Critical Level of Escape

Critical Level of Escape

That level, in the atmosphere, at which a particle moving rapidly upwards will have a probability of 1/e, where e is base of natural logarithm, of colliding with another particle on its way out of the atmosphere. It is also the level at which the horizontal mean free path of atmospheric particle equals the scale height of the atmosphere. The critical level of escape is the base of the exosphere. Also called level of escape, critical level. See cone of escape, fringe region.
On Earth, estimates of the height of the critical level of escape range from about 500 to 1000 kilometers. This large range of estimated values is due primarily to the general uncertainty about the temperature distribution in the ionosphere.

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use