MAGNETIC DISTURBANCES AND AUKOKA. 



CHAPTEE XI. 



COMPARISON OF MAGNETIC DISTURBANCES AND AURORA. 



109. Aurora in England seems always accompanied by more or less magnetic disturbance. Auroras 

 and large magnetic disturbances are lioth rare events in the south of England, so that when a conspicuous 

 aurora and a largo magnetic storm occur there simultaneously, the inference that the coincidence is not 

 a mere accident is almost inevitable. In the Arctic and Antarctic regions, however, magnetic disturbance 

 is the rule rather than the exception, and if the same is not equally true of aurora, still aurora is so often 

 visible that even if no physical connection existed between the two phenomena, accidental coincidences 

 would naturally occur. During several Arctic expeditions i-.y. those in the polar year 1882 elaborate 

 auroral observations have been carried out, and special attention has been given to the <|ucstion of the 

 relationship to magnetic storms. The general conclusion reached seems to be that many Arctic auroras 

 are unaccompanied by any noteworthy magnetic disturbance, but that auroras of a specially vivid and 

 rapidly changing character are usually accompanied by marked magnetic disturbance. 



In considering the incidence of auroras there is the serious complication that to be visible an aurora 

 must occur when there is no other source of light sufficiently bright to render it invisible. Thus aurora 

 is seldom if ever seen until the Sun is below the horizon, and even moonlight, when the Moon is near the 

 full, suffices to render any but specially bright auroras invisible. The state of the sky as to clouds is also 

 important. Faint auroras of limited extent have a better chance of being seen when clouds are few than 

 when they are many. 



Again, there is the fact that while the record of magnetic storms, thanks to magnetographs, goes on 

 equally well in the absence of trained observers, this is not the case with auroras. At Winter Quarters 

 the Meteorological Observers were on the outlook for aurora at the two-hourly observation hours, right 

 through the 24 hours. Specially bright auroras would also naturally attract the attention of the watch 

 on deck, whose instructions under these circumstances were to call Mr. BEKXACCHI. Still, during the 

 night, a display of less than two hours' duration, if faint or only moderately bright, might fail to be noted. 

 Thus the fact that at an observation hour when aurora was noted the magnetic curves were quieter than 

 half an hour before or after, when no aurora was noted, may not possess any real significance. 



110. The magnetic curves, when existent, were examined at all the times at which auroras appear on 

 Mr. BERNACCHI'S list.* 



Before considering the results of this comparison, the following particulars of the auroral statistics may 

 be mentioned.! The auroras recorded, with two exceptions occurring late in March, were limited to the 

 six months April to September, the days on which they were observed being distributed as follows : 



Thus the auroras seen were practically limited to the four months April to August, and so to the season 

 of the year when magnetic movements, both regular and irregular, were smallest. 



* " Physical Observations," p. 101. 

 t Cf. " Physical Observations," p. 126. 



2 C 



