402 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



tions "stars." Many of you are aware of the tremendous amount of 

 work which is going on all over the world in studying the nature of 

 the electrical signals which we get from these interstellar spaces. 



Interplanetary space, that is, space within our own solar system, 

 which on the average happens to be kept warmer because we have the 

 hot sun in our vicinity, reaches temperatures of around 10,000 degrees 

 K. In interplanetary space the number of particles per cubic meter, 

 except when we actually get to the surface of a planet, is fairly small, 

 between a million or perhaps 10 million particles in each cubic meter. 

 This is fairly transparent space. The interplanetary plasmas are of 

 extreme importance to modern science because it is through this me- 

 dium that we must travel if we are to go out any distance from the 

 surface of the earth into interplanetary space, where we have already 

 sent a fair number of probes. The physicists and engineers who are 

 involved in space research are studying the mechanisms and the inter- 

 actions of the plasma state in these interplanetary regions. 



Perhaps the one astronomical area that has been studied most is the 

 Earth. Around it there is a charged blanket resulting from the fact 

 that the atmosphere attached to the earth is being bombarded by solar 

 radiation and the solar radiation produces a plasma from the neutral 

 gases which make the earth habitable. This layer is called the iono- 

 sphere and always insulates us from the outside space. This plasma 

 blanket around the earth has been well known for a long time. It is 

 relatively cold, 1,000-10,000 degrees K, and the electron density can 

 get fairly high, up to about 10"-10" electrons per cubic meter. There 

 are some very interesting and important phenomena which occur as a 

 result of this ionospheric blanket. Well known to radio engineers is 

 the fact that you can bounce radio waves off the ionosphere. The in- 

 teraction of electromagnetic radio waves with the ionosphere has been 

 a major study for many years by both electrical engineers and 

 physicists. 



Recently the newspapers have been full of another phenomenon 

 which was predicted theoretically long ago but actually found experi- 

 mentally only a few years ago when we started sending up rockets 

 and high-altitude balloons. This has been called the "Van Allen 

 belts." These belts are areas of plasma concentration which have 

 been caught in the inhomogeneous magnetic field around the earth. 

 If you put a moving electron in a magnetic field, it has a tendency to 

 go around in a circle, the diameter of which is inversely proportional 

 to the magnetic field. If a charged particle is high above the earth 

 somewhere near the equator, where the earth's magnetic field is not 

 very strong, it goes around slowly in a big circle, but as it gets closer 

 to the pole, where the strength of the magnetic field is greater, it must 

 move in smaller and smaller circles. In shortening the radius of the 



