4o0 



REPORT — 1884, 





:i 



1; 



Ijijj; ; I 



I'M- '' 



In addition to tlio more regular changes in the magnetic needle we 

 observe occasionally greater magnetic disturbances, which have been called 

 magnetic storms by Humboldt. These magnetic storms are both more 

 violent and moro frequent at times of maxima than of minima of sunspot 

 fiequency. Loomis ' has tried to determine whether particular magnetic 

 storms are coincident in time with increased solar activity. He took over 

 a hundred magnetic disturbances, and determined the spotted area on the 

 sun on the days of the storms as well as on the preceding and succeeding 

 days, and found an increased area on the same, and the four or five 

 preceding days. It would tlius seem that a greater number of spots was 

 accompanied or shortly followed by magnetic disturbances. 



One or two occurrences, though not directly connected with our subject, 

 deserve mentioning. On September 1, 1859, Mr. Carrington at Retlhill, 

 and Mr. Hodgson at Rodhill, both obsex'ved two brilliant patches of intense 

 white light in the neighbourhood of a great spot. The phenomenon was 

 an altogether exceptional one, and was accompanied by a magnetic dis- 

 turbance, which was repeated with greater energy the same evening, and 

 brillant aurora) were seen over a large part of the terrestrial globe. 



Professor Young ^ has also on one occasion noticed an exceptionally 

 str g outbreak of protuberances on the sun to be accompanied by a 

 magnetic disturbance. 



Connection between Smispots and the Aurora Borealis. 



It was suspected in the first half of last century already that the 

 anrora appeared more frequently in years in which the number of spots 

 on the sun was peculiarly great. After the discovery of the periodicity of 

 sunspots the connection was placed beyond doubt by Wolf,^ Fritz,"* and 

 Loomis.'' We owe to Fritz ^ the most detailed investigation of the subject, 

 and we shall here give a short summary of his results. In the first place, 

 the eleven-yearly period of sunspots can be distinctly traced in the fre- 

 quency of aurora? observed hi our atmosphere. This holds for the anroni) 

 seen in Europe as well as in America. It is true for those observed in 

 the arctic circle as well as those observed in the temperate zone, and, as 

 far as the scai-city of the material allows us to judge, the same connection 

 exists for the aurora austrulis. The sunspot curve presents the appear- 

 ance of a phenomenon similar to, if not identical with, that of beats, 

 which at certain intervals raises the maxima values high above their 

 means. This phenomenon appears in an exaggerated form in the curv>' 

 of frequency of the anrora borealis. It is, indeed, much more marked 

 than the eleven years period itself, a fact which it is important to bear 

 in mind in any attempt to speculate on the nature of the connection, foi' 

 it shows that the number of auroroe is not proportional to the luiniber of 

 sunspots, like the increase in amplitude of the daily variation, but that 

 while a small number of sunspots hardly shows any effect, a slight in- 

 crease in them is often accompanied by a very large increase in tlio 

 number of aurorne. The inequalities in the sunspot curve show thus a ten- 

 dency to bo exaggei'atcd in tlie curve of aurora?, and the exaggeration is 

 the greater, the greater the number of sunspots. Plato I., which is copicil 



' American Journal of Science, vol, I . 

 ••• Woif's Mitthcihnigen, No. X. (1859;. 

 » Amer. Mep. (1865). 



= The Sun. 



* Ibid. No. XV. (1863). 



« Ueber die lieziehungen, &c. 



