The Earth is constantly being bombarded by chunks of rock of various sizes from grains to fist sized rocks. But mostly those meteors are tiny.
The number of meteors incoming per day averages 8,000. The highest meteor density occurs in June when about 10,000 discrete meteors per day enter the Earth’s atmosphere in the northern hemisphere. Meteors are always incoming.
The Earth’s spin combined with the gravitational pull on the meteors acts like a giant scoop, sweeping up meteors. The meteors vary in velocity – the greater the velocity, the greater effect on the ionisation. And since the Earth’s linear velocity is greatest at the equator, meteor relative velocity (and hence ionisation) is maximum toward the equator and reduces with latitude.
As the meteors descend towards the surface of the Earth, they collide with the oxygen and hydrogen and other lesser density atoms in the atmosphere. Rock atoms are ablated or stripped off from the meteors and join the normal atoms to make an atmospheric soup or plasma.
The process is shown in the attached diagram.
The sun beats down on the plasma. This energy input to the plasma causes ionisation – electrons are split from their atoms. Normally, when an electron is split off, it simply recombines again quickly as the energy is lost as the atom and electron descend. Since the upper atmosphere comprises various atoms, ions and electrons, distances between atoms and electrons are increased. The recombination is slowed down, allowing the electrons to exist for between hours and days.
Useful ionisation occurs at around 125km up, with more stable ionisation at about 100km. Then as the ions descend, they recombine and it’s all over below 95km. The rock atoms join the normal benign atmosphere as a lesser density component.
This process is normal and as I note elsewhere, the normal electron density of the E region at around 100km is insufficient to support propagation. The F region dominates. Something special must happen to densify the E region and enable sporadic E, and this densification is discussed on other pages.