NORTHERN LIGHTS— EVE J 57 



Canada, proved tliat the E region is made conducting, or is ionized 

 by the ultraviolet light from the sun, but it is not yet possible to 

 assign a cause to the F region. 



It should be clear now that it is necessary to determine in due course 

 the different types of radiation responsible for («) the ozone layer, 

 (h) the Kennelly-Heaviside laj^er, (<?) the Appleton layer, {d) the 

 more occasional and local auroral displays, all of which are attribut- 

 able to the sun's activity. There is a yet more difficult problem with 

 respect to the cosmic rays and the bursts or showers of ions to which 

 they give rise. Sometimes a hundred million ions occur at a single 

 outburst. 



In the upper atmosphere the pressure is so low that the molecules 

 are quite far apart, and if an electron is detached from a molecule by 

 some type of radiation, it may have to wander a long way before it 

 can find a partner in a positive ion ; or it may find a resting place on 

 a neutral molecule, so that the pair become a negative ion. While 

 free, the electrons are so small and light compared with their electric 

 charge that they are readily made to oscillate, or dance in rhythm, 

 with any electromagnetic waves that are passing them. Curiously 

 enough, the group of waves travels the faster in consequence, so that 

 an electromagnetic wave entering these ionized regions obliquely 

 has the upper part wheeling faster than the lower, until the wave 

 front is turned round and proceeds downward to the earth agaii». 

 However, much the same sort of thing happens every time you look 

 into an ordinary mirror or looking glass. There also the free elec- 

 trons in the mercury at the back of the glass are able by their stimu- 

 lated motion to return to you a fairly faithful image of your face. 



Eadio signals will also bounce to and fro between the earth and the 

 reflecting regions, proving that the earth also is an admirable radio 

 reflector. The total path for eight such reflections, which have been 

 obtained from the F region, must exceed 2,000 miles. 



SUNSPOTS 



In the old days the heavens were deemed to be eternal, changeless, 

 and perfect, so that the discovery in the days of Galileo that there 

 were spots on the sun came as a shock to medieval thought. The face 

 of the sun is a turbulent place at the high temperature of 6,000° C. 

 Black spots appear on it, sometimes large enough to be seen through 

 a darkened glass with the unaided eye, and much broader than the 

 diameter of the earth. The number of these spots follows the same 

 11-year cycle as the frequency of the aurora. At the beginning of 

 such a period the face of the sun may be practically without spots 

 (pi. 3). In due course a few appear in middle latitudes on both sides 

 of the sun's equator. There is a steady increase in nimiber, the spots 



