Geocoronal Hydrogen

Abstract

Atomic hydrogen is a minor constituent of the atmosphere, created mainly by photodissociation of H₂O and CH₄ between 60 and 100 km of altitude. It diffuses upwards through the thermosphere (Figure 1) until it reaches the upper boundary of the thermosphere, called the exobase or critical level. Atoms which have at this altitude an upward velocity larger than the escape velocity F _esc will leave the earth on hyperbolic trajectories, since they are entering the exosphere which is a collisionless medium. Those with an upward velocity lower than F _esc will return to the exobase after a ballistic flight in the exosphere. Eventually a collision in the exosphere will transfer the atom onto a satellite orbit.