284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



produce the magnetically disturbed condition wliich allows the par- 

 ticles to escape from the belt and enter the atmosphere down the 

 earth's own lines of force. 



The phenomenon of the traveling solar magnetic field is of great 

 interest in many other aspects of physics, particularly in cosmic ray 

 physics. The primary cosmic ray particles are believed to be gener- 

 ated in the galaxy and it has been known for a long time that their in- 

 tensity, incident on the earth's atmosphere, decreased when there was 

 a severe magnetic disturbance. This ejffect was so closely linked with 

 the modulation of the earth's field that it had been assumed to be 

 a local terrestrial effect, in that the variation of the cosmic ray inten- 

 sity was governed by the changes in the earth's field. During 1960 

 and 1961 Simpson of Chicago discovered, by cosmic ray-counting 

 experiments in the American space probes Explorer VI and Pioneer 

 V, that this decrease observed on earth was accompanied by a simul- 

 taneous decrease in the counting rate of the apparatus in the probes 

 when they were many millions of kilometers away in interplanetary 

 space. Clearly, it is the variation in the magnetic field in interplane- 

 tary space itself which is controlling this intensity variation and not 

 the local field of the earth, and the variable interplanetary fields of 

 tliis nature must arise through the magnetic field trapped in the 

 material streaming away from the sun. 



In addition to the material which streams away from the solar 

 corona, and the high energy protons which are ejected at the time of 

 solar flares, the sun frequently ejects large quantities of low energy 

 protons. All these radiations present a serious hazard to the astro- 

 naut intending to travel in interplanetary space and considerable 

 thought is already being given to possibilities of predicting the nature 

 and timing of these solar outbursts. 



Whereas the space probes have become avenues through which we 

 are learning about the influence of the sun in interplanetary space, 

 the observations of the solar radio emissions have revolutionized 

 our picture of the sun itself. The powerful emissions of radio waves 

 during smispots and solar flares are the most obvious radio phenomena 

 associated with the sun. But the observation of the less intense radio 

 emissions from the solar corona has revealed an interesting situation. 

 If we imagine ourselves looking at the sun with radio eyes instead of 

 with ordinary eyes, then we would observe quite a different object in 

 the sky. At a wavelength of 21 cm., mstead of the uniform disk 

 which we usually see, its appearance would be that of a disk that was 

 brighter toward the edges and was flattened instead of circular. It 

 would extend much farther into space than it does when visually 

 observed. If our eyes were tuned to look at a rather longer wave- 

 length in the meter waveband then we should begin to think that the 

 sun was monopolizing the whole sky. On wavelengths of several 



