THE SOLAR SYSTEM — LOVELL 283 



belonging to a magnet whose magnetic field moved with it. Astronom- 

 ically it was believed that magnetic fields were localized in bodies like 

 the earth, the sun, and some of the stars. Now it is realized that if there 

 is a gas in a liighly ionized condition, like the material of the solar wind, 

 moving in interplanetary space where the free path is thousands of 

 kilometers, then this material carries its own magnetic field. This 

 concept of the trapped magnetic field — contained in a stream of gas, 

 coming from the solar corona or from the shell of a star, moving 

 away into space so that the ionized particles and the magnetic field 

 move together, actually being transported through space to another 

 part of the solar system or another part of the cosmos — has become 

 of great importance in theoretical astrophysics. The possibility of 

 magnetic fields moving in this way with the gas appears to be one 

 of the controlling influences which may govern the organization of the 

 interplanetary material and indeed of the interstellar gas in the cos- 

 mos as a whole. The fact that this gas streaming from the sun carries 

 with it a magnetic field is a matter of great importance as far as the 

 earth is concerned, because when this solar wind reaches the neighbor- 

 hood of the earth then the earth's magnetic field is disturbed. The 

 electrons which are streaming away from the sun can then be injected 

 into the earth's own magnetic field ; they become trapped in it and in 

 this way the outer layer of the Van Allen belts is probably formed. 

 These ideas provide a good explanation of the situation whereby this 

 outer belt of electrons is so subject to solar control, why it disappears 

 in a magnetic storm, and the processes by which it is repopulated after 

 a matter of some days. In principle the earth's field should present 

 a fairly solid barrier against the injection of particles of comparatively 

 low energy from outside, but it is tliis distortion of the magnetic field 

 by the traveling fields coming away from the sun which facilitates 

 the injection. 



The whole phenomenon of the earth's magnetic storms and the au- 

 rora borealis, or tlie northern lights, must be tied up with these par- 

 ticles which are trapped in the Van Allen belts. The aurorae were 

 known to be associated with solar flares and it was believed that the 

 phenomena were caused by the particles streaming out from the region 

 of the flare on the sun and reaching the neighborhood of the earth after 

 a period of 24 to 30 hours, when they entered the atmosphere and 

 gave rise to ionization at a height of 100 kilometers or so. This 

 explanation is now obviously incorrect because it is known that the 

 particles from the sun are trapped in the outer Van Allen belt. The 

 formation of the aurora seems to be associated with the draining of 

 the particles from the outer belt during a magnetic storm. Appar- 

 ently the primary aurora particles do orginate in tliis outer belt and 

 the function of the magnetic storm in the aurora phenomenon is to 



