236 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



Carrington remarked that a great magnetic storm commenced at an 

 interval of about 17 hours after he had observed the flare. Many other 

 instance of magnetic storms following flares after an interval of 

 16 to 30 hours have since been obvServed. But not every flare is fol- 

 lowed by a magnetic stonn ; for a storm to develop the flare must occur 

 in a region of the sun that is not at a great distance from the central 

 meridian. The time interval between the two phenomena indicates 

 that the storm is produced by an effect that travels from the sun with 

 a speed of the order of 1,000 to 1,500 miles a second. This is now 

 known to be corpuscular or particle emission from the sun. The 

 stream of particles is emitted in a direction nearly normal to the sur- 

 face and will not meet the earth unless the region from which it is 

 emitted is near to the central meridian. 



A magnetic storm usually begins suddenly and at the same time all 

 over the earth. Its duration is normally between one and two days. 

 The intensity of the magnetic disturbance during a magnetic storm 

 increases with magnetic latitude. In sufficiently high latitudes a 

 bright auroral display is invariably seen during a magnetic storm 

 and the most brilliant periods of the display synchronize with the 

 most rapid and violent movements of the magnetic needle. Aurorae 

 occur most frequently in a belt with a radius of about 23°, centered 

 at each magnetic pole, a further indication of their relationship with 

 the earth's magnetism. The frequencies of both magnetic storms and 

 of aurorae increase and decrease with the increase and decrease in the 

 frequency of sunspots. The corpuscular stream emitted from the sun, 

 though on the whole electrically neutral, contains charged particles. 

 As these particles approach the earth, they are deflected by the earth's 

 magnetic field toward the magnetic poles. When they enter the earth's 

 atmosphere, the electrical effects produced are seen as an auroral dis- 

 play, while the magnetic effects give rise to the phenomenon of a 

 magnetic storm. During the magnetic storm there is widespread 

 interference with radio propagation, which, unlike the radio f adeouts 

 associated with solar flares, is not limited to the sunlit half of the 

 earth. 



The magnetic storm disturbance is attributed to an intense electric- 

 current system circulating mainly within the ionosphere, but possibly 

 in part also outside the earth's atmosphere. This current system is 

 about one thousand times more intense than that which produces the 

 normal diurnal variation. The currents are intense across the polar 

 cap and particularly so over the zones of maximum auroral frequency, 

 and they do not show tlie marked difference of intensity between the 

 sunlit and dark hemispheres that characterizes the current system 

 responsible for the diurnal variation. There is also evidence of an 

 intense current flow over the magnetic equator. The phenomena which 



