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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.

The ionosphere is a region of the upper atmosphere of Earth with substantial partial ionization. Its mobile electrons and ions constitute a plasma mixed with neutral gas. For radio communication, the crucial question is whether a chosen radio wave returns toward the ground, is absorbed, or passes through. A skywave can connect stations beyond their direct view across the curved surface of Earth, while wave interference between a ground wave and a skywave can make reception fade even when the radio transmitter remains steady.

The ionosphere is useful both as a route for radio communication and as a remote laboratory: varying the probing frequency changes how deeply the radio wave penetrates the plasma. The Appleton-Barnett radio interference experiment turned an unwanted fluctuation in reception into a measurement of the overhead route. Cold-plasma radio dispersion explains why this is a tunable medium, while ionospheric virtual height explains why a measured delay is not automatically an altitude.

A physicist whose work with Edward Victor Appleton tested the return of radio waves from the upper atmosphere. Their experiments supplied evidence for reflecting ionized layers and distinguished returns from different heights. The Appleton-Barnett radio interference experiment thus connects an observed ground-level interference signal to the structure of the ionosphere, explaining how radio communication can reach beyond the direct line of sight.^[ref]