Geomagnetism— Knap p and Roberts 167 



As an illustration of the role played by the auroral zones 

 in the distribution of S d , Olsen (10) found that the amplitude 

 of this constituent at a station in Greenland just north of the 

 auroral zone showed a decided periodicity connected with the 

 sun's rotation. This effect could not be discerned at all for 

 stations lying south of the auroral zone. Whether similar effects 

 are observable at other high-latitude stations remains to be seen; 

 in the instance just mentioned it was found that several years' 

 data were needed to establish the effect. 



It is clear that the study of magnetic storms and of the asso- 

 ciated variation known as S d calls for data from points in the 

 auroral zone and on both sides of it. Moreover, it has been 

 found that the auroral-zone effects cannot be isolated by com- 

 paring records obtained at different latitudes if at the same time 

 there is a divergence of longitude; the tilt of the Earth's mag- 

 netic axis causes the auroral zone to react differently on stations 

 in widely different longitudes. To avoid the complication of 

 longitude effect it is desirable to have observatories linked in 

 north-south chains across the auroral zone. Such a chain is 

 now provided in Alaska by the Sitka, College, and Barrow 

 observatories, each of which secures continuous graphic records 

 of the time variations of the declination, horizonal intensity, 

 and vertical intensity of the Earth's magnetic field, with appro- 

 priate control by means of frequent absolute observations. 



The recording apparatus used at each of these observatories 

 is planned to meet the rather stringent requirements imposed 

 by a chain of this character. The magnetograph at Barrow is 

 a special low-sensitivity one, chosen to guarantee a complete 

 record despite the frequency and severity of the magnetic dis- 

 turbances encountered there. Furthermore, it has a strip 

 recorder which permits records to accumulate for several days 

 during protracted snow storms when the instrument cannot be 

 tended. At both Sitka and College there are standard high- 

 sensitivity instruments supplemented by low-sensitivity auxil- 

 iary instruments to safeguard the continuity of the record dur- 

 ing severe magnetic storms when the regular trace goes off the 

 edge of the paper. 



