and for a temporal change oersteds per second. However, it is normal 

 to use more convenient units such as gammas per foot, gammas per 

 meter, gammas per mile, etc., for spatial gradients, and gammas per 

 second, gammas per minute, gammas per hour, etc., for temporal 

 change rates. 



6. Anomaly 



A spatial disturbance in the geomagnetic field, where the intensity 

 of the field departs significantly from the surrounding values, is an 

 anomaly. The significance of such departures is determined by the 

 intended application of the data. Anomalies are accompanied by gradients 

 and other higher derivatives of the field, and their causes are natural 

 or man-made. 



7. Daily variation 



Solar-daily variations in the magnetic elements are daily periodic 

 variations. The daily range, which varies with the latitude, time of year, 

 and sunspot cycle, is not entirely consistent, however, and can vary 

 widely at a given place, regardless of the time of year, etc. Lunar- 

 daily variations also occur and are periodic over lunar days. These 

 latter variations are minor. 



8. Annual variation 



Small variations periodic over a year are annual variations. 

 Their magnitude is less significant than the solar-daily variations. 



9. Annual change 



The year-to-year change rates in the magnetic elements are 

 annual changes. These changes are associated with Secular Variations 

 which are impressively slow variations and which are probably periodic. 

 Secular Variations apparently require centuries for their full develop- 

 ment. 



10. Irregular disturbances 



At irregular intervals disturbances occur which can, from time 

 to time, attain considerable magnitude. The larger disturbances are 

 called Magnetic Storms and may last several hours or several days. 

 They may cause a change of several degrees in declination and of 

 several percent in horizontal intensity and occur almost simultaneously 

 over the whole world. 



XIV- 3 



