsible for the majority of radio propagation phenomenon at upper-HF frequencies, such frequencies as the common CEPT and UK CB radio specification and the 10, 6 and 4 meter amateur radio band.