Reversing Layer

reversing layer [ri′vərs·iŋ ‚lā·ər]

(astrophysics)

A layer of relatively cool gas forming the lower part of the sun's chromosphere, just above the photosphere, that gives rise to absorption lines in the sun's spectrum.

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Reversing Layer 

a thin layer of the solar atmosphere that is about 300 km in depth. It is in the reversing layer that the absorption lines in the solar spectrum are formed. The lines arise because the energy radiating outward from the photosphere below, where the continuous spectrum is produced, is absorbed at certain frequencies by atoms of the chemical elements found in the reversing layer. The energy is then reradiated in all directions, and, as a result, the outgoing stream of radiation is attenuated.

The division of the lower layers of the solar atmosphere into the photosphere and the reversing layer is somewhat arbitrary. This is because a certain amount of the energy in the continuous spectrum of the sun is emitted in the reversing layer, and absorption lines partially originate in the photosphere.