Fringe region
From Glossary of Meteorology
fringe region
(Also called spray region.) The upper portion of the exosphere, where the cone of escape equals or exceeds 180°.
In this region the individual atoms have so little chance of collision that they essentially travel in free orbits, subject to the earth's gravitation, at speeds imparted by the last collision.
See critical level of escape.
See critical level of escape.