1903.] Solar Prominences and Terrestrial Magnetism. 247 



Comparison of the Prominences with the Magnetic Disturbance Curves. 



If a comparison of the curve representing the number of days of the 

 "great" magnetic disturbance is made with those representing pro- 

 minence frequency (Plate 4), it will be seen that the former is as unlike 

 the curves representing the prominence frequency about the solaf 

 equator as it is like those near the poles ; in fact, the polar prominence 

 outbursts, and great magnetic disturbances occur almost simul- 

 taneously. 



The peculiar form and general similarity of the curves can be best 

 seen from the accompanying illustration (fig. 1). In the figure com- 

 parison is made between the epochs of the crossing of the known and 

 unknown lines, the percentage frequency of prominences about the 

 solar poles and Ellis' " great " magnetic disturbances. 



Two curves representative of prominence frequency are given, one 

 to indicate the abrupt nature of the curves representing the frequency 

 in a zone near the pole 10 degrees in width (in this case 60° — 70° 

 north), and the second to illustrate polar action as a whole; this latter 

 was obtained by making a summation of prominence frequency for the 

 two zones 60° — 90° north and south. 



The simultaneous occurrence of the maxima suggests that, when the 

 prominence action takes place at the polar regions of the sun, one 

 effect on the earth is that we experience our greatest magnetic 

 disturbances. 



Further, according to Mr. Ellis,* " unusual magnetic disturbance is 

 frequent about epochs of sun-spot maximum, and nearly or quite 

 ■absent about epochs of sun-spot minimum." 



We find that not only do these " great " disturbances occur at the 

 same time as the polar prominences, but the spectroscopic observations 

 of sun-spots show that they take place not only " about " the times of 

 spot maximum, as stated by Mr. Ellis, but when the sun-spot curve is 

 approaching a maximum and at the dates of the widened line crossings,! 

 when the curve representing the " unknown " lines is on the rise, and 

 crosses the " known " line which is descending. At the other epoch 

 of " crossing," i.e., when the curve showing the " known " lines is on 

 the rise and the "unknown" is falling, there is practically no magnetic 

 disturbance recorded. Attention is again drawn to these crossings, as 

 it is desired to indicate that it is only at those particular times when 

 the sun is increasing his temperature that these disturbances occur. 



•are not the same, and in dealing with, the curve representing this variation for the 

 whole hemisphere, this fact should be borne in mind. 



It may further be noted that the epochs of minima may be practically con- 

 sidered the same for both hemispheres. 



* ' Monthly Notices R.A.S.,' vol. 60. p. 118. 



t ' Rov. Soc. Proc' vol. 67, p. 412. 



