Geocorona The term "geocorona" refers to the solar far-ultraviolet light that is reflected off the cloud of neutral hydrogen atoms that surrounds the Earth (see figure ). This cloud of neutral hydrogen--known as the exosphere--is the extremely tenuous extension of the Earth's neutral atmosphere into space, with densities ranging from a thousand or so atoms per cubic centimeter at the inner edge of the ring current to less than a hundred at geosynchronous orbit (6.6 Earth radii ). Solar far-ultraviolet photons scattered by exospheric hydrogen have been observed out to a distance of approximately 100,000 km (~15.5 Earth radii) from Earth. The theoretical outer boundary of the exosphere lies roughly another 100,000 km beyond this, at ~31 Earth radii, the distance at which the influence of solar radiation pressure on particle velocities exceeds that of the Earth's gravitational pull. Neutral hydrogen densities in this region are on the order of one or less than one atom per cubic centimeter, however, and are too low to be
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