186 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



tions observed in signals from the ionized layers. These layers are far 

 from uniform. They have a granulated structure, and the motion of 

 the granules, which are quite large, may be deduced from analysis of 

 signal irregularities. 



The ionized layers are produced by electromagnetic radiations from 

 the sun. A given ionizing radiation will produce electrons at an in- 

 creasing rate per unit volume as it penetrates into denser and denser 

 atmospheric regions, until absorption begins to be large; after this 

 point it will decrease to zero, when the absorption is complete. The 

 particular radiations responsible for the main ionized layers are still 

 not identified with certainty. It is of interest to note, however, that by 

 sending up in rockets counters sensitive to X-rays, definite evidence 

 has been obtained that, at E-region levels, there are sufficient soft 

 X-rays in the solar radiation to produce the E-region ionization. It 

 was suggested some time earlier by Hoyle and Bates [2] that X-rays 

 emitted from the solar corona might be responsible for the E layer. 

 The F ionization may be due to ionization of atomic oxygen by short 

 ultraviolet radiation, but this is far from certain. There is no doubt, 

 however, from evidence obtained by observing the delay between ob- 

 scuration of the solar disk and the onset of ionospheric changes during 

 a solar eclipse, that the main layers are due to radiations traveling 

 from the sun with the velocity of light. 



The tidal motion of the upper atmosphere exerts dynamical forces 

 on the electrons and ions in the ionized layers, producing a current sys- 

 tem, the nature of which is complicated by the earth's magnetic field. 

 According to the dynamo theory, first suggested by Balfour Stewart 

 [3] as early as 1883, the tidal flow across the field produces a current 

 in a direction perpendicular to each. The magnetic field at ground 

 level includes a contribution from the field of this current system, and 

 this will vary periodically with the tidal motion. These magnetic 

 variations exhibit components varying with the lunar and solar tidal 

 periods. Recent theoretical work has produced strong confirmation 

 of the dynamo theory. Singer, Maple, and Bowen [4] performed a 

 remarkable experiment with a rocket carrying a total-field magne- 

 tometer so that the variation of field with altitude could be measured. 

 In passing through a current layer there should be a sudden decrease 

 in field. Such a decrease was found to occur at an altitude of 93 km. 

 (fig. 4), about the expected altitude of the base of the dynamo current 

 layer. 



THE AURORA AND AIR-GLOW 



There is a clear correlation between the onset of an auroral display 

 and the occurrence of disturbed conditions on the sun. However, 

 unlike the ionospheric-solar relation, there is a time lag of one or two 

 days between the onset of the solar disturbance and that of the aurora. 



