TERRESTRIAL MAGNETISM 



59 



ing layer. Table I may serve to show how the average monthly state 

 of the sun's activity has varied since the year, 1923, of minimum 

 sunspottedness. The numbers for 1923, 1924, and 1925 are final ones 

 and depend upon the records of sunspots from observatories distrib- 

 uted over the entire globe. Those for 1926 are provisional ones, 

 since they depend only on the records of the observatory at Zurich, 

 Switzerland; on account of cloudy weather a month's record may be 

 incomplete. The expectation is that the period of maximum sun- 

 spottedness for the present solar cycle is close at hand. 



Table I — Monthly Mean Sunspot Relative Numbers 

 (according to Prof. A. Wolfer of the Zurich Observatory) 



In spite of numerous observations from polar expeditions the 

 precise relationship between aurora and disturbances in the earth's 

 magnetic state, as shown during periods of magnetic storms, is not 

 yet clearly understood. The results from different expeditions are at 

 times even contradictory. For example. Dr. Sverdrup from auroral 

 and magnetic observations made by the Maud expedition at the 

 winter quarters. Four Pillar Island, September, 1924, to the beginning 

 of April, 1925, reached the conclusion^'' that auroral displays are 

 practically always accompanied by a magnetic disturbance whose 

 severity increases, in general, with the intensity and movement of the 

 aurora. He further found that the intensity of the magnetic dis- 

 turbance increased with increasing altitude of the aurora. At this 

 Arctic station, which was located about 6° south of the zone of maxi- 

 mum auroral frequency, both the maximum of magnetic disturbance 

 and the greatest frequency of aurora occurred around midnight, where- 

 as at the British Antarctic station of 1910-1913, Cape Evans, the 

 middle of the day was most disturbed magnetically and the frequency 

 of the aurora showed a maximum in the night hours. ^^ For this Ant- 

 arctic station Wright found no relation between the altitude of the 

 aurora and the magnetic character at the hour of observation and onl)^ 

 a slight relation between the brilliancy of the aurora and the magnetic 

 disturbance. 



1' Dr. Sverdrup's observations and results will be published in the volume cited above in foot- 

 note 4. 



1^ Chapter 14 of the second work cited above in footnote 12. 



