Theory of Magnetic Storms. 679- 



estimate o£ the duration of an average magnetic storm is 

 20 hours or 1~2. 10 4 seconds. In general also the less time 

 it takes a cloud to reach the earth, the denser it will be and 

 the less time will the earth take to pass through it. Thus 

 one would expect the more intense storms to last a shorter 

 time than the milder storms, as is borne out by experience. 



It appears certain therefore that such clouds of ionized 

 gas can exist, and that they would be projected radially 

 from the sun at such a speed that they would naturally 

 spread out enough by the time they reached the earth to 

 account for the observed duration of magnetic storms. The 

 next point to be examined is whether they would produce 

 the phenomena on the earth associated with such storms. 

 The question of the influence of the earth's magnetic field 

 is difficult to treat. A slight separation of the two charges 

 would take place owing to the different ratio -of charge to 

 mass, but any large effect would be prevented by electro- 

 static attraction. Further, as was pointed out above, a slight 

 excess positive charge of the whole cloud is probable owing 

 to the escape of the more mobile electrons. This would be 

 very small and the ratio of e/m would be of the order 

 2'5.10 3 E.S.U. Whether this would give rise to sufficient 

 deflexion to cause the particles to predominate in polar 

 regions is difficult to say, since in the interior of such an 

 ionized cloud displacements of charge might be compensated 

 by induced displacements at the edges. 



A more serious difficulty appears to be formed by the fact 

 that aurorse occur on the average at about 10' cm. height 

 above sea-level. The range of a hydrogen ion moving at 

 about 8 . 10 7 cm. /sec. would be of the order 3*7. 10~ 4 cm. in 

 air at standard temperature and pressure. This means that 

 it would be stopped after passing through a column of air 

 containing ]0 16 molecules per cm. 2 



It is usually assumed in working out the composition of 

 the upper air that the isothermal layer extends to an in- 

 definite height. Making this assumption the number of 

 molecules at height Alt above the bottom of the stratosphere 

 is obviously 



n \, n 2-> e tc. being the numbers where isothermal equilibrium 

 commences. The total number per cm. 2 through which an 

 ion must pass, therefore, when approaching from the sun 

 would be 



a Ui 6 + A 2 € 



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