E layer

The E layer is the lower main region of the ionosphere, below the F layer. Its free-electron population is supplied largely by solar ultraviolet photoionization and reduced by recombination. Thus its ordinary daytime ionization rises after sunrise and weakens at night, although sporadic enhancements can occur. Its electron population changes the refractive index experienced by a radio wave.

The Appleton-Barnett radio interference experiment supplied direct evidence for this returning path. On 11 December 1924, Edward Victor Appleton and M. A. F. Barnett varied the transmitted frequency at Bournemouth and observed alternating signal maxima and minima at Oxford. Interference between a ground wave and a skywave gave their path difference, implying an equivalent return height about $90\mathrm{km}$. Before dawn, reduced E-region ionization later allowed penetration to the higher F layer.^[ref]

This is a frequency-dependent propagation region, rather than a permanent reflecting material wall. The experiment measures an equivalent path height; refraction, collision losses and wave polarization matter when interpreting it in detail.