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BELL SYSTEM TECHNICAL JOURNAL 



II NOON 



EASTERN STANDARD TIME 



Fig. 3 — Critical ionization frequencies during the partial eclipse of February 3, 1935 



was a bona fide eclipse effect, as the decrease began within a few min- 

 utes after the first contact, and the density attained its lowest value 

 shortly after the maximum of the eclipse and recovered to a more or 

 less constant higher value a few minutes after the last contact. At 

 no time during these measurements on the eclipse day or the days 

 after was there any other variation of a comparable magnitude. 



These results therefore indicate that ultraviolet light ^ is an impor- 

 tant ionizing agency in the E, M, Fi and F2 regions of the ionosphere. 



' While ultraviolet light is probably the ionizing agency responsible for the eflfects 

 noted, any other solar emanation which travels substantially at the velocity of light, 

 should not be precluded from consideration. See E. A. W. Miiller, Nature, Febru- 

 ary 2, 1935, who suggests Roentgen type radiation. 



