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 detected. The exosphere's lower boundary is termed the critical level or "exobase". It is conventionally placed at an altitude of 500 km. At and below this altitude, the atmosphere is sufficiently dense that collisions dominate the motion of the gas molecules and atoms; above the exobase, on the other hand, collisions are so infrequent that atoms moving with sufficient velocity have a high probablility of escaping from the Earth's gravitational field into interplanetary space. Escaping atoms constitute only a portion of the exospheric hydrogen, however. There is also a gravitationally bound component that consists both of atoms following ballistic trajectories and of "satellite" atoms that orbit the Earth for some period of time before returning to the denser atmosphere.

The density and structure of the exosphere are influenced by a number of factors: variations in the temperature and density of the atmosphere below the exobase, photoionization and ionization by impact with solar wind particles, charge exchange with the plasma of the plasmasphere, and radiation pressure exerted by solar far-ultraviolet photons. This last influence, solar radiation pressure, creates the exosphere's satellite component and pushes the exospheric hydrogen away from the Earth in an antisunward direction to form a "geotail" of neutral hydrogen.

The exosphere plays an important role in the plasma budget of the Earth's inner magnetosphere as a "sink" for ring current charged particles, because charge exchange with exospheric neutral hydrogen is the principal mechanism by which the ring current loses plasma and can return to its "ground state" following a geomagnetic storm . The energetic neutral atoms created by this process can be imaged to construct global pictures of the ring current as it grows and decays in response to changing magnetospheric conditions.