The extremely tenuous cloud of neutral hydrogen that surrounds the Earth to a distance of perhaps 1–2 Earth radii. It originates in the upper atmosphere, where water molecules are dissociated into hydrogen and oxygen atoms by the action of sunlight. Scattering of solar radiation by the geocorona interferes with ultraviolet observations at the Lyman-α‎ wavelength. The geocorona can be regarded as an extension of the exosphere.