PART I — THE SOLAR-TERRESTRIAL ENVIRONMENT 



sphere in the anti-solar direction is 

 stretched out by the action of the 

 solar wind into a long "tail," much 

 like the tail of a comet. The geo- 

 magnetic field lines are straight, with 

 the field itself directed away from 

 the earth (and the sun) in the south- 

 ern half and toward the earth in the 

 northern half. 



The geomagnetic tail is now rec- 

 ognized to play a vitally important 

 intermediate role as a reservoir of 

 stored solar-wind energy. Its for- 

 mation requires some form of energy 

 transport across the boundary be- 

 tween the magnetosphere and the 

 solar wind, but whether this transport 

 is accomplished by a process analog- 

 ous to viscosity in a fluid, or by the 

 coupling together of geomagnetic and 

 interplanetary magnetic fields, or by 

 some more exotic process is not yet 

 known. 



Equally mysterious are the pro- 

 cesses by which the tail releases en- 

 ergy. While some of the enormous 

 energy stored in the tail is continually 

 being drained into the earth's at- 

 mosphere, the most dramatic releases 

 are associated with relatively short 

 bursts, known as auroral substorms, 

 which can recur at intervals of a 

 few hours. They are accompanied 

 by disruptions of radio communica- 

 tions and surges on long power lines 

 that can result in power outages. 

 Associated increases in radiation-belt 

 particle fluxes shorten the lives of 

 communication satellites by degrad- 

 ing the performance of the solar cells 

 on which their power supply depends. 



As noted earlier, the substorms are 

 thought to have many analogies to 

 solar flares. An understanding of 

 their mechanisms may thus lead to 

 an understanding of the flare mech- 

 anism. This understanding is vital 

 to our future ability to predict the 

 whole gamut of solar-terrestrial phe- 

 nomena that affect communications 

 and power supplies and may also 

 provide some insight into the plasma- 

 confinement mechanisms that are 

 needed to achieve controlled thermo- 



nuclear fusion. Fortunately, the sub- 

 storm mechanism can be studied di- 

 rectly through satellite probes of the 

 tail region in which the release of 

 energy takes place. 



Radiation Belts — The great en- 

 ergy released in the form of an 

 auroral substorm also serves to re- 

 plenish the radiation belts that sur- 

 round the earth with magnetically 

 trapped particles. The discovery of 

 these belts was the first dramatic 

 result of the Space Age in terms of 

 exploration of our near-space envi- 

 ronment. A broad mapping of their 

 structure and behavior has now been 

 obtained, although no complete ex- 

 planation yet exists of the sources 

 of the belts or of their dynamic 

 behavior. At first the belts were 

 thought to be fairly static and well- 

 behaved. Nature seemed to have pre- 

 sented us with an example of stably 

 confined high-temperature plasma. 

 It is now clear, however, that the 

 individual particles in the outer por- 

 tions of the belts are continuously 

 experiencing a variety of processes, 

 including convection in space, accel- 

 eration, and precipitation into the 

 atmosphere. Plasma instabilities of 

 some kind associated with the growth 

 of hydromagnetic and electromag- 

 netic waves in the magnetosphere 

 seem to be of major importance. 

 Similar instabilities have prevented 

 the confinement of high-temperature 

 plasmas in the laboratory. 



The Plasma-pause — In addition to 

 confining the magnetosphere to a 

 sharply bounded cavity on the sun- 

 ward side, and stretching it out into 

 a long tail in the anti-solar direction, 

 the solar wind apparently generates 

 a vast system of convection that 

 affects the plasma throughout the 

 outer magnetosphere. This convec- 

 tion system pulls plasma from the 

 sunward side of the magnetosphere 

 over the top of the polar caps into 

 the tail, where a return flow carries 

 it back toward the earth, around the 

 sides, and back out to the front of the 

 magnetosphere. 



Another of the great boundary 

 surfaces of the magnetosphere, known 

 as the "plasmapause," marks the di- 

 viding line between plasma that is 

 influenced by this convection and 

 plasma that is tightly bound to the 

 earth and corotates with it. The 

 plasmapause generally lies some 4 

 earth-radii out from the center of 

 the earth above the equator, and 

 follows the shape of the geomagnetic 

 field lines from there to meet the 

 ionosphere at about 60° magnetic 

 latitude. In common with other mag- 

 netospheric boundaries, such as the 

 magnetopause, it is extremely well 

 marked, and the properties of the 

 magnetospheric plasma change 

 abruptly in crossing it. 



Although the close relationship be- 

 tween the plasmapause and the bound- 

 ary of the convection region has been 

 fairly well established, several fea- 

 tures of the plasmapause remain un- 

 explained. These include the sharp- 

 ness of the plasma changes on either 

 side, the shape of the plasmapause 

 at any given instant, and its radial 

 motions in time. There is some 

 evidence that inward movements 

 of the plasmapause during magnetic 

 storms have a bearing on the so- 

 called ionospheric storms, when the 

 density of the mid-latitude iono- 

 sphere drops sharply, leading to a 

 deterioration in radio communication. 

 Experiments aimed at probing the 

 plasmapause are presently being 

 planned with the aim of improving 

 our understanding of the mechanisms 

 influencing its formation and its dy- 

 namic behavior. 



The Ionosphere 



The ionosphere (see Figure 1-4) is 

 defined here as the electrically charged 

 component of the earth's upper at- 

 mosphere, consisting of free elec- 

 trons, heavy positively charged ions, 

 and a relatively small number of 

 heavy negatively charged ions. The 

 non-charged component — i.e., the 

 atmosphere itself — will be consid- 

 ered in the next section. 



