PART I — THE SOLAR-TERRESTRIAL ENVIRONMENT 



fluxes of extreme ultraviolet (EUV) 

 radiation, X-rays, and energetic par- 

 ticles (chiefly protons and electrons), 

 and (c) the "solar flares" that burst 

 forth from within these active re- 

 gions. (See Figure 1-1) 



None of these three phenomena is 

 well understood and the outstanding 



questions about the sun at present 

 are: 



1. What is the basic reason for the 

 11-year solar-activity cycle? 



2. What are the mechanisms un- 

 derlying the emission of the 

 more "exotic" portions of the 

 spectrum — i.e., X-rays and 



Figure 1-1 — SOLAR FLARE 



Solar flares usually lasting only a few minutes form very rapidly in disturbed re- 

 gions around sunspots. Flares occur quite frequently near the maximum of the solar 

 activity cycle and are related to catastrophic changes in the powerful magnetic fields 

 that are associated with sunspots. 



EUV radiation at the short 

 wavelength end and radio 

 waves at the long end? Is 

 there anything that we should 

 know about such relatively un- 

 explored regions of the spec- 

 trum as the infrared and mil- 

 limeter-wave radiations? 



3. What is the basic mechanism 

 responsible for solar flares? 



The Eleven-Year Solar-Activity 

 Cycle — The basic mechanism that 

 produces this cycle is not known. It 

 is almost surely bound up with the 

 internal structure of the sun, which 

 will not be accessible to direct ob- 

 servation for the foreseeable future. 

 Thus, the answer to the first question 

 is not likely to be reached with any 

 degree of certainty for a considerable 

 time, although theoretical mechanisms 

 to explain solar activity should be 

 generated and tested as far as pos- 

 sible against observation. It remains 

 the most basic of all outstanding 

 questions of solar physics. 



Active Regions — There is more 

 hope that a solution will ultimately 

 be found to the problem of growth 

 of individual active regions, as well 

 as the occurrence of flares within 

 these regions. It is known, for ex- 

 ample, that magnetic fields play an 

 important role in the associated en- 

 hanced ultraviolet and X-ray emis- 

 sions, in the growth of sunspots 

 (around which magnetic fields at- 

 tain strengths as great as 1,000 

 gauss), and in the sudden birth of 

 flares. Observations suggest that re- 

 gions of strong magnetic field are 

 carried outward by convection from 

 the interior of the sun. When these 

 magnetic fields break through the 

 visible surface, we see their effect 

 in the form of active regions and 

 sunspots. As the magnetic fields ex- 

 tend outward into the solar atmos- 

 phere, they encounter less and less 

 material. Flares originate at some 

 location within this outer solar at- 

 mosphere. 



It is, thus, likely that answers to 

 the second question posed above will 



