524 



HIRKKLAND. THE NORVEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



SUN-SPOTS AND STORMINESS. 



98. In fig. 190 the storminess is compared with the occurrence of sun-spots. The storminess is 

 that of the horizontal Component at AxelOen put up for each day. The sun-spot curves B, C and D are 

 deduced from the "Results of Measures made at the Royal Observatory, Greenwich, of Photographs of 

 the Sun taken at Greenwich, in India and in Mauritius". 



The curves D give the total visible area of sun-spots for each day during the period of our 

 observations. 



If the radiation started perpendicular to the surface of the sun, it would not be the total visible 

 area of sun-spots that would be significant with regard to magnetic storms and aurora ; but only the 

 spots which at the time under consideration were near to the central meridian of the sun. Curve (' 

 represents for each day the number of sun-spots for which 7 S /. <; 10, where )., is the heliographic 

 longitude of the sun-spot centres as given in the Greenwich records. The dotted curves in (' and I) 

 represent the area of the umbra, the curves drawn in ful indicate the area of the whole spot. 



Finally the graph B represents the time of passage of the central meridian of the various groups 

 of sun-spots given in the Greenwich records. At the time of the passage, an ordinate is drawn whose 

 length is proportional to the largest total area which the group has attained during the time it lias been 

 observed. Thus the graph does not give the area that the group actually had at the time of the passage. 

 We have even gone so far as to put up groups, which have not been visible at all at the passage of 

 the central meridian. The reduction to central meridian has been done by interpolation, or if the group 

 only appears on one side of the central meridian, we have extrapolated by means of the synodic period 

 of the revolution of sun-spots. 



A comparison between magnetic storms and sun-spots shows thai the af>/>earcnce of Inrgf i;m/<f>s n/ 

 sun-spots (foes not take place so regularly as the principal maxima of storminess. Very often large maxima 

 of storminess are not accompained by any sun-spots at all. 



In the following table are given a number of sun-spot groups for which there seems to be an 

 undoubted coincidence with magnetic storms. 



TABLE XCII. 



