ANALYSIS OF THE IONOSPHERE 487 



recapture by the molecules.) Of this theory it may be said that the 

 major facts confirm it, though at night the E-ionization persists so 

 tenaciously that we are obliged to seek for a separate agent (meteors, 

 perhaps?) ; while numerous minor discrepancies can be explained away 

 by making special assumptions which can neither be confirmed nor 

 refuted because there is so little independent knowledge of the upper 

 atmosphere. Not a very satisfactory situation for the present, but 

 at any rate one which offers endless promise ! 



Thus it can readily be seen that as the ionizing light descends from 

 heaven through the upper air, the ionization per unit volume should 

 at first increase (because the air is getting denser) and then decrease 

 (because the light is getting to be used up). This offers an explanation 

 for a layer; and the mathematical working-out of the idea — due in 

 the main to Chapman — shows that not only the existence but the shape 

 of either peak in the curve of Fig. 3A is compatible with the theory. 

 But there are several layers and peaks, not just one; how does this 

 come about? Well, the atmosphere is a mixture of several gases, 

 differently susceptible to the ionizing light; one can attribute a peak 

 to each gas (indeed more than one to a gas, by invoking different 

 states of the molecules). The height of a peak, the iV- value at the 

 crown of a layer, should rise and fall as the sun rises and sinks in the 

 sky. This is true of the layers E and Fi, as we saw from Fig. 11, and 

 again there is a quantitative theory by Chapman, which is borne out 

 in some though not in full detail. Of F2 it is not always true, as 

 Fig. 11 proclaims; there is a minimum at noon in summer, and the 

 highest iV- values of all are attained in winter! One tries to cope with 

 the discrepancy by assuming that as the sun climbs higher in the sky, 

 the F2 region expands so much in the heat that although the total 

 number in the region is properly increasing, the number in unit volume 

 suffers a decline. The layers do not disappear at night, though the 

 A^-values shrink. There seems to be plenty of time for recapture of 

 all the electrons between sunset and sunrise, and one is driven to 

 hunt for other causes of ionization which emerge when the sunlight 

 is gone. These remain mysterious. Inrush of meteors into the high 

 atmosphere has been suggested as one of the causes, and also incessant 

 streams of charged particles similar to those which become intense 

 during magnetic storms. 



Sunlight is therefore not the only, yet apparently the major factor 

 in maintaining the ionosphere. Not, however, any sunlight that we 

 ever feel! This portion of the sun's outpourings is so thoroughly 

 consumed above that it never reaches down to the levels where we live. 

 Were it not so consumed, we should not be able to communicate by 



