THE SUN AND THE ATMOSPHERE STETSON 



155 



This is an illustration of the way in which aurorae are formed in 

 the upper atmosphere of the earth under the electrical excitation 

 that appears to be caused by the sun's radiation as molecules of 

 oxygen and nitrogen are ionized in the regions where the pressure 

 is very low. 



If we make a chart of the numbers and occurrences of these aurorae 

 we find there seems to be a curious connection between the frequency 

 and brightness of auroral displays and the state of the sun as marked 

 by the appearance of sunspots. Professor Brooks, Director of the 

 Blue Hill Observatory, has kindly allowed me access to the records 

 made of aurorae at that station for the last 30 years. Utilizing the 

 observations of the brighter aurorae, we have here a graph showing 

 the variations in auroral frequencies occurring in years distributed 

 with respect to the maximum occurrences of sunspots. The fewest 



5 4 3 2. 

 YEARS BEFORE 



I I 



SUNSPOT 

 MAXIMA 



2 3 4 5 

 YEARS AFTER 



Figure 1.— Relative frequency of auroral display at Blue Hill Observatory from records 1885-1937. 



number of aurorae appear to occur from 4 to 6 years before or after 

 the years marking sunspot maxima. The time when aurorae appear 

 most frequently would seem to be about 2 years after the passing of 

 the maximum of sunspots. The very high column occurring 1 year 

 before sunspot maximum is due largely to an unusually large number 

 of observations in 1893. These results corroborate rather well 

 those of a longer series of observations tabulated by Dr. Chree extend- 

 ing for over 100 years, or from 1750 to 1877. 



The fact that aurorae therefore appear to occur with greater fre- 

 quency at the times when sunspots are most numerous suggests an 

 electrical effect in the upper atmosphere for which sunspots may be 

 responsible. Much of our present knowledge of aurorae is due to the 

 exhaustive studies and mathematical calculations of Dr. Stormer of 

 Blindern, Norway. By careful analysis of the motion of charged par- 

 ticles in the magnetic field of the earth he has been able to deduce 

 tracks of ionization so simulating auroral forms as to indicate very 



