ECLIPSE EFFECTS IN THE IONOSPHERE 165 



there are in effect 0-component curves for four different frequencies 

 available for plotting the contour map. The dotted lines represent re- 

 gions of maximum ionization. These curves show in a rather striking 

 manner the sharp decrease in ionization of the E and Fi regions near 

 the time of the eclipse maximum. 



The uncertainty of the 1932 results as regards the F'2 region led us 

 to concentrate our efforts on this region during the 1935 eclipse. Im- 

 proved technique now made it possible to measure the critical ioniza- 

 tion frequencies directly instead of makmg virtual height measure- 

 ments on fixed frequencies as had been done during the 1932 eclipse. 

 The critical frequencies for the E and M regions were measured in 

 addition to those for the F2 region. 



W^e found that this eclipse was accompanied by a decrease in the 

 maximum ionic density in all three regions and that the minimum ioni- 

 zation occurred at or very shortly after the eclipse maximum. The 

 percentage decrease was progressively greater from the lowest to the 

 highest region, being approximately twenty per cent for the E region, 

 twenty-two per cent for the M region, and twenty-five per cent for 

 the F2 region. 



Some such progressive change might be expected from the fact that 

 the eclipse had a magnitude of forty per cent at the ground and ap- 

 proximately forty-three per cent in the F2 region (250 kilometers over 

 Deal). These magnitudes are in terms of the sun's diameter, and on 

 the basis of eclipsed area these figures become twenty-nine and thirty- 

 one per cent, respectively. 



Figure 3 gives the critical ionization frequencies for the three days 

 on which data were obtained. The curves for the E and M regions 

 are for the O-component while the curves for the F2 region are for the 

 X-component. For the 0-component, the ionic density, N, is pro- 

 portional to /c^ where fc is the critical ionization frequency, while for 

 the X-component curve in Fig. 3, iV, is proportional to {fc — 750 kc.).' 



The decreases in ionic density of the various regions may be com- 

 pared with a fifty to sixty per cent decrease in the E region ionization 

 during the eclipse of August 31, 1932, when the eclipse magnitude was 

 ninety-five to one hundred per cent. 



The 1935 measurements give a more definite synchronism than those 

 of 1932, between the eclipse occurrence and the time of decrease in 

 ionic density 'of the F2 region. 



In view of the variable nature of the F2 region, it is a possibility 

 that the decrease in ionic density at the time of the eclipse was a mere 

 coincidence and was actually due to some noneclipse agency. We be- 

 lieve that this was not the case and that the decrease in F2 ionization 



