GENERAL ASPECTS OF UPPER ATMOSPHERIC PHYSICS 
drop is still not known with any reasonable degree of 
certainty. 
Thanks to the work of Taylor [100] and of Pekeris 
[86], the long-standing problem of semidiurnal baro- 
metric oscillations and, along with it, the difficulty of the 
dynamo theory of quiet-day (solar) magnetic variations 
appears to have been solved. But the lunar semidiurnal 
barometric oscillations and the corresponding magnetic 
variations still have their puzzles. The barometric oscil- 
lations have unaccounted-for variations both in regard 
to amplitude and phase. 
Our knowledge of upper atmospheric ionization has 
been extended remarkably by radio exploration utilis- 
ing the powerful pulse technique. Notable additions to 
our knowledge have also been made by the adaptation 
of radar technique for this purpose. The ionosphere, 
however, is still full of mystery. Very plausible hy- 
potheses have been put forward regarding the produc- 
tion of the different ionospheric regions, but it is safe to 
say that the last words on the subject have not yet 
been spoken. 
Part, at least, of what is known as sporadic H is now 
known certainly to be due to meteoric ionization. The 
sudden bursts of ionization that had been noticed by 
many ionospheric workers have been traced to ioniza- 
tion produced by individual meteor trails [55]. It may 
be recalled, m this connection, that 450 kg of meteoric 
material burn up every day in the atmosphere near the 
level of Region EH. Still, there is clear evidence that 
meteors cannot be assumed to be responsible for all 
sporadic E [91]. Also the imerease of meteoric echo 
frequency, when the wave length is increased from 6 to 
8 m (the large background rate), is still not quite 
explained [55, 89, 105). 
An interesting observation on radio echoes from 
meteoric trails requires further clarification. Echoes of 
long duration show, besides a regular decay, a periodic 
fluctuation in intensity. It has been suggested that 
such a fluctuation may be caused by variations of 
wind with height [45]. These winds deform the straight 
column left by the meteor. Further observations im 
different latitudes are needed to test this suggestion 
and also to find out how far the magnetic field of the 
earth influences the cross section of the ionized column. 
But perhaps the central problem in the study of radio 
echoes from meteors is to find the mechanism by which 
sufficient electron density, for periods of more than a 
minute, could be maintained against the forces of diffu- 
sion [18, 45, 56]. 
The reason for the bifurcation of the F-layer into F, 
and F. during daytime is not yet understood. Associa- 
tions between F, ionization and tropospheric condi- 
tions have been observed. But what is the origin of 
such association? 
It is now suspected that many of the anomalous 
behaviours of Region F, may be traced to the effect of 
the terrestrial magnetic field, because ions and electrons 
in Region F, unlike those in Region E, have long mean 
free paths. For example, a geomagnetic control of 
Region F, in the form of a belt of low ionization round 
the geomagnetic equator, has been observed [4]. A 
257 
plausible explanation of this has also been given [71] 
but the phenomenon needs much fuller investigation 
[53]. 
An attempt has also been made to explain some of the 
F,-region anomalies by action of atmospheric tidal 
movements [59]. For example, the variations of h’ 
(minimum height) and Aya: (height of the region of 
maximum ionization) have been explained as due to 
simple rising and falling of isobaric surfaces, caused by 
tides. Variation of Naz (maximum electron density) 
cannot, however, be so simply explained. For this a 
theory has been developed in which it is shown that 
horizontal tidal motion of ionized masses gives rise to 
electrodynamic forces which produce a vertical com- 
ponent everywhere except near the magnetic equator. 
Many of the observed anomalous behaviours of Region 
F, have been explained by this theory. However, a 
complete theory of the F.-region is still lacking. 
An ionospheric region is by no means smooth in its 
ionization. Patches or clouds of more intense ionization, 
against a general background of uniform ionization, 
have been detected by workers in different countries 
[91]. As a matter of fact, the existence of winds in the 
high regions of the ionosphere has been established by 
systematic study of the movements of these clouds. 
The clouds are generally believed to be produced by 
meteor ionization. But their exact nature and life his- 
tory are still little known. More observational data in 
different latitudes are needed. 
The coefficient of recombination of electrons and 
ions in the high ionized regions, as deduced from ob- 
servations, is found to be several orders higher than 
the theoretical value. This discrepancy appeared to 
have been removed by the hypothesis of effective re- 
combination coefficient [7, 61]. Further, the identifica- 
tion of the nocturnal Region F with the luminescent 
layer of the upper atmosphere emitting the O lines and 
N>» bands has helped to unify the effective recombina- 
tion hypothesis with the emission process of these lines 
and bands [89, 69]. Contemporary work, however, ques- 
tions the soundness of the effective recombination hy- 
pothesis. It has been pointed out by Martyn [59] that 
if account is taken of the electrodynamic forces de- 
veloped as a result of the tidal motions, a term appears 
in the expression of the recombination equation which 
becomes important in Region F. The computed result, 
takmg into account the contribution of this term, 
agrees with the observed value of the recombination 
coefficient. A closer comparative study of the effective 
recombination hypothesis and Martyn’s hypothesis is 
needed to estimate the relative importance of the two 
in bringing agreement between observed and theoreti- 
cally computed values. 
Evidence of association between weather conditions 
near the ground and ionization density in Region F, has 
been obtained. No theoretical explanation of such asso- 
ciation has yet been given. 
It is highly desirable that more systematic observa- 
tions on ionospheric data, particularly in regard to 
ionospheric absorption and ionospheric tides, be made 
in different parts of the world. With regard to the tidal 
