30 SECTIONAL ADDRESSES. 



in the daytime. However, Chapman points out that if we assume the 

 convective motion of the upper air to be of the same general character as 

 at ground level, the current foci should be on the meridian at 1 p.m. or 

 2 p.m. instead of 10 a.m. or 11 a.m. as is observed. The conductivity of 

 the medium is attributed to ionisation due to the sun, and Chapman is of 

 opinion that the total electric conductivity must be of the order 10~^ e.m.u. 

 This appears to be rather high, since P.O. Pedersen's analysis of the 

 propagation of radio waves leads to a value of only 5.10"''. However, 

 more recent observations on the reflection of wireless waves indicate a 

 conductivity of the order 10"* as the number of electrons per c.c. in the 

 reflecting layer, and for a thick layer a conductivity of the right order is 

 possible. 



Appleton has shown that the height of the ionised layer gradually 

 rises after sunset and reaches a maximum value about one hour before 

 sunrise, after which a somewhat rapid lowering results. Normally, the 

 height at night varies from 90 to 130 kilometres, but occasionally in winter 

 heights as great as 250 to 350 kilometres have been measured. Appleton 

 suggests that on some nights the ionisation in the lower layer has been 

 so much reduced by recombination that penetration by wireless waves is 

 possible. In such cases, however, deviation of wireless waves takes place 

 at an upper layer which is richer in ionisation. The lower layer is formed 

 again at sunrise and deviation by it again results. As the solar influence 

 increases ionisation likewise increases, and the outer boundary of the 

 lower layer falls, and evidence has been obtained of the formation of yet 

 another lower region of ionisation. 



As a first and crude approximation we may, therefore, imagine a 

 spherical conducting layer to surround the earth, and in addition a con- 

 ducting hemispherical cap over the hemisphere facing the sun, the height 

 of this cap being a few hundred kilometres. Neither the complete 

 spherical conducting shell nor the hemispherical cap are of uniform con- 

 ductivity, and the matter constituting these layers moves with the earth, 

 so that ionisation and recombination are always taking place. 



While we have from wireless measurements fairly good evidence of 

 the height of the lower conducting layers, our knowledge of the extent 

 of the ionisation is not sufficiently good to enable us to do more than 

 speculate on the merits of the theories advanced, for in addition to the 

 dynamo theory there is one due to Ross Gunn known as the diamagnetic 

 layer theory, and a third called the drift current theory. The difierences 

 between the theories are best brought out by considering the ionisation 

 effects in the hemispherical conducting cap facing the sun. Pederson 

 has calculated the number of electrons and ions per cubic centimetre at 

 various heights, and he and Ross Gunn have considered the nature and 

 magnitude of the conductivity of the upper ionised regions. They have 

 shown that the conductivity varies with the direction of the magnetic 

 field, the conductivity at right angles to the field being at times very small, 

 and under certain conditions it approaches zero, while the conductivity 

 in the direction of the field is unaffected by the field's intensity. It is 

 pointed out that the transverse conductivity is reduced in the ratio 

 N2/(N2+o)^) where N is the number of collisions per second and co is the 

 angular velocity of the electrons in their spiral paths round the magnetic 



