SOLAR KADIATIOX AND ATMOSPHERE — STETSON 159 



sun's surface have been made on many different frequencies of the 

 sun's radiation. These cinematographic records promise more material 

 for the intense study of the behavior of the sohir surface than has ever 

 before been available. Some of the movements in the high solar at- 

 mosphere over the regions of sunspots revealed by this new process of 

 recording continuous motion at present defy explanation and may yet 

 completely revolutionize our ideas of the sun's behavior pattern. 



Perhaps the terrestrial effect that has most nearly paralleled the sun- 

 spot cycle is the variation in the state of the earth's magnetic field. For 

 over 100 years, it has been definitely known that the direction of the 

 compass needle and the intensity of the earth's magnetic field show 

 definite relationships. In the years when sunspots are most numerous 

 magnetic disturbances are most frequent and appear with marked in- 

 tensity. The years when sunspots are most numerous follow with 

 more or less regularity an interval of somewhat over a decade between 

 the times of maximum sunspot activity. This solar cycle, or sunspot 

 period as we sometimes call it, is usually conceded to be on the average 

 of about 11.3 years duration. An examination of a graph will show 

 that sometimes the interval between maxima may be as short as 9 

 years and on occasion as long as 17 years. 



The last maximum of sunspots was passed in 1937 and we are well on 

 our way on the down side of the cycle. It was not until the more 

 recent discovery of an ionized region in the upper atmospliere of the 

 earth that any real explanation appeared as to why sunspots and 

 changes in the earth's magnetic field should show so close a parallelism. 



Everyone knows in a general way that the earth is a magnetic sphere. 

 That the compass needle does not point true north except in various 

 restricted parts of the globe is also a fact which is generally recog- 

 nized. Perhaps comparatively few who are not geomagneticians 

 realize that the compass needle is constantly wandering back and forth 

 every day by a slight amount. When the sun rises in the east, the north 

 end of the compass needle turns slightly toward that direction. By 

 noon when the sun is south, it is pointed in its normal position. Then 

 in the afternoon as the sun wanders and sets in the west, the compass 

 needle wanders likewise to the west, coming back again to its normal 

 position about midnight when the sun is below the northern horizon. 

 This goes on day after day, month after month — but during the years 

 when sunspots are most numerous these daily excursions of the com- 

 pass needle will on the average be twice as great as during the years 

 when sunspots are lacking. These diurnal wanderings of the compass 

 needle can now be roughly explained as due to the effects of ionization 

 of the upper atmosphere by sunlight. As the electric charges become 

 separated in the process of ionization f the omlecules of nitrogen and 

 oxygen under the bombardment of ultraviolet light from the sun, 

 the movements of these ions create a perceptible current, deflecting 



