Theory of Magnetic Storms. 671 



radioactive substance it could not escape spectroscopic 

 •detection. 



Any attempt to weaken these arguments by assuming the 

 radioactive substance to be spread over a larger area entails 

 a perfectly definite connexion with sun-spots which lias 

 certainly not been observed. If one spot causes a radio- 

 active patch another should, and the earth should experience 

 a magnetic storm each time a spot appeared within 17° of 

 the critical plane. 



An even more serious difficulty arises when one calculates 

 the velocity of the a particles from Dr. Chapman's figures, 

 though he probably lays no great stress on these. If the 

 total charge is 3.16 17 E.S.U. as he assumes, the mass of the 

 particles must be 2 . 10 3 grammes. In order to account for 

 an energy of 10 23 ergs the velocity must therefore be 

 10 10 cm./sec. No a particles of such velocity are known, 

 but if they did exist there is reason to believe that they 

 would penetrate to within 4*5. 10 6 cm. of the earth's surface, 

 ?'. e. about half the height of the aurora. Further, the life 

 of the radioactive substance which produced them would be 

 of the order of 10 _]23 second. It could therefore not occur 

 in appreciable quantities. 



Even assuming enough a particles were emitted it appears 

 certain, as pointed out by Prof. Schuster, that their mutual 

 electrostatic repulsion would prevent their remaining to- 

 gether in the form of a beam for any length of time. The 

 order of magnitude of the force causing the cloud of particles 

 to expand may be estimated by considering a sphere of 

 helium atoms carrying two charges each. If there are v 

 particles per cm. 3 the radial acceleration on a single particle 



4 o 



is -77- — . At the earth's orbit r is of the order 

 6 m 



2'2. 10 12 and z; = 44.10 -7 , according- to Dr. Chapman. 



Near the sun r cannot be greater than 7.10 10 , the sun's 



radius, so that v must be about 4*4. 10 -4 . Since n = 2, 



£ = 4'78.10~ 10 and m = 4 . 1*65 .10~ 24 gramme, the accelera- 



tion is 1*8 . 10 13 cm. /sec. 2 It is clear that no beam of 



anything like the required density could persist for the time 



required to reach the earth, which must exceed 500 seconds. 



The only alternative would be to assume that the a. particles 



carried an equivalent number of electrons with them. This 



would approach the theory propounded below, but reasons 



will be given to show why it is improbable. 



Suppose, however, that these a rays could be produced 



and dispatched from the sun in the required beam. Assume 



3 A2 



