The return of radio waves from the middle atmosphere—I

Abstract

One of us introduced the name “Ionosphere” to designate that region of the upper atmosphere of which the most prominent physical characteristic was the occurrence of sustained high ionization densities, and which was, in consequence, of fundamental significance in the propagation of radio waves. The name, after finding its way into many languages, has been formally adopted by the Union Radio Scientifique Internationale for international use, and is now commonly applied to the region of the atmosphere above the first 90 km. It is an object of this present paper to show that this sharp distinction, although very broadly justified, is less happy than might have been hoped. It has been customary to regard the return of radio waves of measurable intensity from regions sensibly below 90-100 km. as very improbable in any save exceptional conditions, the collisional damping at lower levels being believed to ensure severe attenuation of such waves as might otherwise be returned from any temporarily densely ionized regions at moderate levels, while at still lower levels the rate of recombination seemed likely to prevent the maintenance of substantial ionization densities.