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The Kennelly–Heaviside layer, named after Arthur E. Kennelly and Oliver Heaviside, also known as the E region or simply the Heaviside layer, is a layer of ionised gas occurring between roughly 90–150 km (56–93 mi) above the ground — one of several layers in the Earth's ionosphere. It reflects medium-frequency radio waves, and because of this reflection, radio waves can be propagated beyond the horizon.
Propagation is affected by time of day. During the daytime the solar wind presses this layer closer to the Earth, thereby limiting how far it can reflect radio waves. Conversely, on the night (lee) side of the Earth, the solar wind drags the ionosphere further away, thereby greatly increasing the range which radio waves can travel by reflection, called skywave. The extent of the effect is further influenced by the season, and the amount of sunspot activity.
Its existence was predicted in 1902 independently and almost simultaneously by the American electrical engineer Arthur Edwin Kennelly (1861–1939) and the British physicist Oliver Heaviside (1850–1925). However, it was not until 1924 that its existence was shown by British scientist Edward V. Appleton, for which he received the 1947 Nobel Prize in Physics. Physicists resisted the idea of the reflecting layer for one very good reason. It would require total internal reflection, which in turn would require that the speed of light in the ionosphere would be greater than in the atmosphere below it. But the latter speed is essentially the same as the speed of light in vacuum, so how could the speed in the ionosphere possibly be higher. It was one of those situations where the more physics theory you knew, the surer you were that it couldn't happen. And yet Marconi had received signals in Newfoundland that were broadcast in England! The paradox was resolved by the discovery that there were two velocities of light, the phase velocity and the group velocity. The phase velocity can in fact be greater than c, but the group velocity, being capable of transmitting information, cannot, by Special relativity, be greater than c. The phase velocity for radio waves in the ionosphere is indeed greater than c, and that makes total internal reflection possible, and so the ionosphere can reflect radio waves. The geometric mean of the phase velocity and the group velocity cannot exceed c, so when the phase velocity goes above c, the group velocity must go below it. In 1925, Americans Gregory Breit and Merle A. Tuve first mapped its variations in altitude.
The "Heaviside layer" is used as a symbol for heaven (in the afterlife sense) in Andrew Lloyd Webber's musical Cats. This reference is based on a quotation found in a letter written by T. S. Eliot, whose book Old Possum's Book of Practical Cats forms the basis of the musical. In the musical, one cat is chosen each year by Old Deuteronomy to go to the Heaviside Layer and begin a new life. In the song "The Journey to the Heaviside Layer", it is stated that the Heaviside Layer is "past the Russell Hotel" and "past the Jellicle moon", indicating that it is very far away and difficult to access.
In the end of the musical, Grizabella is chosen to go the Heaviside Layer. She does so by ascending on a flying tire until she reaches a structure resembling clouds, into which she disappears, although in the film version of Cats she ascends with Old Deuteronomy and then she walks up a giant metal hand to the Heaviside layer.
Eliot also refers to the "Heaviside Layer" in his play The Family Reunion which explores issues around the afterlife, heaven and hell.