METEORS AS PROBES OF THE UPPER ATMOSPHERE 
ing ‘the velocities of meteor streams. The strengths of 
individual pulses are recorded on a rapid time base 
triggered by early echoes from the meteoric ion trail. At 
nearly perpendicular incidence the echoes from the 
lengthening ion column vary in strength according to 
the Fresnel pattern presented from instant to instant. 
The time rate of the variations measures the angular 
velocity, which, combined with range data, gives the 
meteoric velocity. The method was proven on the 
Geminid shower of 1947 and has been used by Ellyett 
[18] to demonstrate the heliocentric character of a 
number of the daylight radiants. The radar technique 
is being used for radio meteors by Bateman, McNish, 
and Pineo [5] at the U.S. Bureau of Standards. 
The study of meteors by the continuous-wave, 
Doppler, or ‘‘whistling meteor” method was initiated 
by Chamanlal and Venkatamaran [10] in 1941. Hey, 
Parson, and Stewart [27] showed that the rapid changes 
in pitch must arise from changing range rather than 
deceleration of the meteoroid. A number of investigators 
listened to the whistles during the Giacobinid shower in 
1946. Manning and Villard, heading a group at Stanford 
University, developed, with Peterson [49], the tech- 
nique of recording ‘“‘whistle” oscillations in 1948, a 
technique that was proven by excellent velocities de- 
rived for the 1948 Perseid shower. An important mete- 
orological development by Manning and Villard [48] is 
that of determining wind velocities and directions in the 
upper atmosphere by the ‘body doppler” motion of 
the meteor ion columns. In eighteen observations from 
May to September 1949 wind velocities have varied 
from 47 to 210 km hr (mean = 105) with a predomi- 
nant direction towards SSH, but with a number of 
motions oppositely directed. This method of observing 
high-altitude winds shows exceptional promise. 
The electronic group of the Canadian Research Coun- 
cil, under the direction of McKinley and cooperating 
with Millman of the Dominion Observatory, have done 
some excellent work on meteors, combining radar tech- 
niques [53, 54] with visual and photographic techniques 
(Millman, McKinley, and Burland [58]) and have made 
most remarkable progress in measuring meteoric veloci- 
ties by the Doppler method. McKinley [52] has re- 
ported on some 3000 meteor velocities measured by 
this method; the data show no indication of extra-solar- 
system bodies. The group have succeeded in measuring 
the deceleration of a meteor by radio techniques alone,’ 
a significant step forward in meteoric and upper-atmos- 
pheric research. Since the Canadian-group plan to 
combine radio techniques with visual and photographic 
meteor observations, using Super-Schmidt cameras 
when available, their further activities should be scruti- 
nized frequently by those interested in the upper 
atmosphere. 
Theoretical progress in the radio meteor field is still 
somewhat behind the technological progress; several 
of the basic processes have not been clearly demon- 
strated. It is probable that the radio meteor echo arises 
from the coherent scattering by electrons in a long 
2. Private communication. 
363 
column of diameter small compared to the wave length, 
as suggested by Blackett and Lovell [7] and investigated 
more fully by Lovell [45] and Lovell and Clegg [46]. At 
shorter wave lengths this theory appears to be in better 
agreement with the observations than Pierce’s theory 
involving a column diameter finite compared to the 
wave length. Added confidence in the former theory is 
provided by Herlofson [25], who has semiquantitatively 
determined the fraction of the meteoroid energy de- 
voted to electron production as about 10~. 
The marked increase in the number of meteors ob- 
served at wave length 8 m as compared to 4 m is 
partially explamed. The lower limit of wave length 
useful for observing meteors, at roughly 3 m, is set by 
the high electron densities required and by the in- 
frequency of meteors with sufficient energy. The facts 
of an upper useful limit in wave length, around 100 m, 
and the time delay (several seconds) in the formation 
of the observed echoes indicate, however, that an ion 
column of finite width is required at longer wave 
lengths. The upper limit is also dependent upon the 
effects of other atmospheric ionization and the lack of 
resolving power. 
Evidence that the electron diffusion in the ion trail is 
influenced by the earth’s magnetic field has been pre- 
sented by Lovell [45], following a suggestion by Herlof- 
son. The detailed processes of dissociation, ionization, 
detachment, diffusion, turbulence, recombination, and 
attachment of electrons in meteor ion columns, how- 
ever, require much more theoretical elaboration, since 
only a modest advance has been made beyond Opik’s 
meteor theory. Even so, the present theory [45] shows 
clearly that an important fraction of sporadic-H! ioniza- 
tion must arise from some source other than meteors as 
we now understand them and that the ionization nor- 
mally produced by meteors is trivial compared to the 
normal daytime H-layer ionization. During the 1946 
Giacobinid Shower, however, meteors produced an ap- 
preciable H-layer; Pierce [71] calculated an energy flow 
of 3 watts km~ over a 4-hour period. 
Also, there still is needed a detailed theoretical mecha- 
nism for the formation of the ion cap of short duration 
that follows the motion of the meteoroid. It is difficult 
to choose among the hypotheses of electron production 
(1) ultraviolet radiation from the meteoroid, (2) en- 
counters arising from excessive mean free paths of the 
high-velocity atoms from the meteoroid, or (3) en- 
counters arising from crumbling or spraying effects from 
finite particles of the meteoroid. 
Nevertheless, these present uncertainties only serve 
to stress the enormous technological progress that has 
been made in the field of radio meteors. The future 
possibilities of upper-atmospheric research by radio 
meteors appear brilliant. 
REFERENCES 
1. Appreron, BH. V., NatsmrrH, R., and Ineram, L. J., 
“British Radio Observations During the Second Inter- 
national Polar Year 1932-33.’’ Phil. Trans. roy. Soc., 
London, (A) 236: 191-259 (1937). 
2. Appieron, E. V., and Pippineron, J. H., “The Reflexion 
