SUX AND THE EARTHS MAGNETIC FIELD FLEMING 175 



clouds escaping into surrounding space is not readily traced because 

 of the rarefaction and decreased luminosity of the gas. 



The areas involved in these rapidly changing granulations, sun- 

 spots, and the long bright streaks or faculae, usually branched, and 

 other phenomena accompanying sunspots are so small as contrasted 

 with the whole area of the sun that the direct effect of their variations 

 is rarely more than a fraction of 1 percent of the total solar radiation. 

 Thus these phenomena must be indices of some more fundamental solar 

 change rather than direct factors causing any large measurable changes 

 in the amount of solar radiation — as will appear later in the discus- 

 sion of their effects on the magnetic field of the earth. 



Tlie unsettled and troubled conditions on the sun extend outward 

 for vast distances into adjacent space. The brightly illuminated 

 gaseous material forms the beautiful solar corona, formerly observable 

 only during eclipses but now photographed at times other than eclipses 

 by means of special optical instruments designed some years ago by 

 the French scientist Lyot at Pic du Midi. The coronal envelope varies 

 in a marked way with variation in frequency of sunspots. The stream- 

 ers and plumes near the poles of the sun suggest in shape the lines 

 of force of a spherical magnet, and near the equator sometimes extend 

 outward many solar diameters. 



In 1908 Hale, at the Mount Wilson Observatory, showed that sun- 

 spots have intense magnetic fields, which for large sunspots may be 

 of intensity 3,000 to 4,000 gausses or more — comparable to those be- 

 tween the pole pieces of large dynamos. Small spots 200 to 300 miles 

 in diameter have field strengths about one-thirtieth as great. In gen- 

 eral, the field strength at the center of a spot is roughly proportional 

 to the logarithm of the radius of the dark and cooler central part or 

 umbra and diminishes toward zero at the outer and lighter part or 

 penumbra. The discovery of these magnetic fields was made through 

 study of characteristic features of spectra of sunspots utilizing the 

 Zeeman effect which reveals that when light is passed through a strong 

 magnetic field each single spectral line is turned into a doublet or trip- 

 let — the doublet when the light is viewed in the direction of the lines 

 of magnetic force and the triplet when viewed in the perpendicular di- 

 rection. The strength of the magnetic field is determined from the 

 differences in the wave lengths of the separate components. These 

 observations show that the lines of force are normal to the surface at 

 the umbra of a spot but that toward the penumbra they spiead out- 

 ward. 



Many sunspots are surrounded by hydrogen vortices as shown by 

 photographs of the sun's surface by the light of hot hydrogen or cal- 

 cium. From these photographs, estimates are made of the great 

 rapidity of vortical motion. The direction of rotation of these vor- 

 tices is generally counterclockwise in the northern hemisphere and 



