PART I — THE SOLAR-TERRESTRIAL ENVIRONMENT 



Figure 1-2 — THE INTERPLANETARY MEDIUM 



In addition to visible radiation, both steady and sporadic electromagnetic emissions 

 from the sun extend over a large range of wavelengths (radio to X-ray). Low energy 

 charged particles in the expanding outer corona form the solar wind which, together 

 with the extended solar magnetic field, dominates the environment of the interplane- 

 tary medium. Occasionally, great flares in active regions emit charged particles of 

 cosmic ray energy. 



produce under laboratory conditions, 

 and their properties even harder to 

 measure, because the probes used 

 have generally been larger than the 

 thickness of the shock wave itself. 

 Now, however, the shock wave on 

 the sunward side of the earth's mag- 

 netosphere provides a natural labora- 

 tory for studying collisionless shocks; 

 space-probe techniques, in which the 

 probe dimensions are much smaller 

 than the shock thickness, are likely to 

 produce a great deal of valuable 

 information. 



The Interplanetary Magnetic 

 Field — The solar-wind material is 

 permeated by a weak magnetic field, 

 also of solar origin. This interplan- 

 etary magnetic field plays an impor- 

 tant role in guiding the highly 

 energetic flare particles toward or 

 away from the earth. The detailed 

 behavior of the field is exceedingly 

 complex, however, and not well un- 

 derstood. Furthermore, the picture is 

 complicated by the often irregular, 

 or "turbulent," structure of the mag- 

 netic field, which causes particles to 



diffuse outward from the sun much 

 as chimney smoke diffuses in the tur- 

 bulent atmosphere. This turbulence 

 is highly variable and depends on the 

 general background of solar activity 

 at any particular time. 



Since the effects of energetic par- 

 ticles reaching the vicinity of the 

 earth are generally undesirable, an 

 ability to predict their arrival would 

 be useful. One fact that helps in 

 their prediction is that, because the 

 sun rotates, interplanetary magnetic 

 field lines stretch out in a spiral, 

 much like water drops from a rotating 

 garden sprinkler. Hence, the earth is 

 connected magnetically to a point 

 well to the western side of the sun's 

 visible disc rather than to the center, 

 and intense flares originating in the 

 western portion of the disc are more 

 likely to produce serious effects than 

 those erupting in the eastern portion. 

 Nevertheless, a great deal more work, 

 both observational and experimental, 

 is needed to lay the foundation for 

 accurately predicting the arrival of 

 potentially harmful particles. 



Blast Waves — Major solar flares 

 are accompanied by blast waves 

 which move out from the sun at 

 speeds of the order of 1,000 kilo- 

 meters a second, sweeping the am- 

 bient solar-wind plasma ahead of 

 them and bringing in their train a 

 greatly enhanced solar-wind flow. 

 The more intense blast waves are 

 not appreciably affected by inter- 

 planetary conditions. As the blast 

 waves encounter the earth, they pro- 

 duce major effects on the magneto- 

 sphere, giving rise to worldwide 

 magnetic storms and visible auroras 

 (often at much lower latitudes than 

 the conventional auroral zones). They 

 also provide the most important 

 sources of fresh material for the 

 radiation belts that surround the 

 earth. 



Ability to predict these effects is 

 a matter of some practical impor- 

 tance, since serious interruptions in 

 radio communications and even in 

 domestic power supplies may result. 



