SECRETARY'S REPORT 149 



in each of the three areas is best advanced by an iterative process 

 in which refinements of the parameters are accomplished simultane- 

 ously or cyclically for a number of satellites. This diverse program 

 is under the broad guidance of Dr. Fred L. Whipple, director of the 

 Observatory. 



From 5 years of investigation since the first artificial satellite, we 

 now know much about the high atmosphere. The past year saw 

 Dr. Luigi G. Jacchia's timely preparation of a survey, "Variations 

 in the Earth's Upper Atmosphere as Revealed by Satellite Drag," for 

 the Reviews of Modem Physics} The comprehensive content of tliis 

 review stands witness to the sensitivity and refinement of the tech- 

 niques developed and employed at SAO. 



Analyses by Dr. Jacchia and Jack W. Slowey have established that — 



(1) Both electromagnetic (extreme ultraviolet) and corpuscular radiation 

 from the sun contribute to the heating of the upper atmosphere. 



(2) Most of the energy carried by these two forms of radiation is absorbed 

 at heights lower than 200 km ; the atmosphere above this level is heated by 

 conduction from below. 



(3) The greater heating in the sunlit hemisphere gives rise to a permanent 

 atmospheric "bulge," at the center of which the temperature is 40 percent 

 higher than it is at the opposite point in the dark hemisphere. Because of 

 the earth's rotation, this bulge travels around the globe at a latitude equal to 

 that of the subsolar point; its longitude is the one for which the local time 

 is 2 p.m. 



(4) The temperature of the upper atmosphere can be correlated with the 

 decimetric (radio) solar flux, which exhibits variations with characteristic 

 cycles of 27 days (caused by the rotation of the sun) and of 11 years (caused 

 by the suuspot cycle). The temperature can be computed and instantaneous 

 density profiles derived from atmospheric models when the decimetric solar 

 flux is known. 



(5) The atmosphere of the earth is heated and expanded during magnetic 

 storms by a factor directly related to the geomagnetic planetary index ap. 



(6) The semiannual effect in upper atmospheric densities is real. This shows 

 that the solar wind contributes substantially to atmospheric heating, even during 

 quiet periods. 



During the past year larger quantities of precisely reduced tracking 

 data, particularly for satellites of quite different inclinations, have 

 become available from the Baker-Nunn system. Imre Izsak, Dr. Yo- 

 shihide Kozai, and their associates have used these enlarged data in 

 new determinations of the coefficients in an expansion of the gravi- 

 tational field of the earth in spherical harmonics.^ 



Mr. Izsak has given particular attention to determination of coef- 

 ficients of higher-order tesseral and sectorial harmonics. The per- 

 turbation theory of these effects being well developed, the problem 

 actually consists of the construction of extensive computer programs 

 that would analyze the large number of observations available. Sev- 



See footnotes on p. 164. 

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