SECTIONAL TRANSACTIONS. 
(For reference to the publication elsewhere of communications entered in the 
following lists of transactions, see end of volume, preceding appendix.) 
SECTION A. 
MATHEMATICAL AND PHYSICAL SCIENCES. 
Thursday, September 6. 
Discussion on The ionosphere (10.0) :— 
Prof. E. V. APPLETON, F.R.S.—Introduction. 
In the absence of data derived from measurements in situ, such as are 
possible for the lower strata of the atmosphere, information concerning the 
nature of the ionosphere (80 km. and above) is derived from ground obserya- 
tions on (1) terrestrial magnetism, (2) luminous manifestations such as the 
aurorz, meteorites, etc., and (3) wireless wave exploration. Although the 
first indications of pronounced upper-atmospheric ionisation came from (1), 
the prosecution of (3) has proved, on the whole, the most fruitful. Wireless 
methods possess the marked advantage in that an exploration can be made at 
any time and it is not necessary to wait for natural sequences or irregularities. 
Wireless exploration consists in projecting waves (usually) vertically 
upwards and noting the characteristics of the returned energy. The 
quantities measurable are (a) the time of flight on the up and down journey, 
(6) the polarisation and phase changes, and (c) the intensity of the returned 
waves. Each type of measurement has been made to yield information. 
From measurements of (a) at different wave-lengths the somewhat compli- 
cated structure of the ionosphere has been broadly worked out and its tem- 
poral variations studied. From (6) conclusive evidence has been derived 
that free electrons exist throughout the whole of the ionosphere and are 
active electrical agents in causing the deviation of the waves ; while from (c) 
the frictional effect of air pressure on the free electrons may be estimated. 
Regular features—The ionosphere is divided into two main divisions, 
Region E at an equivalent height of 100 km. and Region F at an equivalent 
height of 230 km. and above. In both regions the ionisation is replenished 
daily at a rate dependent on solar altitude, and during the night steadily 
decreases. (During the day a lower ‘shelf’ is also formed on the main 
Region F.) The diurnal and seasonal variations are such as can be explained 
by assuming solar ultra-violet light as the ionising agency and recombination 
of electrons and ions as the dissipative influence. 
Irregular features —(a) There is often formed a thin reflecting sheet of 
ionisation about the height of Region E. This may occur by day or night, 
Possible influences to be discussed in connection with the origin of this 
“Abnormal Region E’ are : 
