354 



NATURE 



[May 19, 1910 



MAGNETIC STORMS.' 



T^HE magnetic needle has been described with poetic 

 ■*■ licence as "true to the pole," and few, 1 suspect, 

 are aware how little it deserves this reputation. The 

 earliest known information on this point in England dates 

 from 1580, when Boroughs, observing at Limehouse, found 

 the needle to point ii^° to the east of geographical north. 

 During the next 2^ centuries it kept moving to the west, 

 reaching its extreme position of 24^° to west of north in 

 1818. It has since retraced its path, and now at Kew 

 points only a little more than 16° to west of north. 



Besides this slow secular change, there are daily 

 changes, which are continuously recorded at a number of 

 observatories. At a complete station there are three mag- 

 netographs, recording, respectively, declination, horizontal 

 force, and vertical force changes. In the Kew pattern 

 instrument each magnetograph has a separate drum and 

 a separate sheet of paper, but the three drums are driven 

 by a single clock, and two days' traces are usually taken 

 CM! the same sheet. 



In some foreign types of magnetograph, e.g. the 

 Eschenhagen, which was used in the National Antarctic 

 Expedition of 1901-4, the three elements are recorded on 

 one drum, but only one day's record is taken on each 

 sheet. 



In my subsequent remarks I am obliged to employ a 

 term having more than one meaning. It will be simplest 

 to explain these by reference to the familiar daily varia- 

 tions of temperature. Suppose that in March we record 

 the temperature at Kew at every hour and take a mean 

 value for each hour of the twenty-four from all days of 

 the month. We shall then find a regular rise from a 

 minimum, probably at 6 a.m., to a maximum, probably 

 at 3 p.m., and then a gradual fall to the minimum. The 

 difference between this maximum and minimum is known 

 as the range of the regular diurnal inequality for the 

 month. On individual days, however, the hours at which 

 the highest and lowest temperatures occur will vary, and 

 if we take the mean of the differences between the highest 

 and lowest temperatures of each individual day, irrespective 

 of the hour at which they occur, we get a totally distinct 

 range, which I shall call the mean absolute range. 



The absolute range in any element cannot be less, arttl 

 must usually be considerably greater, than the range of 

 the regular diurnal inequality. At Kew, for instance, the 

 mean absolute daily range of declination derived from 

 the eleven years 1890 to 1900 was 13-6', while the corre- 

 sponding range of the regular diurnal inequality was 

 only 8-o'. 



The range of the regular diurnal inequality varies with 

 the season of the year. Table I. shows its amplitude in 

 the case of the declination at Kew, Batavia, and the 

 Discovery's winter quarters. 



Table I. 

 Range of Regular Diurnal Inequality (Declination). 



Remembering that in the southern hemisphere June 

 represents mid-winter, it will be seen that the range is in 

 all cases larger in summer than in winter. 



Allowance must be made for the fact that the disturbing 

 force required to displace the needle i' out of the magnetic 

 meridian is proportional to the horizontal component H 

 of the local magnetic force. Now the values of H in 

 C.G.S. measure, at the epochs to which the data refer, 

 were 0183 at Kew, 0-367 at Batavia, and only 0065 at 

 the Antarctic station. Thus the disturbing force required 

 to produce a range of i' at Batavia would produce a range 

 of 2' at Kew and of nearly 6' at the Discovery's winter 

 quarters ; but, even allowing for this, the Antarctic range 

 is much the largest of the three. 



1 From a discourse delivered at the Royal Institution on Friday, March 4, 

 hy Dr. C. Chree, F.R.S., Superintendent Observatory Department, Natiotial 

 Physical Laboratory. 



The great increase apparent as we pass from temperate 

 to Arctic or Antarctic latitudes is even more conspicuous 

 in the irregular movements, which, when sufficiently pro- 

 nounced, are known as magnetic storms. This is illus- 

 trated by Table II. 



Table II. 

 Absolute Ranges of Declination. 



At Kew from 11 years 



Percentage of Days when Range 



o -10 10 -20 20 -40 over 40 



Antarctic (77° 51' S.) from 2 years 



Percentage of Days when Range 



°'~3o' 3o'-6o' 6o'-no' over 120 



As already explained, the forces required to displace the 

 needle i' out of the magnetic meridian at Kew and 3' 

 out of the magnetic meridian at the Antarctic station are 

 approximately equal. If, then, the disturbing forces at 

 the two places were of similar magnitude, we should 

 expect ranges of less than 30' in the Antarctic to be as 

 common as ranges of less than 10' at Kew, and ranges 

 above 40' at Kew to be as common as ranges above 120' in 

 the Antarctic. This, it will be seen, is exceedingly wide of 

 the mark. A single year's records in the Arctic or 

 Antarctic is likely to supply as many large disturbances 

 as the records of a generation in the south of England. 

 This is one reason why so much importance attaches to 

 continuous magnetic observations in high latitudes. 



The daily amplitude of irregular magnetic changes, like 

 that of the regular diurnal inequality, is variable through- 

 out the year, but the seasonal variation is usually different 

 in the two cases. This is shown by Table III. 



Table III. 1 



Annual Variation in Inequality and Absolute Declinatio^ 

 Ranges at Kew, omitting Highly Disturbed Days (1890- 

 1900). 



Winter 



Equinox Summer 



Inequality range 

 Absolute range 



5'2S 

 10 35 



9 30 

 13-81 



10-80 

 1 3 "56 



• NO. 2 II 6, VOL. 83] 



Each of the three seasons contains four months, March, 

 April, September, and October being included under 

 " Equinox." 



If the days of large disturbance^ averaging nineteen a 

 year, had been included in Table III., the preeminence of 

 the equinoctial value of the absolute range would have 

 been greater. Kew, it should be added, is fairly repre- 

 sentative of all stations in temperate latitudes. 



When we pass to days of large disturbance, the promin- 

 ence of the equinoctial season in temperate latitudes be- 

 comes accentuated. TWs is shown by Table IV., which 

 gives the seasonal distribution of the 721 magnetic storms 

 recorded at Greenwich from 1848 to 1903, as calculated 

 from the lists drawn up by Mr. W. Ellis and Mr. E. W. 

 Maunder, with corresponding results for Batavia from 1883 

 to 1899, obtained by Dr. Van Bemmelen. 



Table IV. 

 Seasonal Distribution of Magnetic Storms. 



Place 



Epoch 



Greenwich 

 Bat" via 



1848-1903 



1E85-18QQ 



Percentage of all Recorded 



Winter Equinox Summer 



1 



Out of every 100 storms recorded at Greenwich, forty- 

 two occurred in the four equinoctial months. 



The seasonal variation seems to diminish as we approaci 

 the magnetic equator, and but little remains of it a- 

 Batavia. 



