PROCEEDINGS OF THE IRE Relations Between the Character of Atmospherics and Their Place of Origin* J. CHAPMANt AND E. T. PIERCET Summary-From recent experimental work at Cambridge, Eng., it is shown that atmospherics originating from different geographical localities are systematically different in character, even when the distances of propagation are the same and there is no reason to an- ticipate appreciable dissimilarities in the ionospheres along the re- spective propagation paths. Detailed and precise information is given of how these "geographical" effects may be traced by recording and classifying types of waveforms. It is also shown that the effects are apparent for observations of atmospherics at fixed frequencies be- tween 0.65 and 27 kc. No attempt is made to assign a reason for the geographical phenomena, but the most promising approach would seem to be by considering differences in the conductivity of the earth's surface, and in particular, those between land and sea. THE WAVEFORMS OF ATMOSPHERICS T IS NOW clear that the main factors influencing the waveform of an atmospheric are the distance of the source from the observing station and the con- dition of the lower ionosphere; information is, however, becoming increasingly available'-' that even when these two factors are the same, sources in different geo- graphical localities produce quite dissimilar types of waveform. Much of the evidence for these "geographical effects" has been presented in a rather vague and indef- inite manner; the object of this note is to add precision to, and to somewhat extend, the previous conclusions. It is first necessary to classify waveforms into types. This kind of subjective division is unfortunately de- pendent upon the receiver characteristics and display employed; differences are often found between classifica- tions of two groups of workers, although results obtained by individuals within the same group are usually con- sistent. Here, a division is made into four types of wave- form, all originating in the return stroke of the lightning flash; it is hoped that with the assistance of the brief notes and Fig. 1 the relation of this classification to others previously given may be easily recognized. The waveform types are: 1) Reflection-Contains a series of five or more pulses whose spacing fits the concept of successive re- flections from an ionosphere of constant height. * Original manuscript received by the IRE, December 11, 1956. Paper presented at Symposium on Propagation of Very-Low-Fre- quency Electromagnetic Waves, Boulder, Colo.; January 23-25, 1957. t Courtald's Ltd., Coventry, Eng. Formerly with Cavendish Lab., Cambridge, Eng. t Cavendish Lab., Cambridge, Eng. 1 F. E. Lutkin, 'The nature of atmospherics VI," Proc. Roy. Soc., vol. 171A, pp. 285-313; June, 1939. ' P. G. F. Caton and E. T. Pierce, 'The waveforms of atmos- pherics," Phil. Mag., vol. 43, pp. 393-409; April, 1952. 3 F. Horner and C. Clarke, "Some waveforms of atmospherics and their use in the location of thunderstorms," J. Atmos. Terr. Phys., vol. 7, pp. 1-20; January, 1955. REFLECTION PEAKY SHORT SMOOTH SHORT QUASI-SINUSOIbAL Fig. 1-These waveforms were recorded using a continuously running time-base. Triggering could therefore occur at any point on the time-base sweep and the actual starts of the waveforms are in- dicated by arrows. The total time of sweep is 10 milliseconds for the reflection type waveform, and 2 milliseconds for the other types. 2) Peaky Short-Contains five or fewer pulses which decrease rapidly in size; in view of the small number of pulses and the broad nature of some of the peaks, it is difficult to demonstrate a fit to the reflection mechanism on any stringent criterion. 3) Smooth Short-Contains five or less smooth oscil- lations of rapidly decreasing size, but which join to give a continuous variation in electric field. Does not fit the reflection concept. 4) Quasi-sinusoidal-Contains six or more smooth oscillations, several being of comparable amplitude. Does not fit the reflection concept. All waveforms, recorded by standard methods2 at Cambridge during the past few years, for which Sferics4 estimates of the place of origin were available, were classified, as far as possible, on the above system. Sixteen regions, defined by four zones of distance and four quadrants, centered on Cambridge, were then con- sidered: the quadrants were North to East, East to South, etc.; the zones of distance, 0-1000 km, 1000-2000 km, 2000-3000 km, and beyond 3000 km. The results were separated into "day" and "night" categories, all waveforms received at times less than three hours re- moved from ground sunset or sunrise at Cambridge I C. V. Ockenden, "Sferics," Met. Mag., vol. 76, pp. 78-84; April, 1947. 804 June
Transcript
PROCEEDINGS OF THE IRE
Relations Between the Character of Atmosphericsand Their Place of Origin*
J. CHAPMANt AND E. T. PIERCET
Summary-From recent experimental work at Cambridge, Eng.,it is shown that atmospherics originating from different geographicallocalities are systematically different in character, even when thedistances of propagation are the same and there is no reason to an-ticipate appreciable dissimilarities in the ionospheres along the re-spective propagation paths. Detailed and precise information is givenof how these "geographical" effects may be traced by recording andclassifying types of waveforms. It is also shown that the effects areapparent for observations of atmospherics at fixed frequencies be-tween 0.65 and 27 kc. No attempt is made to assign a reason for thegeographical phenomena, but the most promising approach wouldseem to be by considering differences in the conductivity of theearth's surface, and in particular, those between land and sea.
THE WAVEFORMS OF ATMOSPHERICS
T IS NOW clear that the main factors influencingthe waveform of an atmospheric are the distance ofthe source from the observing station and the con-
dition of the lower ionosphere; information is, however,becoming increasingly available'-' that even whenthese two factors are the same, sources in different geo-graphical localities produce quite dissimilar types ofwaveform. Much of the evidence for these "geographicaleffects" has been presented in a rather vague and indef-inite manner; the object of this note is to add precisionto, and to somewhat extend, the previous conclusions.
It is first necessary to classify waveforms into types.This kind of subjective division is unfortunately de-pendent upon the receiver characteristics and displayemployed; differences are often found between classifica-tions of two groups of workers, although results obtainedby individuals within the same group are usually con-sistent. Here, a division is made into four types of wave-form, all originating in the return stroke of the lightningflash; it is hoped that with the assistance of the briefnotes and Fig. 1 the relation of this classification toothers previously given may be easily recognized.The waveform types are:1) Reflection-Contains a series of five or more
pulses whose spacing fits the concept of successive re-flections from an ionosphere of constant height.
* Original manuscript received by the IRE, December 11, 1956.Paper presented at Symposium on Propagation of Very-Low-Fre-
quency Electromagnetic Waves, Boulder, Colo.; January 23-25,1957.
t Courtald's Ltd., Coventry, Eng. Formerly with CavendishLab., Cambridge, Eng.
t Cavendish Lab., Cambridge, Eng.1 F. E. Lutkin, 'The nature of atmospherics VI," Proc. Roy. Soc.,
vol. 171A, pp. 285-313; June, 1939.' P. G. F. Caton and E. T. Pierce, 'The waveforms of atmos-
pherics," Phil. Mag., vol. 43, pp. 393-409; April, 1952.3 F. Horner and C. Clarke, "Some waveforms of atmospherics and
their use in the location of thunderstorms," J. Atmos. Terr. Phys.,vol. 7, pp. 1-20; January, 1955.
REFLECTION PEAKY SHORT
SMOOTH SHORT QUASI-SINUSOIbAL
Fig. 1-These waveforms were recorded using a continuously runningtime-base. Triggering could therefore occur at any point on thetime-base sweep and the actual starts of the waveforms are in-dicated by arrows. The total time of sweep is 10 milliseconds forthe reflection type waveform, and 2 milliseconds for the othertypes.
2) Peaky Short-Contains five or fewer pulses whichdecrease rapidly in size; in view of the small number ofpulses and the broad nature of some of the peaks, it isdifficult to demonstrate a fit to the reflection mechanismon any stringent criterion.
3) Smooth Short-Contains five or less smooth oscil-lations of rapidly decreasing size, but which join to givea continuous variation in electric field. Does not fit thereflection concept.
4) Quasi-sinusoidal-Contains six or more smoothoscillations, several being of comparable amplitude. Doesnot fit the reflection concept.
All waveforms, recorded by standard methods2 atCambridge during the past few years, for which Sferics4estimates of the place of origin were available, wereclassified, as far as possible, on the above system.
Sixteen regions, defined by four zones of distance andfour quadrants, centered on Cambridge, were then con-sidered: the quadrants were North to East, East toSouth, etc.; the zones of distance, 0-1000 km, 1000-2000km, 2000-3000 km, and beyond 3000 km. The resultswere separated into "day" and "night" categories, allwaveforms received at times less than three hours re-moved from ground sunset or sunrise at Cambridge
I C. V. Ockenden, "Sferics," Met. Mag., vol. 76, pp. 78-84; April,1947.
804 June
Chapman and Pierce: Character of Atmospherics and Their Origin
being rejected in order to avoid possible twilight effects.Tables I and II, showing the relative percentages of thedifferent types recorded from each region by night andby day, were then constructed; they are based on rec-ords of over a thousand individual waveforms. Somediscretion was involved in the preparation of the tables;in particular, no entries are given for the regions fromwhich few fixes were reported, and for which the per-centage figures would accordingly be very misleading.
TABLE IPERCENTAGE OCCURRENCE OF DIFFERENT
WAVEFORM TYPES BY NIGHT
Distance Direction Type of Waveform
(K(M) (Quadrant) Reflection Peaky Smooth Quasi-Short Short sinusoidal
It is apparent from Table I that at night reflectionwaveforms are received from all distances to the South-East, but in the South-West there is a change from re-
flection to smooth types between 1000 and 2000 km,most of these smooth waveforms being of the quasi-
sinusoidal kind. There is some evidence indicating firstlythat at any instant the alteration in waveform tyrpewith distance occurs very sharply; i.e., within one or twohundred kilometers, and secondly, that the transitiondistance varies from time to time; thus, averaging overa long period results in a transition zone rather than in asudden changeover distance. It is therefore probablymisleading to specify any single figure for the change-over distance, but nevertheless, values of 1600 km and3000 km have been suggested by Caton and Pierce2 andHorner and Clarke.3 The limits 1000-2000 km, given inTable I for the transition zone, are in accord with thefigure of 1600 km but not with that of 3000 km.By day, for all directions, peaky short waveforms are
the most common variety from within 1000 km, but atgreater distances there is a preponderance of smoothwaveforms of the smooth short kind. The effect firstnoted by Lutkin' that waveforms from the West aresmoother and contain more oscillations than those fromthe East is confirmed by the results represented in TableII. At comparable distances, South-Westerly regionsgive more smooth types with a greater proportion ofquasi-sinusoidal atmospherics than do the South-Eastern areas.
THE FREQUENCY CONTENT oF ATMOSPHERICSThe lack of precision involved in using subjective
methods of waveform classification may be avoided byemploying tuned receiver techniques as used, for exam-ple, by Bowe.5 In order to ascertain whether the geo-graphical effects found for waveforms are also evidentin observations on fixed frequencies, Table III was con-structed. This represents the averaged ratio of the re-sponses on certain frequencies for daytime atmosphericsoriginating within 1500 km, from sources to the East,
TABLE IIIRATIOS OF RESPONSES AT CERTAIN FIXED FREQUENCIES FOR DAY-
TIME ATMOSPHERICS FROM EASTERLY AND WESTERLY SOURCES
and to the West, respectively. Table III may be con-sidered as indicating the relative content at the dif-ferent frequencies for waveforms originating, respec-tively, from the two directions; it is apparent that thespectrum is comparatively broad for atmospherics fromthe East, but narrow and peaked around 10 kc for thosefrom the West.
5P. W. A. Bowe, "The waveforms of atmospherics and the propa-gation of very low frequency radio waves," Phil. Mag., vol. 42, pp.121-138; February, 1951.
1957 05
PROCEEDINGS OF THE IRE
CONCLUSIONIt has been shown that relations between the charac-
ter of atmospherics and their place of origin may beidentified both from the study of waveforms and fromrecords on fixed frequencies. This is, of course, not un-expected, since waveform and frequency content are ofnecessity interrelated, and both methods of observationreflect the modifications introduced during propagation.A paper by Chapman and Pierce6 shows how particulartypes of waveform are associated with characteristicspectra; in consequence, all phenomena observed forwaveforms may be expressed in terms of frequencies.The reasons for the geographical effects are still not
entirely clear. Suggested explanations include differ-ences in propagation over land and sea paths, a de-pendence of propagation on orientation with respect tothe earth's magnetic field, a possible distinction betweenlightning flashes to land and to sea, and differences be-tween oceanic and continental thunderstorms. It is dif-ficult to distinguish between these possible influences
6 J. Chapman and E. T. Pierce, "The waveforms, frequencyspectra and propagation of atmospherics," Proc. 1956 Paris Conf. onWave Prcpagation. Also Onde Electriqgue; May, 1957.
from observations in the British Isles, and further workin a more favorable locality is therefore highly desirable.It seems most likely, however, that the important factoris the difference in propagation over land and over sea.This interpretation is supported by experimental obser-vations on long wave radio stations, e.g., Tremellen,7and by the recent theoretical work of Wait.8 It is per-haps noteworthy that the geographical phenomena aremost pronounced at night when, the differences be-tween the conductivities of the earth and the lowest ion-osphere being less than by day, variations in the con-ductivity of the earth's surface, e.g., between land andsea, may be expected to have their maximum effect.
ACKNOWLEDGMENT
We are grateful to Miss Shelagh M. Cussen forchecking some of the data in this paper.
7 K. W. Tremellen, "A study of ionospheric ray field intensity inthe 10-30 and 500-1100 kc/s bands," Marconi Rev., vol. 13, pp. 153-166; Fourth Quarter, 1950.
8 J. R. Wait, "On the mode theory of vlf ionospheric propagation,"Proc. 1956 Paris Conf. on Wave Propagation. Also Geof. Pura. Ap-plicata (Milan)., vol. 37, June; 1957.
A Technique for the Rapid Analysis of Whistlers*J. KENNETH GRIERSONt
Summary-This paper discusses the design of a new type ofsound spectrograph, intended for analyzing whistlers. This instru-ment is of the single-channel scanning type, but its basic action is toscan the frequency-time plane in frequency at a fixed time ratherthan, as in existing instruments, in time at a fixed frequency. Theprinciple is as follows. First, a very short section of the signal tobe analyzed (roughly equal in duration to the reciprocal of thebandwidth of the analyzing filter) is stored in the instrument elec-tronically. The stored signal is then read out repeatedly many timesfaster than its original speed, and this repeated waveform is ana-lyzed by a variable-tuned filter which sweeps once very rapidlythrough the expanded frequency band which the signal now occupies.The varying output from the filter, representing the variations of theamplitude of the signal with frequency at one particular time, isrecorded as one line of a scan across a continuous strip display.Finally, the stored sample of signal is erased, replaced with the nextsample, and the whole process repeated. An audio frequency spectro-graph of this type appears to combine speed of operation with fineresolution in frequency, rendering it useful for the rapid analysis ofwhistlers.
INTRODUCTION
N\XJHISTLERS are naturally occurring radio signalsof audible frequency. They may be receivedon a vertical wire or loop antenna, amplified by
* Original manuscript received by the IRE, February 13, 1957.Paper presented at Symposium on Propagation of Very-Low-Fre-quency Electromagnetic Waves, Boulder, Colo., January 23-25,1957.
t Defence Research Board, Ottawa, Ont., Canada.
a straightforward audio-frequency amplifier, and madeaudible by connecting the output of the amplifier to aset of headphones or a loudspeaker; they may also berecorded on tape for subsequent analysis. Whistlersconsist of gliding tones; that is, tones whose frequencieschange with time; some forms of whistlers are relativelypure tones, while others (swishes) sound more like bandsof noise with progressively changing mean frequency.There is interest attached to the way in which themean frequency of the tones depends on time, and it wasthe need to be able to graph this dependence rapidly andautomatically that initiated the present study.
This paper describes the principles involved in thedesign of a new type of audio-frequency spectrograph,for making a visible record of the distribution of theenergy of a signal, in time and frequency. Although thetechnique proposed might have other uses in the audio-frequency field, it will be considered here only in appli-cation to whistlers.
PREVIOUS USE OF SPECTROGRAPHS IN WHISTLERRESEARCH
The spectrograph, in various forms, has been themain research tool in the investigation of whistlers.
