176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



and a duration of a few minutes. Tlie granulation bears witness of 

 the existence of turbulent gaseous layers in the photosphere. 



Local disturbances of the equilibrium are the sunspots and all the 

 phenomena associated with the formation of the spots. The fre- 

 quency of the spots follows the well-known 11-year period. In the 

 beginning of a new cycle the spots appear in high heliographic lati- 

 tude, and then, as the cycle proceeds, the spots appear closer and 

 closer to the Equator, The electromagnetic fields associated with 

 sunspots are of extremely great interest. The spots often appear in 

 pairs of opposite magnetic polarities, or in groups in which a line 

 may be drawn between spots of opposite polarities. During a certain 

 cycle there is a fixed rule according to which the poles follow each 

 other in the direction of rotation. If a south pole follows a north 

 pole in the northern hemisphere, the opposite order will hold in the 

 southern hemisphere. From one cycle to the next the order changes 

 in both hemispheres. This shows that the true sunspot cycle em- 

 braces two 11-year periods, thus 22 years. 



The most important instrument for investigating the surface of 

 the sun is the spectroheliogi'aph. In this instrument, which was de- 

 signed over 50 years ago independently by G. E. Hale and H. Des- 

 landres, the solar surface is photographed in light of a very small 

 interval in wavelength. Most often the spectroheliograms are taken 

 in light which is contained within a certain absorption line in the 

 solar spectrum. The color band of the solar spectrum is not a con- 

 tinuous one, but is broken by an immense number of absorption lines, 

 the Fraunhofer lines, which correspond to the characteristic lines of 

 the atoms of various elements that are present in the solar atmosphere. 

 The interpretation of these lines as belonging to various elements 

 opened up the field of solar physics. About 25 years ago H. N. Russell 

 showed how the intensity of the lines allow us to draw conclusions as 

 to the amounts in which the various elements occur in the atmosphere 

 of the sun. Usually the spectroheliograms are taken in one of the 

 heavy lines of ionized calcium, the Fraunhofer lines H and K, or in 

 the hydrogen lines, for instance the red Ha. The photographs in H 

 or K show the wide and bright "flocculi," or chromospheric faculae, 

 of ionized calcium which are often conspicuously dense about the sun- 

 spots. Spectroheliograms in the hydrogen lines are of very special 

 importance. They show often narrow, bright areas which develop 

 rapidly, often within a few hours, the so-called solar "flares." These 

 eruptions are often connected with disturbances in the terrestrial 

 magnetism and in the ionospheric layers, and are often accompanied 

 by the appearance of bright aurorae. The disturbances in the iono- 

 sphere often cause interruptions in the transmission of radio waves. 



The spectroheliograms in Ha show narrow, dark "filaments" of 



