TART I — THE SOLAR-TERRESTRIAL ENVIRONMENT 



(Thomson) scatter radar technique, 

 which allows direct ground-based 

 investigation of high-lying ionization. 

 This has added immensely to our 

 knowledge of the mechanisms in- 

 fluencing the F region. It has been 

 possible to measure not only electron 

 concentration, but also temperatures 

 of the different plasma components 

 and effects of electric fields in causing 

 motion of the plasma. Results from 

 existing and planned scatter radar 

 installations will undoubtedly add a 

 great deal to our knowledge of this 

 outermost region of our atmosphere, 

 and will, hopefully, lead to improve- 

 ments in our ability to predict 

 changes in radio propagation condi- 

 tions. 



Two F-region phenomena are of 

 special interest at this time. These 

 are the "polar wind" and the "iono- 

 spheric storm," both of which are 

 more than mere curiosities. The 

 polar wind, which has been predicted 

 on theoretical grounds but not yet 

 adequately verified observationally, 

 arises because of the existence of the 

 long geomagnetic tail described in 

 the previous section. The F-region 

 plasma can diffuse quite freely along 

 the direction of the earth's magnetic 

 field, and as long as the field lines 

 loop back into the opposite hemi- 

 sphere of the earth no plasma is 

 lost thereby. In the polar regions, 

 however, the field lines are greatly 

 stretched by the solar wind and 

 eventually become lost in interplan- 

 etary space. When F-region plasma 

 travels out along these field lines, it 

 ultimately disappears. This outward 

 flow is expected to be at least partly 

 supersonic; it plays a large part in 

 the loss of the lighter constituents 

 (hydrogen and helium) from the at- 

 mosphere. 



Ionospheric storms, by contrast, 

 have been recognized observation- 

 ally for many years but still have 

 no adequate theoretical explanation. 

 Over most of the earth they appear 

 as a rather rapid decrease in the 

 electron concentration of the F region, 

 accompanied by a corresponding in- 



ability to propagate radio signals that 

 normally propagate freely. Recovery 

 from this effect is much less rapid 

 than its onset. The ultimate explana- 

 tion for ionospheric storms may lie 

 in a combination of inward motion 

 of the plasmapause, discussed in the 

 last section, movement of the F region 

 caused by electric fields, and changes 

 in the photochemistry of the region. 



The £ Region — As one moves 

 downward from the peak of the F re- 

 gion, photochemistry becomes stead- 

 ily more important relative to motions 

 in determining the characteristics of 

 the ionosphere. The E region, which 

 is largely formed by solar X-radiation 

 together with some EUV radiation, 

 shows quite different characteristics 

 from the F region, and many of these 

 differences arise from photochemical 

 causes. Movements of the ionization 

 are still important, however, in that 

 they give rise to very substantial 

 electric fields and currents because 

 of the difference between the colli- 

 sion characteristics of electrons and 

 ions. In fact, the whole situation is 

 analogous to a dynamo, in which an 

 electrical conductor moves in a fixed 

 magnetic field and thereby generates 

 an electric field. For this reason, the 

 region is often referred to as the 

 "dynamo region" of the ionosphere. 



The E region is the seat of the 

 major current systems responsible for 

 surface magnetic-field variations. The 

 latter are particularly pronounced 

 near the magnetic equator, where the 

 magnetic field lines are horizontal, 

 and in auroral zones, where irregular 

 changes in ionospheric conductivity 

 are associated with particle bombard- 

 ment. The great concentrations of 

 ionospheric current in these regions 

 are known respectively as the "equa- 

 torial electrojet" and the "auroral 

 electrojet" by analogy with the jet 

 streams of the lower atmosphere. 

 While the broad reasons for the 

 existence of these electrojets are 

 fairly well understood, they still pre- 

 sent many puzzling features. The 

 growth of small but intense irreg- 

 ularities within and near the electro- 



jets, in particular, presents a chal- 

 lenge in geophysical plasma physics 

 that has not yet been fully met. 



The development of the thin, dense 

 layers of electrons known collectively 

 as "sporadic E," once an outstanding 

 problem, now appears to be largely 

 explicable in terms of the interaction 

 of vertical wind shears with metallic 

 ions of meteoric origin. This problem 

 is not completely solved, however, 

 and is still an active field for theory 

 and experiment. The continuous in- 

 flux of meteoric material to the at- 

 mosphere has turned out to be quite 

 important to both the E and D re- 

 gions of the ionosphere. There are 

 many unanswered questions con- 

 nected with this meteoric material, 

 including its chemical composition, 

 its distribution within the atmos- 

 phere, and its ultimate fate. 



The D Region — After years of 

 relative neglect, a great deal of inter- 

 est is presently focused on the D 

 region, which is the real meeting 

 ground between the lower and upper 

 atmosphere. It now appears certain 

 that many of the strange facets of 

 this region's behavior are basically 

 due to meteorological effects con- 

 nected in as yet unknown ways with 

 the lower atmosphere. Thus, on cer- 

 tain winter days the D-region elec- 

 tron concentration rises abnormally; 

 this "winter anomaly" is associated 

 with sudden warmings of the strato- 

 sphere and mesosphere which are 

 probably connected with the break- 

 down of the polar winter vortex 

 of the general atmospheric circula- 

 tion. D-region electron concentra- 

 tions usually display a high degree 

 of variability during winter, while 

 they are relatively stable from day 

 to day in summer. These effects, 

 and others of a similar kind, are 

 currently arousing a great deal of 

 interest both among meteorologists, 

 who are extending their concepts 

 upward into this unexplored region 

 of the atmosphere, and among iono- 

 spheric workers, who are bringing 

 their interests downward. 



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