80LAR EADIATION AND THE STATE OF THE 

 ATMOSPHERE ' 



By H\KL.^N True Stetson 

 Alassachusetts Institute of Technology 



[With 4 plates J 



For a week before the great aurora of September 18, 1941, with the 

 attendant wide-spread disturbances to radio communication, the snn 

 had been heralding the event. Although the sunspot cycle has been 

 definitely on the wane since 1937, the week preceding the 18th pre- 

 sented one of the largest groups of spots seen in recent years, so con- 

 spicuous, in fact, that it was visible to the naked eye through suitable 

 dark glasses. The forecasting of interruptions to radio communication 

 is of increasing significance in these days of national defense, but the 

 ability to foresee communication interruptions has been a recent residt 

 of intersified interest in the stduy of relationships between solar dis- 

 turbances and terrestrial atmospheric conditions. 



The sun is a typical star of rather ordinary proportions, but because 

 of our proximity to it, the sun is the one star upon whose radiations 

 activities on the earth are unalterably dependent. Military campaigns 

 as well as agriculture since the dawn of civilization must be governed 

 by the seasons for which the changing declination of the sun is the 

 major astromonical factor. With the conquest of the upper air for 

 transportation and the use of atmospheric electric waves for world- 

 wide radio communication, the study of the sun's radiations in relation 

 to our atmosphere has become an astronomical problem of far more 

 than academic interest. 



Not a day passes but that our United States Naval Observatory in 

 cooperation with other observatories has a complete photographic 

 record of the conditions of the sun's surface. When a huge solar storm 

 is in the making, communication agencies are forewarned days in 

 advance of probable periods of interrupted radio reception. The ques- 

 tion of the relation of sunspots to the forecasting of weather is still 

 in a controversial stage, but meteorologists are beginning to realize the 

 increasing significance of conditions in the high atmosphere to the 



> lieprinted by permission from The Scientific Monthly, vol. 54, June 1942. 



151 



