ELEMENTS OF THE SOLAR-T: F 



The practical importance of the 

 D region arises from the fact that it 

 efficiently absorbs radio waves at the 

 higher frequencies (MF and HF), and 

 reflects them at the lower frequencies 

 (LF and VLF). Both of these proper- 

 ties are greatly modified by solar dis- 

 turbances, since the energetic radia- 

 tion and particles emitted from the 

 sun at these times penetrate through 

 the thin upper regions of the iono- 

 sphere and deposit most of their 

 energy in the D region. When in- 

 tense solar flares give rise to fluxes 

 of energetic solar protons, for ex- 

 ample, the protons are funnelled by 

 the earth's magnetic field to the 

 polar regions, where they enter the 

 atmosphere and create very intense 

 ionization in the 50- to 100-kilometer 

 altitude range. The consequent strong 

 absorption of HF radio waves (known 

 as "polar cap absorption") com- 

 pletely disrupts short-wave radio 

 communication over the polar regions, 

 sometimes for days on end. The X- 

 rays emitted from the same flares 

 cause mild, brief fadeouts that extend 

 over the sunlit hemisphere of the 

 earth. 



While the problem of predicting 

 these effects is ultimately the prob- 

 lem of predicting intense solar flares, 

 it is also important to learn as much 

 as possible about the relationship 

 between the detailed characteristics 

 of individual flares and the nature 

 and magnitude of the ionospheric 

 response. A great deal has already 

 been achieved in this area through a 

 combination of ground-based radio 

 observations and direct rocket and 

 satellite measurements of the radia- 

 tions and particles responsible. 



Ionospheric Modification — One in- 

 teresting recent development is the 

 possibility of artificial modification of 

 the ionosphere. Research on this 

 problem is still in its infancy, but 

 success could lead to a major increase 

 in our ability to use the ionosphere 

 for radio-propagation purposes. Un- 

 controlled modification has been pro- 

 duced artificially by high-altitude 

 nuclear detonations; attempts are 



now being made to modify the iono- 

 sphere in more sophisticated ways 

 by releasing ion clouds from rockets 

 and by use of high-power radars on 

 the ground. This approach is likely 

 to lead eventually to greater insight 

 into the mechanisms that control 

 the natural ionosphere as well as 

 provide us with a new range of pos- 

 sible practical uses. 



The Upper Atmosphere 



This section deals with the neutral 

 gas of the upper atmosphere, as dis- 

 tinct from the electrically charged 

 component that forms the iono- 

 sphere. In terms of altitude, the two 

 overlap; indeed, they are closely 

 coupled together in many ways, so 

 that several of the problems men- 

 tioned in the preceding section are 

 inseparable from the problems of 

 the neutral upper atmosphere. The 

 neutral upper atmosphere also shows 

 a range of properties not directly 

 related to the ionosphere, however, 

 and those are the questions of con- 

 cern here. 



Like the ionosphere, the neutral 

 atmosphere has long been divided 

 into altitude regions, based mainly on 

 thermal structure. (See Figure 1-5) 

 The very lowest region of the atmos- 

 phere, in which the earth's weather 

 systems are located, is known as the 

 troposphere; here the temperature 

 generally decreases with increasing 

 altitude. Above the tropopause the 

 temperature first remains constant 

 and then increases with increasing 

 altitude through the stratosphere, 

 terminating at a temperature maxi- 

 mum near 50 kilometers altitude 

 known as the stratopause. Above 

 this lies the mesosphere, a region of 

 decreasing temperature with height, 

 which extends to about 85 kilometers. 

 The temperature at the mesopause is 

 lower than anywhere else in the at- 

 mosphere, and can be below — 150° 

 centigrade. Above the mesopause 

 lies the thermosphere, in which the 

 temperature steadily increases with 

 altitude, eventually reaching a fairly 



steady value in excess of 1,000° cen- 

 tigrade. The warm regions of the 

 upper atmosphere owe their high 

 temperatures to the absorption of 

 solar ultraviolet radiation, by ozone 

 near the stratopause and by EUV 

 radiation in the thermosphere. 



Tlic Thermosphere — The intense 

 heating experienced by the thermo- 

 sphere must set up some kind of 

 circulation pattern, analogous to the 

 circulation of the lower atmosphere 

 but differing in many important re- 

 spects because of the extreme rarity 

 of the medium and the influence of 

 the ionosphere. Little is known about 

 this circulation, but the effects of the 

 variable heat input on the density 

 of the thermosphere can be directly 

 detected through changes in the or- 

 bital period of satellites that travel 

 through the upper thermosphere. As 

 solar activity increases, the thermo- 

 sphere heats up, expands outward, 

 and increases the frictional drag on 

 satellites, thereby appreciably short- 

 ening their lifetimes. 



Thermospheric heating depends on 

 the structure of the sun's EUV spec- 

 trum and its variability with solar 

 activity, neither of which is known 

 adequately, and on the constitution 

 of the upper atmosphere and the 

 manner in which the various atoms 

 and molecules absorb the radiation. 

 The non-uniformity of the heating 

 from equator to poles causes strong 

 temperature gradients which in turn 

 give rise to very strong winds. Some 

 of the properties of these thermo- 

 spheric winds have been inferred 

 from their influence on the F region 

 of the ionosphere, which is amenable 

 to exploration by ground-based ra- 

 dio sounding, but this information is 

 still very sparse. 



The principal chemical components 

 of the thermosphere are atomic ox- 

 ygen, helium, and hydrogen; the two 

 latter, being the lightest constituents 

 of the atmosphere, tend to diffuse 

 toward the higher regions; atomic 

 hydrogen, in particular, is so light 

 that appreciable numbers of atoms 



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