ELEMENTS OF THE SOLAR-TLRK 



come eventually from a gradual ex- 

 tension of present work, in the form 

 of refinement of satellite and space- 

 probe observations and the continua- 

 tion of the ground-based observations 

 that have provided the core of our 

 knowledge about the sun. 



Solar Flares — Solar flares are 

 cataclysmic outbursts of radiation, 

 similar to those generally observed 

 from active regions, but in immensely 

 greater quantities and with much 

 higher energies. Fortunately, individ- 

 ual flares are short-lived (of the order 

 of an hour at most), and the most 

 intense ones are quite rare, even at 

 the peak of the solar cycle. 



The effects of flares on the near- 

 earth environment make them by far 

 the most important solar phenom- 

 enon. The sudden surges of radiation 

 they produce constitute a major haz- 

 ard to manned space flights and a 

 hazard of uncertain magnitude to 

 the passengers and crew of super- 

 sonic-transport aircraft on transpolar 

 flights, where natural solar-radiation 

 shields are less effective than else- 

 where. Flares also increase the elec- 

 trical conductivity of the lower part 

 of the earth's ionosphere, giving rise 

 to severe interruptions of radio and 

 telegraph communications, particu- 

 larly at high latitudes. 



Considerable progress in predict- 

 ing major solar flares has been made 

 through observations of time varia- 

 tions of the magnetic field configura- 

 tion within known active regions in 

 the lower solar atmosphere. While 

 improvements in empirical forecast- 

 ing techniques of this kind can be 

 expected, truly accurate predictions 

 must await an understanding of the 

 basic physical mechanisms respon- 

 sible for the development of a flare. 

 Many promising suggestions have 

 been put forward, but none has proved 

 entirely satisfactory. Some think a 

 flare is caused by the annihilation of 

 magnetic fields. Another interesting 

 possibility has emerged from satellite 

 probes of the "auroral substorms" 

 that occur in the earth's outer mag- 



netosphere (see page 8). There is 

 an apparent analogy between many 

 of the observed radiation character- 

 istics of these substorms and those 

 of solar flares, opening up the pos- 

 sibility that their mechanisms are 

 basically similar, though with modifi- 

 cations appropriate to the different 

 solar environment. 



Other Research Needs — Solar 

 EUV radiation is largely responsible 

 for the existence of the earth's iono- 

 sphere, and the broad nature of that 

 responsibility is now fairly clear. 

 Many of the details, however, re- 

 main beyond our grasp. The de- 

 tailed structure of the sun's radiation 

 spectrum in the EUV and X-ray 

 regions, the points of origin of these 

 radiations at the sun, and the mech- 

 anisms responsible for producing 

 them are still areas of considerable 

 uncertainty. Much progress is likely 

 to come from the satellite and space- 

 probe programs aimed at long-term 

 monitoring of solar radiation in these 

 wavelength regions with high an- 

 gular resolution. 



The Interplanetary Medium 



The broad features of the inter- 

 planetary medium are known and 

 understood. (See Figure 1-2) Inter- 

 planetary space is in fact tilled with 

 material, or plasma, from the outer 

 reaches of the solar atmosphere (the 

 "corona"). It is made up for the 

 most part of electrons and protons 

 (hydrogen nuclei), with small quan- 

 tities of helium and traces of heavier 

 nuclei. As a result of the instability 

 of the outer solar corona against 

 expansion, this material is rushing 

 outward from the sun at speeds of 

 the order of 400 kilometers per sec- 

 ond, forming the "solar wind." 



Many important details are still 

 missing from this picture, however. 

 For example, solar-wind matter is 

 believed to constitute a sample of 

 the material that exists in the upper 

 solar corona. After a solar flare has 

 erupted, however, the nuclear com- 



position of the solar wind has 

 seen to change quite suddenly to one 

 that contains up to 20 percent helium, 

 with appreciable amounts of heavier 

 elements. This material is probably 

 that of the lower solar atmosphere, 

 near the base of the corona or in the 

 chromosphere, where flares originate. 

 Spacecraft are providing an oppor- 

 tunity to study fairly directly these 

 interesting differences in the chemical 

 composition of different regions of 

 the sun itself. Comparison of solar- 

 wind compositions with the terres- 

 trial composition may produce in- 

 sights into how the earth and solar 

 system were formed. 



"Collisionless" Shock Waves — 

 Another important area for study is 

 the reason for the fluid-like behavior 

 of the solar wind. In conventional 

 fluids, particles interact by collisions, 

 but collisions between individual 

 solar-wind particles are extremely 

 rare. Nevertheless, the solar wind 

 displays many of the properties of 

 a continuous fluid. In particular, the 

 wind's outward expansion is super- 

 sonic, in the sense that its speed 

 relative to the sun and planets is 

 greater than the speed with which 

 waves can propagate through the 

 medium. As it sweeps past any solid 

 body in the solar system, the wind 

 forms a standing shock wave up- 

 stream of the body, analogous to the 

 shock wave ahead of an aircraft in 

 supersonic flight. The width of the 

 wave that forms around the earth 

 is determined by the outward extent 

 of the earth's magnetic field, rather 

 than by that of the solid earth itself, 

 because the material in the solar 

 wind, being a good electrical conduc- 

 tor, is strongly affected by magnetic 

 fields. The earth's shock wave is 

 much larger than that formed around 

 other, more weakly magnetized bod- 

 ies like the moon, Venus, and Mars. 



Collisionless shock waves are phe- 

 nomena that may have an important 

 bearing on our understanding of the 

 basic plasma physics that holds the 

 key to controlled thermonuclear fu- 

 sion. They have been difficult to 



