 E layer (source code)

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= E layer
{c}

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>.<The Ionosphere by Edward Victor Appleton>[Printed pp. 81–84: 1924 interference experiment, upper layer and measured daily ionization variation]{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.

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