224 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



accepts the startling concept that we He below the edge of the sun. 

 But Chapman goes even further and suggests that the conduction of 

 heat from the solar surface through the corona cannot be neglected and 

 that perhaps our ionosphere derives some of its high temperature from 

 direct thermal contact with the corona through which the earth is 

 moving. Shock waves may even be able to traverse the entire distance 

 between the sun and the corona. On contact with the earth, the wave 

 could distort the earth's magnetic field, producing ionospheric and 

 magnetic storms, as well as certain types of auroral phenomena. 



But let us return to the more important "edge" of the sun, defined 

 by the photosphere and chromosphere and other visible appendages 

 of the solar neighborhood. Around sunspots and also around other 

 areas that presumably have intense magnetic fields associated with 

 them, we observe increased activity. We sometimes see matter ejected, 

 usually in the form of surges. In all probability additional surges 

 occur that are invisible to us in the hydrogen radiations we ordinarily 

 employ for observation. The mere fact that we observe more matter 

 falling back toward the solar surface than we do leaving the sun 

 clearly indicates that some of the material must go up invisibly, prob- 

 ably in a stage of high temperature and high excitation. 



Several different processes are undoubtedly responsible for the ejec- 

 tion of the material as well as for the form and structure of down- 

 falling masses. As I have already indicated, the hydrogen convec- 

 tive zone, especially with the activity enhanced in the neighborhood 

 of magnetic fields, is a major force. But the electric currents responsi- 

 ble for such fields probably give a marked assist. Although the mag- 

 netohydrodynamics of the situation are too complicated for detailed 

 calculation, argument based on simple engineering models reveals 

 that the electromagnetic forces are ample to eject material. The forces 

 acting on the simplest of all configurations, a single loop of current, 

 are well known. The loop tends to expand indefinitely in radius, and 

 the diameter of the current-carrying wire tends to diminish, if the 

 wire should happen to be compressible. 



In the solar atmosphere, of course, no solid wires exist. The cur- 

 rents are free to move and change their position. However, as Cowling 

 pointed out long ago, the strength of the currents and the size of 

 the current-carrying elements indicate that magnetic fields will not 

 change rapidly in the solar atmosphere. A sunspot, for example, must 

 have an electric current of 10 12 -10 13 amperes circulating about it. We 

 cannot break such a current arbitrarily in less than thousands of years. 

 A magnetic field may be transported from one place to another in the 

 highly conductive volume, but it can scarcely arise spontaneously 

 except in rare instances. 



The physical nature of certain forms of prominences presents an in- 

 teresting puzzle. Downward-moving gas may suddenly become visible 



