APRIL 1946 111

RADIO INVESTIGATION OF THE IONOSPHERE

. by C. J. BAKKER. 621.391.11 : 551.510.535

This article gives a survey of investtgations made concerning the layers of ionized air(ionosphere) present in the atmosphere high above the earth which are able to reflectradio waves back to the earth, thus making radio reception possible at a great distancebeyond the horizon of the transmitter. Various phenomena in shortwave reception, suchas fad i n g and the skip dis tan ce, are due to 'the presence of the ionosphere.Tlie density of ions and the so-called "effective" altitude of the ionosphere can be derivedfrom measurements that. have been taken. It has be'en found that particularly the densityof ions, and also the reception of short waves, depends to a large extent upon the sim,namely upon its height above the horizon and its eruptive activity. The main, but not theonly cause of the ionization is in fact the irradiation of the atmosphere by sunlight, inpartienlar by the part of the spectrum in the far ultra-violet. In the process of ionizationthis part of the sun's spectrum is absorbed and cannot, therefore, be observed on the earth.Interesting conclusions as to the intensity of the sun's radiation in the far ultra-violetcan be drawn from data concerning the ionosphere obtained experimentally by meansof reflected radio waves. •

After the achievement of the fust transatlantic .wireless connection by Marconi ill 1901, a dis-cussion quickly arose as to how it was possible thatthe eletromagnetic waves sent out could reach thereceiving station around the curved surface of theearth. If the propagation of the waves were recti-linear, signals from a transmitter situated beyondthe horizon could never be received, so that i~hadto be assumed that the path of the waves.was not.a straight line but a curved or a broken line; bothhypotheses found supporters..

One year after Marconi's experiments Ken-nelly and Heaviside, independently of eachother, proposed the hypothesis that the high, veryrarefied layers of air"behave as a mirror for elec-trical waves and reflect the radio signalsback to theearth. The propagation of the waves could thentake place along broken lines. The reflective powerof these higher layers of air might be caused by anionization of the air, i.e. a splitting of the moleculesinto electrons and positive ions. Kennelly esti-mated the lower boundary of the .ionizationregion,the so-called ion 0 spher e, to be at an altitude of80 km," which, as we now know, is fairly correct,at least as far as the order of magnitude is con-cerned.In the meantime attempts had also been'made to

explain the· possibility of reception belowthe.hori-zon without the hypothesis of a reflecting layer.Various theorists (among whom Rayleigh, Po in-car é, later Watson, Sommerfeld, Bremmerand van der POll)) showed' that the electro-magnetic waves are bent along the conducting~arth's surface, so that, especially in the case of

1) See: Philips techn, Rev. 4, 245, 1939.

long waves, reception far below the horizon is. possible.,In the courseoffurther investigations it liasbëen

found that this theory ö.f refraction must not beconsider~d as an -argument for making the. hypo-thesis of the ionosphere reflections superfluous,but that the two theories supplement each 'otherexcellently. In radio communications With wavesshorter. than 10 m the .ionosphere plays. no role.In the very important w~ve-Ie~gth :.;cgiori'from10to 50 m, on the other hand, the bridging of longdistances with relatively slight energy would be. impossible without the. reflection of the .wavesagainst the ionosphere.'Until about 1920only wave lengths above 200m

were used. After the possibilities of the wave-length region below 200 m had been proved bythe bridging' of the Atlantic with relatively lowpower transmitters at about that time, shorter a?ldshorter waves began to be used. Several newphenomena were then encountered which did' notoccur on long waves or only sporadically, and thecause of these was thought to lie in the presenceof the ionosphere. It was found, for example, thatreception was generally stronger at night than inthe daytime. Another phenomenon was that radioH-location on short waves is subject to largererrors at night than in the daytime. To these pheno-mena could be ádded that of fading and the factthat with increasing distance reception from an.ultra-short wave transmitter does not decrease inintensity continuously, but after having passed a"dead zone" (skip distance) may abruptly attain agreater intensity' again. Another interesting pheno-menon, and one which is of technical importance,is that the northern lights, the magnetic storms

~. :

112

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PHILIPS TECHNICAL REVIEW VOL. 8, No. 4

(i.e. great changes in the .magnitude and directionof the magnetic field of the earth) and certain kindsof fading are found to be intimately related to eachother. Thus, for example, in 1928 it was at first'impossible to make - wireless contact with theNobile expedition which had come to grief nearSpitsbergen, since the attempts to do so happenedto coincide with magnetic storms in the northernlights zone.

The phenomena mentioned have been the subject -of profound investigations carried out in manycountries, and ,these have now reached a stage wherenot only has a fairly complete technical insight beenobtained into the causes and the mechanism of manyof the phenomena referred to, but also various newfields of geophysical, meteorological and nstro-physical investigation have been opened.

Refraction and reflection of radio waves by theionosphere

A radio wave is an electromagnetic wave whichdiffers from a light wave only in its wave length.It may therefore be expected that the refractionand reflection of radio waves by-the ionosphere willin many respects follow the knownIaws of optics.We shall therefore try to characterize the propertiesof''the inosphere in the manner customary in optics,by assigning 1:0 every point in space a definite indexof refraction. What do we mean by this index ofrefraction? For the case when dissipatlon can bedisregarded it can be derived in a simple way.-According to a familiar -relation of Maxwell

the index of refraction of a substance n is relatedto the, dielectric constant E by the equation:

1£2= ê ••••• ' ••••

We can therefore determine the index of refractionby studying how a gas of the nature of the ionos-phere behaves as a dielect~ic. In doing so we shallconsider the ionosphere for the present a~_a homo-geneous gas which in addition to the moleculescontains if electrons and N positive ions per cm3:

Suppose that in such a gas electrical vibrationsare excited with an angular frequency wand anamplitude Eo.' This electrical, alternating field in adielectric gives rise to electric currents whose den-, sity we shall call j. The dielectric 'constant is thendefined by

e = ili«where i, represents the displacement current for avacuum., The electric current j. consists in generalof twocomponents, the first of which is the above-men-

tioned displacement current for a vacuum. This isgiven by:

. 1 dE11 = -' - . . . . . (2)

4'71: dt

The second component j2 is a convection current,to be ascribed to an actual displacement of charges.If N is the number of charged particles per cm'',e the charge and v the velocity, then

j2 :::= Ne v

and the dielectric constant is thus:

ê=4:0 Nev

UI + j2)/jl = 1 + dE/dt (3)

For free electrons such as are present in the. ionos-phere the velocity, v can easily be calculated 2).The acceleration of particles with charge e and mass., m is determined by the equation ' '

dvnL- = eE ...

dt (4)

When E changes sinusoidally with an angularfrequency w, then the same is true for v, i,e. .

d2v,-=-w2v.dt2

From this and equation (4), , after differentiating t, ,it then follows that:

e dE/dt'V:::r:: - cj?

- lILOY

and by substituting this' in (3)·one obtains :

(1)

.4nNe2

n2 = ê= 1----mw2 (5)

The apparent dielectric constant is thus: less tha!lunity, because the convection current is in oppositephase to the displacement 'current, and the same istrue of the index of refraction. Thus, speaking inoptical. language, the ionosphere is a less densemedium than empty space.

Now: it is known that a wave which passes froman optically dense medium into an optically lessdense medium is not only refracted but mayalso betotally reflected. If n is the index of refraction ofthe less dense medium, n' that of the densermedium, the condition for total reflection- isn < n' sin c, where a is the angle of incidence on the:

2) In addition to free electrons the ionosphere contains anequally large number of positive ions. Since, however, thesepossess a mass about 50 000 times as large, they!are accel-crated to a much smaller extent by electric fields, so thÏttin practice "wc' need only consider as mobile particles thefree electrons. , '. ' , ' ' .

APRIL 194,6 RADIO INVESTIGATION OF THE IONOSPHERE 113

surface of reflection. In our case n' = 1. If we nowfirst consider the case where a = 0 (signal directedvertically upward), the condition is then n < 0,or, according to (5),

4n Ne2---'~ l.mco2 -

, .For sufficiently low values of co this condition isalways satisfied; low-frequency signals are thus.totally reflected by the ionosphere. Above a certainfrequency, however, this ~s no longer the case; forthe critical frequency, according to equation (6),the following holds:

4n Ne2 Ne21, or f2crit = -.

nmThe critical frequency fcrit is thus a measure of thcelectron density N i~ the ionosphere.

The signals directed vertically upwards for whichwe -have calculated the critical frequency are ofgreat importanèe in the study of the ionosphere,as will be discussed below, In practical radio trans-mission, however, one is usually concerned withwaves directed obliquely upwards. The chance oftotal reflection is thereby increased. If a is the angleof the wave to the vertical then n < 0 sin a holds

- for total, reflection, or according to equation (5)

4n Ne'l-1-· ~ Si1l2 a = 1-, cos- a , , . (8)

mw2

Ifwe set a'= 0 we again obtain equation (6) for thccritical frequency. If, conversely, we choose anyarbitrary frequency f greater than fcrit, equation(8) then gives us a critical angle of reflection:

4n Ne~ Ne2 .J~critcosê Ucrit = --- = -- ~ --

mw2 nmf~ P:Reflection only' occurs for a:> acrit, while. waveswith an angle of incidence a < acrit pass throughthe ionosphere. Practically this amounts to thefact that only at a certain minimum -distance fromthe transmitter can the reflected signals bc observed.,, A .slmplified representation öf the situation isillustrated by fig. la, where it is assumed that thealtitude of the ionosphere is so small in relationto the radius of the earth that the earth may be consi-dered flat, while, moreover, the reflection from theionosphere takes place in the form of a sharp 'angle.

The fact that the reflected radiation only becomesobservable at a definite distance rl is familiar inpractice in the existe~ce of a "dead zone"~) (skip

3) Within ihe dead zone signals can sometimes be receivedat irregular times; these are, however, unsuitable for regular

. ,communication. The reflection in these case.s probably. takes place from irregularly occurring e1ectron clouds in

or between the normallayers of the ionosphere.

distance). If one is concerned with case a in fig. 1the radius of the skip' distance can easily be calcu-lated, and one finds:

(6)

(ï)

where h is the altitude of that layer of the ionospherefrom which the. radiation is reflected. Since thisheight may amount to several hundred kilometers,however, in many.cases the assumption of a flatplane as the earth's surface is no~ sufficiently accu-rate, and consequently the curvature of the earthhas to be taken into account. Furthermore, theangle of reflection is not so sharp, since the boun-daries of the ionosphere are more or less vague andthe deflection is therefore more gradual. Takingboth these factors into account, the picture 'repre-sented in fig. lb is obtained. The region commandedby the radiation reflected at the ionosphere then hasnot only an inner boundary rl (skip distance) butalso an outer boundary r2, namely where the re-flected ray still just touches the earth. Underfavourable conditions this distance amounts toabout 1700 km for signals reflected by a layer at analtitude of 100 km and about 3600 km for a layerat 300 km. Transmission of short-wave signals overgreater distances by single reflection at the ionos-phere is therefore impossible. With mul,:iple re-

~~~L_--~-~~~-----~i~4234.

(9)a)

Fig. 1. Reflection of waves by the ionosphere when the earthis considered flat and reflection sharp (a) and when the earthis curved and reflection more gradual (b). Rays with an anglesmaller than aCTil are not reflected but pass through the ionos-phere. The region commanded by the once-reflected waves islimited by a minimum radius 1"1 (skip distance) and in the caseof the curved surface of the earth also by a maximum radius T2•

114 1'lULIPS TECH)1ICAL B.EVJEW VOL. 8, N J,' 4

flection at the earth and the ionosphere greaterdistances' can of course be bridged.The higher the frequency is chosen, the narrower

the annular region commanded by the reflectedwaves, since the radius Tl of the skip distance zonebecomes steadily larger. For the limiting frequencyat which it shrinks to nothing one finds. about30 megacycles/sec (Ä= 10 m), depending on theactivity of the sun, while fcrit lies ~t about 3 mega-cycles/sec (Ä= 100 m). In the following we shallgo more deeply'into the numerical data and alsodis'cusswhat these empirical values are able to showus about the altitude and condition of theionosphere..

Altitude and electron density of the ionosphere\

Experimental investigation of the ionosphere isbeing carried out in very many.countries, so thatin spite of the fact that it has been in progress only'some 20 or 30 years there is already. considerableempirical material available._The data of most interest are those concerningthe altitude of the ionosphere and the degree of, ionization.

Sever~l methods .of determining the altitudeare in use, A method originally employed byAppIe ton makes use of interferences between theso-called carrier wave and the radiation reflectedby the ionosphere (seefig. 2). The distance betweentransmitter and receiver chosen is such that theintensity of the carrier wave received is about.equalto that of the reflected wave. The two waves willamplify each other when 2 (a-a') = mÄaudatten-uate each other for 2 (a-a') = (m.+ 1/2) Jo (mbeinga whole nuinber and Jo the wave length).If the wave 'length is decreasad continuously

h'

f~,-- 8,' _

42444

Fig. 2. Determination of the effective altitude h' of the ionos-phere from interferences resulting from the difference in patha-a'. It may lie seen that the actual altitude is smaller since,the waves do not experience a sharp reflection but are graduallybent. The dots on the. line indicating the path of the electricwave represent time intervals and show that the rounding-off of the .angle results ill:no decreasein the 'transit time, sinceinthe ionosphere the signal is propagated more slowly. '

from A to Jo', amplified-and attenuated signals arereceived alternately. The number of times thatan amplifiedsignal is received is:

f.l = 2(a--a')/A' -2(a-a')/A.

By determining [J., a, Jo and A', a' and thus also h'can be calculated. This "effective altitude" h' isgreater than the actual altitude of the point where'the reflection takes place, since the velocity withwhich the signal is propagated in the ionosphere isless than the veolcity e of light in vacuum.

8 40674

Fig. 3. The transmitted signal A and the reflected signals Bin the determination of the effective altitude of the ionosphereaccording to the method of Breit and Tuve.

The phenomenon underlying Appleton's me-thod, viz. interference of the waves that have trav-elled different paths from the transmitter to thereceiver, is manifested.dn normal radio receptionas the familiar fading. ', Interpretation of the results of the measurementsby the Appleton method is rather difficult.Therefore a different metho~ is now' usuallyempoyed, the " e ch 0 met hod ,; developed byBr e i t and T~ ve, upon which, as a matter of fact,the now universally known "r a dar" is based.By means of a transmitter short wave trains are

sent oui, directly upwards at intervals of, forinstance, 1150-sec (see fig. 3). A receiver not farfrom the transmitter records the carrier wave andimmediately afterwards the echoes due to reflectionfrom the ionosphere. From the time te elapsing he-. tween the reception of the carrier wave and thatof the reflected wave, [echo time), the effectivealtitude h' of the ionosphere can be calculated bythe relation h' = 1/2 ete•. Some results obtained by the Breit and Tuveecho method are reproduced in fig. 4. The registro-gram shows the echo time of the reflected signal as afunction of the frequency. It is found that up toabout 4 megacycles/sec (Jo = 75 m) an echo .timeof 60. microsecouds occurs, eorresponding to anapparent altitude of 100 km. At frequencies above3.5 megacycles/secthe echo time begins to increasegradually; apparently waves of these frequenciespenetrate deeper into the ionosphere. The criticalfrequency is passed at 4 megacycles/sec.The signalwhich penetrates through the lowest layer of theionosphere above this limit does not, however, dis-appear into space but is refl~cted from a. higher

RADIO INVESTIGATION OF THE IONOSPHERE us

layer (h ~ 200 km). This so-called PI layer ismuch less sharply bounded than the underlyinglayer, which is indicated as the E layer. This isevident in the diagram from the fact that the alti-tude at which reflection occurs increases sharply

Fig. 4. Recording of the effective altitude of the ionosphere inkm as a function of the frequency in megacycles/sec. when thesun is high. Registrogram of the Carnegie Insitution Washi ng-ton; taken from Darrow, Bell Syst. techno J.19, 455,1940.

with the frequency. At a frequency of 5.5 mega-cycles/sec the signal penetrates also through theFI layer, reflection then occurring at the so-calledF2 layer, which has an altitude of 300 km. Notuntil a frequency of 10 megacycles/sec is reachedare the waves able to penetrate through this layer,when the signal disappears into space.

In the frequency region in which the F2 layer iseffective the diagram shows a second echo, whichcan be ascribed to waves which have covered thedistance between earth and the ionosphere and backtwice. Furthermore a remarkable doubling of thediagram lines is visible. This latter effect is con-nected with the terrestrial magnetic field and will bediscussed briefly farther on in this article.The critical frequencies found for the reflections

at the various layers make it possible to calculatethe electron density N at the point where the wavesare reflected. According to equation (7) thisquantity is:

itm f? ,N = cnt = 1.24.10-8 j"l'crit (10)

e2

By the combination of determinations of N withmeasurements of h' a fairly complete picture canbe obtained of the distribution of the electrons overthe different layers of the ionosphere.

Results of thc investigation

Apart from systematic variations with the seasonof the year and the position of the sun, the stateof the ionosphere is found to exhibit rather greatfluctuations. Electron densities occur which are 10

times as great as the average. In general the follow-ing conditions can be determined.

At an altitude of about 50 km in the daytimethere is a slightly ionized layer ("ozone" or' Dlayer). At night the electron density in the D layeris very slight. The' second layer (E layer) is per-manent and lies at about 120 km altitude. Abovethat at 200 and 300 km altitude lie at least two otherlayers (FI and F2) more or less distinct in the day-time but immediately after sunset merged into one,the F layer; sometimes in the daytime in wintertoo the separation between the FI and F2 layerscannot be observed. Between the E and FI layersthere occurs at very irregular 'times another layerof limited extent, which is sometimes called theE2 layer (American: "sporadic E layer"); in thesummer this is often observed in the morning andin the evening; sometimes this E2 layer' occurs inthe evening at all seasons.

The following table gives a survey of the averagecondition of the ionosphere at noon on a summerday for a mean latitude on the earth:

effective criticalmaximum electron

layer altitude frequencydensity (number of

(km) (mega-particles/ C:tp3)cycles/sec)

D SO <0.4 <2.5 X 103E 120 2,5 105

FI 200 5.0 4 X lOSF2 300 8.0 105

For the sake of comparison it may be noted thatfrom the radio inv'estigation of the ionosphere itappears that the molecular density of the atmos-phere at the altitude of maximum ionization of theE layer is of the order of magnitude of 1012 mole-cules per ern", while in the F layer the figure isabout 1011.The question has arisen as to whether there are

still other layers present: It is indeed very' wellpossible that we are unable to observe all thelayers; a layer can only manifest itself when itsionization is greater than that of all the underlyinglayers, so that its limiting frequency is higherthan the lowest frequency of the waves penetratingthrough the lower layers. Faint indications of layersother than those mentioned have sometimes beenfound. It is possible, however, that deceptive phe-nomena may occur which would appear to indicatethe existence of an ionized layer although such alayer does not actually exist. In this connectionit is interesting to note that in 1927 Hals receivedechoes whose echo-time was of the order of magni-tude of 10 to 30 sec, which would point to a reflee-

116 PHILIPS TECHNICAL ,REVIEW VOL. 8, No. 4,

ting layer far beyond the orbit of the moon, whereaccording to Störmer a sphere of charged parti-cles can indeed be expected. Whether this is thecorrect explanation of the origin öf the echoes withlong transit time may be open to doubt. '

The reason for the complex character of the ionos-phere has not yet been determined with certainty.This phenomenon is probably connected with theabsorption of different parts of the sun's spectrum•at different altitudes in our atmosphere. The upperlayers of the ionosphere are probably, 'ionized notonly, by light but also by charged particles origi-nating in the sun.

Which of the gases of our atmosphere are subjectto this ionization is also uneertain. It might bethought that our atmosphere at 200 to 4.00 km alti-tude no longer consists mainly of nitrogen, but oflighter gases, for instance hydrogen and helium.A. direct indication of the presence of these, gaseshas not, however, been found, although they haveoften been sought, for example in the spectrum ofthe northern lights; which occur at the altitudesin question.

One of the bands of the northernIights spectrum, could be ascribed to the positive ion of the nitrogenmolecule, so that the' occurrence of the reactionN2 -::;_.Nt + e is thus proved. This process is prob-,ably one of'.the causes of the ionization in the Flayer.

. Daily, annual and other periodical variations

The connection between the position, of the sunand the state of the ionosphere has already been

, referred to in the foregoing. This connection makesit probable that the ionization is for a large partbrought about by the ultra-violet radiation of thesun. For the E layer this supposition can even beconfirmed quantitatively, by calcularing the elec-tron density as a function of the position of the sunon the basis of the laws of molecular equilibrium.

The following formula holds for the change inelectron density per second:

dN_ = Ij .:': u.N2.dr .'

In this expression .q is the number of electronsformed in the ionosphere per second per cm3 andaN2 the number which per second and per 'cm"reunite with the positive ions; a is the so-calledrecombination coefficient. The fact that thevelocityof the recombination Is determined by the square

. of N is ~lear when remembering t4~t the chanceof recombination is .proportional to the product ofthe concentrations of the' electrons and of the posi-

tive ions. If an equilibrium condition is assumed;r.e, dN/dt = 0, the following applies:

N = 'Vq/a.The quantity q depends upon the height of the sun,which can he expressed by the angle X made by thesun's rays with the perpendicular. If one considers,'for instance, a horizontal'plane' in the ionosphereof 1 CqI~, it is clear that the amount of ultravioletenergy striking that plane is proportional to cos X.Thus N is proportional to (cos X)'I., and accordingto (10) this means that '

fcrit is proportional :to (cos X)'I.. (ll)

This relation is reproduced in jig. 5 for the sake ofcomparison with the results of measurements. Itis found that the formula for the E layer corres-ponds to the observations carried out during a large

Fig. 5. Daily variation of the critical frequency (in megacyclésfsec.) for the E- layer (average from March 18 to 23 incl. 1935;.taken from Applcton, Proc. Roy. Soc. 162, 451', 1937).The line drawn represents the variation of the quantity(cos z) 'I., where X is the zenith distance of the sun. Except inthe hours of night the curve corresponds very well to the varia-tion of the critical frequency. G.M.T. = Greenwich MeanTime. .

part of the day. From this it .may be concludedthat the processes ofionization and recombinationin the E layer in the daytime are indeed in equili-brium with each other ..

Formula (11) does not meet the case so well forthe variation for the FI layer, and it is certainlynot valid for the F2 layer. In these layers thereforethe' equilibrium 'between ionization and recombi-

. nation is absent, which is understandable consi-dering that in the more rarefied atmosphere theprocesses take place much more slowly, because ofthe smaller number of collisions. However, it mayalso be assumed in explanation that the ionizationis determined by phenomena other than the ultra-violet rays. The two causes probably work together.

The other known periodical variations in the,state of the ionosphere are likewise connected with, the sun. In the course of a year the altitudes andespecially the electron densities vary with theseasons' (see -jig. 6). The electron concentrationin the E layer and especially in the FI layer isgreatest in summer, which is understandable after.what has.heen said about the influenee, of the height:

APRIL 1946 RADIO INVESTIGATION OF THE IONOSPHERE 117

h'km ï '~ ,

~ggH=t~:j:v;-:r-l:;!I;;:J;,./~::j:pdfr=,:jj+=ir1 =200:, E '-j~tOO ; r '-rr-I 'H-

tSO&:=,:!:..;;U"!_;,~I~.~::;:B:::;:' ::;::~;::::;:~ ..l~!fl-~I-=: DE~36 j:!l+-+-I-+-~U'HI_+_~fcrit. I[ 11 '--t-+'--II~~+-l--f-+-+g~~.~ Disturbances in the ionosphere (fade-outs)kC/s I=;=I~'R:--'-'- -l~,t~-:=,- I~ v~ "~'--'- .

" ~ I In the year 1935 Dellinger published his de-10000

1= -:-~-I- . -f- f-I-I-! i- -1-1-1-~+III+--HH'H-1-1-- - - I ~ tailed investigation of an important discovcry by'~ - iI- r:- ,_!c.. it H- ~ '~.':1-11-+-+1%'''\-1'--+-1'-+-i',f-"-I-)'~,f-!! ~,', ~I I~I\ Jouast, Mögel ft al. At certain moments it is

! J ' t-> , ~ \ 'IF' 1-.... ' ,-1-1- ;;'H -+--I-+-+'H-~j_.-r- 'suddenly impossible to receive any shortwave

1/ '1 - K: - - -!:t: ~ I ~ f- f\ tF signals passing over the part of earth that is in5OOOIH-lt-JL-,H-, - a' I' 1-1- r::::I-+-I!~"+' , E ......Ï". !-I- I-!-!- i ff 1-1- ~ daylight. This effect occurs so abruptly that it, is

• I'-I-!- _:17!::=A,,,"'f~~H-l f h h h hi . h h1 : I\~-r lif I ' , 0 ten t oug t t at somet mg IS wrong WIt t e

, IJ: ~1 ' receiving set. The disturbance lasts from about teno~~~,-L~~~~·~ ~~~I~A~,L~I~q~~o 41 8 f2 ,16 f10 00 ..,. t8 IJ /6t 20 oh minutestoanhourorlonger.Dellingerpointedout

t, tJ tf taEASTERN S~ANOARO TIME "'''''''''IJ the simultaneous ocqurrellce of these fade-outs over

the whole daylight half of the carth. and showedthat there was a connection with eruptions in' thechromosphere of the sun. The explanation of thefade-outs is to he sought in the fact that when thereis an eruption a large amount of ultra-violet radia-tion is emitted by the erupting spot on the sun,which causes intense ionization in the earth'satmosphere at an altitude. of 50 km' (D layer),especially through the a line of the L y m a n serieswith a wave length of 1216 Á. As a result of the

. relatively high pressure at these altitudes the elec- 'trons set .in vibration in the field of a radio .wavewill undergo continual collisions with gas moleculesand lose their cnergy; in consequence the absorp-tion of the .radio signals is so great that. they dis-appear and 'radio cominunication becomes im-possible.

of the sun. The P2 layer 'behàves differently, thelimiting frequency and thus also the electron densityreaching a greater value in winter than in summer.

-~m._" E :, ~

i?ig. 6. Daily and seasonal variations of the critical frequencyj~rit in kc/sec and of the effective altitude h' for the differentlayers of the ionosphere (borrowed from Smith, Gillilandand Kirby, J. Res. Bur. Stand. 21, 835, 1938): tI sunrise;'2 sunset. Eastern standard time, 75° west longitude (practi-cally the meridian of Philadelphia).

From this it is sometimes concluded that the, F2layer is sensitive to temperature, becoming diffuseand rising in the summer whereas in winter it be-comes denser and falls. ..On the basis of his own experiments and those of

his collaborators, Elias presumed that the nightlyionization at, 350-400 km effective altitude can in-deed be distinguished from that 'in the daytime andthat. it is to he ascribed to corpuscular radiation,'or perhaps partly to eletromagnetic radiation ofcosmic origin.

The eleven-year period in the number of sun-spots observed every year seem~ to correspond toan equal period in. the electron densities of theionosphere. According to Sm ith, Gillil an d .ändKirby, during -the years 1932-1937 the electrondensities of the E, F1 and FiJ. layers increased: withthe number of sunspots (seefig. 7). '

Further of interest are the changes in the ionos-phere during an eclipse of the sun -. It has beenfound- that during ·an. eclipse the electron densityin the E and FI layers decreases sharply, but almostimmediately after the 'eclipse returns to the normalvalue again. The electron densities of these layersare found to he roughly proportional to the non-eclipsed portion of the sun's disc; that of the F2layer behaves differently and varies in a smallerdegree. This may he ascribed partly to the r~co:Ql-

hiaation coefficient being too low, but on the otherhand it seems to confirm the suspicion that theionization of the F2 layer is caused partly. by acorpuscular radiation.Finally in 1939 Appleton and Waekcs dis-

covered lunar times in the ionization' state ofthe ionosphere. This influence ofthe moon, however,is very slight. '

Fig. 7. Comparison of the annual average of the number of .sunspots s' with ·the noon values of the critical frequencyfcrit (in megacycles/sec) for the different layers F2' Fl and Ein the corresponding years (from Smith, Gilliland andKirby, J. Res. Bur. Stand. 21, 835, J938).·, .

----------------~ .._- --------------------.,-------------,---

118 PHILIPS TECHNICAL HEVmW VOL. 8, No. 4,

There is some indication that the reeeption .strength of Ion g waves increasesduring a fade-out,instead of decreasing. Since these, waves are re-flected by the D layer at about 50 km altitude,this would agreewith the explanation that (ade-outis to be ascribedto an increase of ionization in theD-layer.Disturbances due to D layer absorption are also

known which do not occur and disappear so sud-'denly as fade-outs, and which last for ~ longertime,usually several hours. The decrease in intensityof the signals, too, is usually not so great as inthe case of fade-outs. 'Disturbances from another cause occur during

magnetic storms and upon the appearance of theaurora borealis, which again is found to be con-nected with the eleven-year period of the sunspots,During the occurrence of magnetic storms and theaurora borealis material particles from the sunpenetrate into the ionosphere and ,there disturbthe normal state. Investigation has shown that at ahigh geographical latitude the electron densityof the E layer then increases sharply. At lowerlatitudés the F layer seems to be primarily affected;the electron density' decreases and the effectivealtitude rises, thus indicating a diffusion of thelayer.

Practical consequencefor radio reception

From what lias been said in the foregoing apicture can be formed of the influence of the ionos- .phere in the different frequency-regions, In the caseof long-wave daytime broadcasting (kilometrewaves) the waveis, as it were, enclosed between theD layer and the surface of the earth. Owingto therelatively lowaltitude of 'the D layer the differencebetween the path travelled by the carrier wave andthat travelled by the wave reflected àgainst the Dlayer is usually small' compared with the wavelength. These waves are therefore not clearlyseparated from each other but form a singlevibra-tion phenomenon which is propagated parallel tothe earth's surface.Fading seldomoccurs. 'A' peculiar phenomenon, .which may he caused

by the ionosphere ID the case of long waves, is asort of cross-modulation, whereby the modulationof a strong transmitter can be tra:O:sferredto the. ,

carrier wave of other transmitters (the so-called. Luxemburg effect). This effectwas first observedby Butt and Tèllegen, each independently, whoascertained that the modulation ofthe Luxemburgtransmitter could also be heard 'on 'several othertransmitters lying in about the same direction ..from the receiver, hut at greater distances. This

effect is to be ascribed to non-linear phenomena inthe ionosphere. Apparently the state of the ionos-phere above a powerful transmitter on the long-wave is slightly affectedin the' rhythm of the modu-lation, thereby giving rise to a reciprocal actionon the radio waves. " .Furthermore, on the m"termediatewave, between

200 and 600 m fading often occurs, especially atnight and particularly in those regions where the,intensities of the carrier wave and the wave

..reflected against the E layer are equal. Receptioncan only be called perfect, in the vicinity of thetransn:itter (radius about 75 km) where the carrierwave is received more strongly than the reflectedwave. At a very great distance, where only the re-flectedwave is received,no fading would be expec-ted, since the cause of the fading phenomena lies -in interferences between carrier wave and reflectedwave. Nevertheless, even at great distances ,verymarked variations in. intensity may occur due tofluctuations in the reflective properties of the ionos-phere and also due to interference arising from the .fact that when reflected from the ionosphere thesignal may reach the receiver along different pathsof varying lengths. 'During the day the reflected waves of the inter-

mediate region are rather strongly absorbed by theD layer.Wave lengths between 100 and 200m are usually

less suitable for radio' communication over longdistances. With increasing frequency. the carrierwave is more and more strongly damped by ab-sorption in the earth, an' effect which is alreadynotiêeable at 100-200 m. The reflected, wave is,weak,' since this kind of waves is very much ab-sorbedby the ionosphere.The causemay perhaps besought partly in the fact that the frequencies ofthevibrations of the electrons around the lines of forceof the terrestrial magnetic field correspond to thefrequencies of this wave region. Resonance is there-for~ possible, and this, is always accompanied byabsorption. The wave lengths from 100 to 50 mand from 50 to 10 m are indeed very importantfor wireless communication; Especially in the'latterregion, however, the, .possibility of reception islimited by the occurrence of the dead zone. In thedaytime reflection take's place' mainly at the' Elayer, while at night the wave penetrates throughthe E layer and is reflected at the F layer.Fading occurs to a very marked degree in this

wave-length region. For receivers situated on theedge of the dead zone the average signal strengthmay vary considerably owing to the expansion orshrinkage of the dead zone. Reception on these

APRIL 1946 RADIO INVESTIGATION' OF THE IONOSPHERE 119

waves depends very much on the activity of thesun. Especially where the trajects P!lss along thezones of the aurora borealis to earth the receptionis very variable, 'The strong fading phenomena discussed in the

previous section, which are caused by a disturbance ,in the normal state of the ionsophere, are particu-larly pronounced in this' wave region. '

Influence of the earth's magnetic field

In the foregoing it has already been stated that the wavesin the regionof 100-200 m are strongly absorbed by the ionos-phere owing to the terrestrial magnetic field. A closer investi-gation shows that this is not the only effect of the magneticfield. Under the influence of the earth's field a so-called doublerefraction occurs, and one speaks, as in the propagation oflight in crystals, of the ordinary and extraórdinary compo-nents. That something of the sort must exist is easily seenwhen the behaviour at the magnetic equator is studied, wherethe earth's field is directed horizontally North-South, If at

, that spot linearly polarized radio waves 'are sent upwards,whose electrical vector E is also directed North-South, then inthe ionosphere the electrons are set in vihration in the directionNorth-South and they experience in their motion no effectfrom the earth's field, For such polarized waves the propa-gation in the ionosphere is independent of the magnetic fieIaand one-therefore speaks ofthe ordinary component. The extra-ordinary component on the other hand is found when from themagnetic equator radio waves are sent upwards whose elec-trical vector is directed East-West; the electrons in the iono-sphere set in vibration by the radio waves then experience' aLorentz force, with the result that equation (10) for theelectron density at whi,chreflection occurs is changed t.o

3tm " -8N = -. fU=F.fa) = 1,24 X 10 f(f=FfH) (12), e ,

eH:where.rH = - , is the so-called Larmor frequency, i.e.

m 23tc '

the frequency at which the electrons can describe circularorbits' in the magnetic field, which frequency is equal for allcircles. The upper sign holds for f > fa .and the lower forf <fa. For a field strength of about 0.5 GaussfH~ 1.5 mega-cycles/sec., corresponding to a wave length of about 200 m.When radio waves are sent upwards whose electrical vector E

is directed for instance NW-SE, double refraction occurs inthe ionosphere. An ordinary component is received which ispolarized linearly N-S,' and an extraordinary componentpolarized linearly E-W. When f >fa it is found from equations(10) and (12) (upper sign) that the extraordinary componentis reflected at a smaller electron. density ihan the ordinary' .component. As a result the ordinary component is receivedlater than the extraordinary one. At the same time the criticalfrequençy for the extraordinary component then, lies higherthan that for the ordinary one. Thus if the frequency of thesignal transmitted is chosen between these two critical fre-quencies, the extraordinary component is still reflected butnot the ordinary component.Experimenta carried 'out by the Carnegie Institution in

Huancayo; Peru (lying on the magnetic equator) fullyconfirm the above statements. From this it follows in partienlarthat; t~e' e~ective charged particles of the ionosphere are actu-

ally clectrons; if the reflection were cuused by heavier par-ticles no measurable difference could be expected between thereflection of the ordinary and the extraordinary components.

Experiments carried out at higher magnetic latitudes givea somewhat more complicated result; in that case the reflectedwaves a!e polarized elliptically in opposite directions, at themagnetic North pole the ordinary component being polarizedcircularly to the left and the extraordinary component cir-cularly to the left, whiie at the magnetic South pole the polari-zation directions of the 'two components are just the reverse.Observation of the opposite polarization directions in theNorthern and Southern hemispheres clearly confirm thesefacts.

Significance of the study of the ionosphere for thephysics of the sun

I~ the foregoing j.t has already been pointed outthat the investigation of the ionosphere is closelyconnected with the. physics of' the sun, withmeteorology and with geophysics.. We shall herebriefly consider the connection with the' physicsof the sun.Some of the ions in the ionosphere have been

formed by ionization of nitrogen molecules. Thiscan only be accomplished by radiation of a wave 'length shorter than 661 A. If the temperature ,atthe surface of the sun is known, the intensity of theeffective ultra-violet radiation can be calculated,considering the sun as a heat radiator. From thedistribution of intensity in the visible region a valueof at most 6500 oe follows for the temperature ofthe sun, and from this Saha deduced that in the.wave-length region Ä< 661 Á the sun would giveoff to the earth 104light quanta per cm2 per .second.,Since up~n absorption by a nitrogen molecule eachquantum releases one electron, in a vertical columnwith a base of 1 cm2 ~04 electrons are"released persecond. According to Ap pIe to n and Chapman,however, 109 to 1010 electrons per cm2 per secondare necessary for the maintenance of the ióIÎ.lzation.This result indicates that the intensity of the sun's.radiation emitted in the far ultra-violet region isabout 10ij times greater than that of a glowingblack body with the temperature of the sun. It isvery probable that this extra ultra-violet radiationoriginates in the hydrogen and helium present onthe sun. .The foregoing may serve to show what progress

has been made in th~ radio investigation of the 'ionosphere. A much deeper insight has been ob-tained into the physical phenomena 'taking placein-the ionosphere and it is not only radio technicsthat Have profited from this, for the investigationof the ionosphere now also furnishes importantmaterial for geophysics, meteorology and astro-physics.

120 .PHILIP5 TECHNICAL REVIEW VOL. 8, No. 4.

LITERATURE

Among the numerous articles concerning the ionosphere we would mention:

H. R. Mi m no, The physics of the Ionosphere, Rev. Mod. Phys. 9, 1, 1937.

W. Grotian, Sonne und Ionosphäre, Naturw, 27, 555 and 569, 1939.

J. H. De l lin gc r, The role of the. Ionosphere in radio wave propaga tion, Electr, Eng.'I'ransactions. Suppl. 803, 1939.

K. K. Darrow, Analysis of the Ionosphere, Bell Syst, Techn. J. 19, 4.55, 19·10.Sec also:

L. Harang, Das Polarlicht und die Probleme der höchsten Atmosphürenschichten,Leipzig 194·0.

c. J. Bakker, M. Minnaert, A: Pa nn eko ek, J. Veldkamp, De vcr-ultravioletteen de corpusculaire stralingen der zon (The fur ultra-violet and the corpuscular radiationsof the sun). Ned. T. Natnurk. 9, 209, 1942.

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