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ANNUAL REPORT SMITHSONIAN INSTITUTION,, 1942 



followed by 1 day the highest value of sunspottednebs, and that on the 

 average 1 to 2 days after the auroral displays occurred, the weakest 

 field strengths on the 770 kc. frequency were recorded. Since waves 

 in the broadcast band are returned from the E layer of the ionosphere, 

 it would appear that there is a definite lag of from 24 to 48 hours be- 

 tween the disturbances in the auroral zone and the greatest deteriora- 

 tion (ionization) in the E layer. Had we similar measurements of 

 field strengths at higher frequencies representing waves returning from 

 the F layer, we might expect a similar effect to occur at an interval 

 intermediate between the time of maximum auroral display and the 

 time of minimum field strengths from the E layer. Such field-strength 

 measurements are not available, but fortunately through the courtesy 

 of the Bell Telephone Laboratories we had available a record of their 



Figure 6.— Seasonal trend of radio field strengths (night) of WBBM (770 kc.) 

 as received at Boston. Values here represented are not corrected for twilight 

 effect, which depends upon angle of depression of the sun below the horizon 

 during periods of observation. 



transmission disturbances over oceanic paths. Taking the reciprocal 

 of these transmission-disturbance numbers, we have an index of trans- 

 mission conditions comparable to the field-strength measurements in 

 the broadcast band. A plot of these transmission disturbances for re- 

 ception from the F layer indicated a lag of roughly 12 hours after the 

 auroral displays for the minimum transmission conditions. This pro- 

 vides perhaps as clear a confirmation as could have been anticipated for 

 ionization disturbances occurring more promptly in the F layer than 



