ELEMENTS OF THE SOLAR-TERRESTRIAL SYSTEM 



Short-term prediction of blast waves 

 is not too difficult, since the appear- 

 ance of an intense flare on the sun 

 gives one or two days advance warn- 

 ing. Longer-term prediction is in- 

 volved with the problem, discussed 

 earlier, of long-term prediction of 

 flares themselves; this problem re- 

 mains unresolved due to our relative 

 lack of understanding of the basic 

 mechanisms underlying solar activity. 



In general, the major practical re- 

 sult of increasing our knowledge of 

 the interplanetary medium would be 

 an improved ability to predict solar- 

 flare particle effects in the vicinity 

 of the earth. Basic advances in our 

 understanding of the processes gov- 

 erning collisionless plasmas, and of 

 the origin of the solar system itself, 

 are also likely consequences, and 

 should be pursued. 



The Sunward Side — The mag- 

 netopause marks the true boundary 

 between the plasma originating at 

 the sun and that belonging to the 

 earth. On the sunward side of this 

 boundary lies the immense shock 

 wave described in the previous sec- 

 tion, which stands some 15 earth- 

 radii out from the center of the 

 earth, as well as a region about 5 

 earth-radii thick known as the "mag- 

 netosheath"; the latter is made up 

 of solar-wind plasma that has been 

 disoriented by passage through the 

 shock wave, together with tangled 

 irregular magnetic field. 



The existence of something like 

 the magnetopause had been predicted 

 theoretically long before the Space 

 Age; its existence has now been 



verified by satellites and space- 

 probes carrying magnetometers. But 

 many of its observed properties re- 

 main puzzling. Furthermore, most 

 of the observations have been con- 

 fined to near-equatorial regions, while 

 many of the important problems of 

 energy transfer from the solar wind 

 to the magnetosphere hinge on the 

 existence and properties of the mag- 

 netopause over the polar caps. Here 

 practically no information exists. 



The Geomagnetic Tail — The con- 

 figuration of the outer magnetosphere 

 in the direction pointing away from 

 the sun is quite different from that 

 in the solar direction. Instead of 

 being compressed by the solar wind 

 into a volume sharply bounded by 

 the magnetopause, the magneto- 



Figure 1-3 — THE MAGNETOSPHERE 



The Magnetosphere 



The magnetosphere (see Figure 1-3) 

 is the outer region of the earth's 

 ionized atmosphere, in which the 

 medium is sufficiently rarified that 

 collisions between charged and neu- 

 tral particles can be neglected and 

 the behavior of the charged particles 

 is dominated by the earth's magnetic 

 field. It can be regarded as the region 

 in which control of the environment 

 by the solar wind gives way to con- 

 trol by the earth. As such, it is an 

 enormous storehouse of solar energy, 

 extending out to a distance of some 

 10 earth-radii in the direction of the 

 sun and to a much greater distance, 

 perhaps as much as 1,000 earth-radii, 

 in the opposite direction. 



The magnetosphere extends from 

 the "magnetopause," where the geo- 

 magnetic field terminates, down to 

 a height of about 250 kilometers 

 above the surface of the earth, and 

 thus includes a large part of the con- 

 ventional ionosphere. This section 

 will be devoted to the outer regions 

 of the magnetosphere proper; the 

 inner portion will be treated as part 

 of the ionosphere in the next section. 



This conceptual model of the earth's magnetic field is based on years of spacecraft 

 observations. The dot marked "moon" indicates the relative distance at which the 

 moon's orbit intersects the plane containing the sun-earth line and geomagnetic axis. 



