THE SOLAR SYSTEM — LOVELL 281 



is about 4X10" or nearly a billion billion kilowatts. This energy 

 is produced by thermonuclear processes which convert 4 million tons 

 of the solar matter into energy every second. The conversion takes 

 place in the center of the sun where the temperature is about 20 

 million degrees centigrade and the pressures amount to several thou- 

 sand million atmospheres. Under these conditions the atoms may bo 

 stripped of electrons, in which case matter is said to be degenerate. 

 These transmutations in the interior of the sun involve the conversion 

 of 564 million tons of hydrogen to 560 million tons of helium every 

 second. Although the solar material is being used at this rate the 

 processes have already been operative for at least 4,000 million years. 

 The sun's mass is so tremendous that this rate of use of its material 

 represents only about one-tenth percent of its mass evei*y 10,000 million 

 years. 



THE INTERPLANETARY SPACE 



Until quite recently we have tended to think of the space between 

 the sun and the earth and the planets as being empty — a near vacuum, 

 not possessing many factors of interest to geophysics or astronomy. 

 We were aware that the earth was surrounded by the ionosphere — a 

 region of electrons of varying density extending to a few hmidred 

 kilometers above the earth's surface — and that the density of electrons 

 in these regions appeared to be related to the condition of the solar 

 surface and to be generally under solar control. Apart from this, the 

 various bodies seemed to be rather disconnected except for the gravi- 

 tational forces which controlled their motions. One of the most 

 remarkable changes of opinion during the last few years is in respect 

 of this situation in interplanetary space, because it now appears that 

 this space is not empty. On the contrary, the interplanetary space 

 must now be visualized as a medimn where the conflict of a complex 

 of radiation, ionized particles, and magnetic forces is determining 

 the geophysical environment of the earth and the planets. 



THE SOLAR ATMOSPHERE 



Tlie discovery of the ionized particles trapped in the Van Allen 

 belts around the earth has led to a searching inquiry regarding their 

 origin. The inner belt seems to be composed chiefly of protons which 

 are believed to be the decay products of neutrons moving out from 

 the atmosphere of the earth, where they have been produced by cosmic 

 ray bombardment. The outer zone of electrons is mistable and the 

 present theory of the origin of the electrons is that they are part of 

 the stream of material which is being blown away from the sun. It 

 is possible that in the region of about 10 earth radii we have the 

 interfacial bomidary, where the earth's own enviroimaent is coming 

 to terms with these solar forces. The sun has an intensely hot central 



