ELEMENTS OF THE SOLAR-TERRESTRIAL SYS 



The natural environment of man 

 consists of a single, gigantic system, 

 all of whose parts continuously in- 

 teract. It has been customary over 

 the centuries to view certain of these 

 parts in isolation: the atmosphere of 

 winds and moisture; the hydrosphere 

 of oceans, lakes, rivers, and ground- 

 water; the biosphere of living things; 

 and the lithosphere, or the crustal 

 portion of the "solid" earth. Only 

 during recent decades has a general 

 awareness been developing that the 

 behavior of each of these parts is 

 fundamentally influenced, and indeed 

 frequently controlled, by the behavior 

 of the others. Even less apparent to 

 many has been the role of the more 

 remote parts of our environment: (a) 

 the deeper reaches of the earth that 

 lie beneath the crust, and (b) the vast 

 region that extends from the upper 

 levels of the atmosphere to the sun — 

 and even beyond to the sources of 

 much of the cosmic radiation that 

 continues to bombard the earth. The 

 latter is designated here as the "solar- 

 terrestrial system" and forms the 

 starting point of this review of the 

 status of environmental science. The 

 system can be divided conveniently 

 into five parts: 



The Sun — an undistinguished star, 

 but the principal source of the energy 

 that drives our environmental system. 



The Interplanetary Medium — pre- 

 viously considered a vacuum, this 

 enormous region between the sun 

 and the near-earth environment is 

 now known to be filled with matter, 

 largely electrons and protons (hy- 

 drogen nuclei), originating in the 

 outer reaches of the solar atmosphere 

 (the "corona") and rushing outward 

 at great speeds as the "solar wind." 



The Magnetosphere — that region 

 of space in which the earth's magnetic 

 field dominates charged-particle mo- 

 tion. An enormous storehouse of 

 solar energy, the magnetosphere is 



bounded, on the sunward side, by 

 the "magnetopause," which is the in- 

 ner boundary of a transition region 

 (the "magnetosheath") beyond which 

 lies the solar wind. In the direction 

 away from the sun, the magneto- 

 sphere stretches beyond the orbit of 

 the moon in a long tail, like the tail 

 of a comet. 



The lonospliere — a region con- 

 taining free electrically charged par- 

 ticles by means of which radio waves 

 are transmitted great distances around 

 the earth. Within the ionosphere are 

 several regions, each of which con- 

 tains one or more layers that vary 

 in height and ionization depending 

 on time of day, season, and the solar 

 cycle. 



The Upper Atmosphere — an elec- 

 trically neutral region, whose chief 

 characteristics derive from the ab- 

 sorption of solar ultraviolet radiation. 

 The upper atmosphere (the "thermo- 

 sphere" and "mesosphere") overlaps 

 the lowest level of the ionosphere 

 but also extends below it. Near 50 

 kilometers from the earth, at the 

 "stratopause," the upper atmosphere 

 gives way to the atmosphere lay- 

 ers that immediately surround the 

 earth (the "stratosphere" and "tropo- 

 sphere"). 



This enormous volume of space is 

 matched by the great range of physi- 

 cal mechanisms that occur. In the 

 closer regions of the upper atmos- 

 phere, solar-terrestrial science is con- 

 cerned with many of the concepts 

 that meteorologists have evolved in 

 dealing with the weather systems of 

 the lower atmosphere; at the outer 

 extremes, the methods of astrophysics 

 and high-temperature plasma physics 

 must be utilized. The status of solar- 

 terrestrial science is thus strongly de- 

 pendent on the specific phenomena 

 being considered, for scientific prog- 

 ress has not been uniform across this 

 complex system. 



At the same time, the solar-terres- 

 trial system, considered as a whole, 

 is both the source of beneficial radia- 

 tion, without which life itself could 

 never have developed on the earth, 

 and the mechanism for controlling 

 harmful radiation. Without this con- 

 trol mechanism, life could not long 

 survive. The whole range of solar- 

 terrestrial relationships is therefore 

 of the greatest environmental con- 

 cern. 



The past twenty years have pro- 

 duced a wealth of detail and at least 

 partial understanding of the activity 

 going on in this region. The knowl- 

 edge has not produced quantitative 

 models of the dynamical effects on 

 the earth environment. The effects 

 are too complicated — in the same 

 way that weather is still too com- 

 plicated for satisfactory quantitative 

 modeling. But the knowledge informs 

 us about what is happening, allowing 

 us to understand the effects and to 

 avoid them in some cases, thus per- 

 mitting intelligent planning for the 

 future. 



The Sun 



A powerful source of energy, gen- 

 erated by thermonuclear processes, 

 the sun can nevertheless be expected 

 to remain in its present condition, 

 emitting radiation at a more or less 

 constant rate, for an extremely long 

 time. This surmise is based on astro- 

 nomical observations of stars similar 

 to the sun. Scientific attention is 

 therefore directed principally to as- 

 pects of solar activity, and its attend- 

 ant radiation, that are more variable 

 in time. 



Most of the variability in solar 

 radiation is associated with (a) the 

 11-year solar-activity (or sunspot) 

 cycle, (b) the "active regions" that 

 are often displayed at the peak of 

 the cycle and are the source of intense 



