Significant Achievements in Ionospheres and Radio Physics, 1958 - 1964

8/7/2019 Significant Achievements in Ionospheres and Radio Physics, 1958 - 1964

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Foreword

HIS V O L U M E IS ONE OF A SERIES which summarize theT rogress made during the period 1958 through 1964in discipline areas covered by the Space Science andApplications Program of the United States. In thisway, the contribution made by the National Aero-nautics and Space Administration is highlighted againstthe background of overall progress in each discipline.Succeeding issues will document the results from lateryears.

The initial issue of this series appears in 10 volumes(NASA Special Publications 91 to 100) which describethe achievements in the following areas: Astronomy,Bioscience, Communications and Navigation, Geodesy,Ionospheres and Radio Physics, Meteorology, Particlesand Fields, Planetary Atmospheres, Planetology, andSolar Physics.

Although we do not here attempt to name those whohave contributed to our program during these first 6years, both in the experimental and theoretical researchand in the analysis, compilation, and reporting of results,nevertheless we wish to acknowledge all the contributionsto a very fruitful program in which this country may takejustifiable pride.

HOMER. NEWELLAssociate Administrator for

Space Science a nd Applications, N A SA


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PrefaceHE IONOSPHERE, an ionized region around the EarthT apable of reflecting radio waves, is used regularly

in radio communications to guide signals around the cur-vature of the Earth. It is produced by the action of solarradiation on the Earths atmosphere, in which processpotentially harmful portions of the Suns X-ray and ultra-violet radiation are absorbed. The ionosphere is thus animportant and interesting part of our near-space environ-ment. Similar ionized regions may be expected in otherparts of space, but since very little is known about these,this report is confined to the Earths ionosphere.

Apart from the scientific interest in near space and thepractical prospect of improving long-distance radio com-munications, there is another cogent reason for includingionospheres and radio physics in the space science program.The interaction of radio waves with ionized regions pro-vides a powerful tool for the investigation of their ioniza-tion and magnetic fields from great distances. Ionospherictheory and measurements of the radiation flux enable theionization to be related to the static and dynamic propertiesof the local medium. This is a fruitful approach to plane-tary studies, particularly when contamination must beavoided, and to studies of the very hot solar corona. It isaxiomatic that these tools and theories must be developed,tested, and applied to our own ionosphere before we canventure further afield with any confidence. Additionalrequirements come from radio astronomy, where an un-derstanding of the interaction between an ionized mediumand a receiving antenna is important to the measurementof low-frequency signals in space and where ionosphericrefraction and focusing may be put to good use in increas-ing antenna directivity.

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i * LIONOSPHERES AND RAD IO PHYSICSPrior to the advent of rockets and satellites, the Earths

ionosphere could only be investigated by indirect means,and much ingenuity was expended on deducing the proper-ties of the ionosphere from the changes impressed on anexploring radio wave-a procedure similar to investigatingthe topography of a rugged terrain by observing the wayin which a ball bounces from it. In a situation whereneither the neutral atmosphere, nor the ionization, nor thecausative solar radiation could be directly observed, it isnot surprising to note the highly speculative nature ofionospheric physics before the introduction of more directmethods of in situ observation made possible by rockets andsatellites.

Progress in understanding the ionosphere is closely re-lated to progress in a number of other areas : solar physics,responsible for investigating the radiation which producesthe ionosphere ;planetary atmospheres, responsible for de-fining the neutral atmosphere from which it is formed;and meteorology, through the connection recently dis-covered with the lower ionosphere.A necessary prerequisite to understanding the iono-sphere is to obtain an adequate description of it. Becauseof the ionospheres great variability with both time andposition, such a description requires a great deal of data.Until quite recently, these data were essentially limited ingeographical coverage to the more populous parts of theglobe, and in altitude to below the peak of the F-region(about 300 kilometers). I t is a sign of real progress thatsatellite-borne topside sounders have now obtained datain quantity from above the F-region. The importance ofthis is made clearer by the realization that there is morethan twice as much ionization above the F-region peakthan there is below. These data, however, have only beenobtained during the recent period of low solar activity,and this work must be continued to at least the next solarmaximum to complete the description.vi


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., * * . PREFACE' Considerable progress has been made in relating particu-

lar portions of the solar spectrum to the ionization producedat different altitudes. Much more work needs to be done,however, before the state of the ionosphere can be com-puted from a detailed measurement of the incoming solarflux, since the processes of ionization removal are not yetclear and the relative roles of chemical recombination andbodily transport often cannot be distinguished. Indeed,much more work is needed, both on rates of reaction andon the dynamics of the ionosphere.

Unexpectedly, helium was discovered to be an importantconstituent in the upper atmosphere. Its presence waspostulated from a study of satellite drag measurements andfirmly established from an investigation of the ionized con-stituents of the ionosphere. The variations of helium con-centration with altitude, latitude, and solar activity arecurrently under study and have not yet been fully explained.

The development of techniques for measuring both neu-tral and charged particle temperatures resulted in the dis-covery that the charged particles, produced by photoioniza-tion of the neutral atmosphere, can maintain temperaturesin excess of the neutral gas. These investigations haveshown that the excess temperature can vary with altitude,season, time of day, and latitude, but have not as yet re-vealed any seasonal changes. Spaceborne instrumentsand ground-based backscatter sounders played importantroles in this discovery. It remains to be seen whether theexisting theories correctly predict the variations with solaractivity which should be observed in the next few years.

Intensive work has been done on a phenomenon knownas sporadic E. It has been shown that sporadic E is a verythin, intense layer of ionization associated with wind shearsin middle latitudes, but is a different phenomenon overthe magnetic equator where it is associated with an insta-bility of flow in the intense electrojet current.


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IONOSPH ERES AND RADIO PH YSICSA number of new phenomena have been discovered in

the interactions between electromagnetic waves and plas-mas in the ionosphere. Resonances have been observed atdifferent frequencies associated variously with the Earth'smagnetic field, the electron density, and the ionic composi-tion. In addition, an examination of VLF signals receivedin satellites has resulted in the discovery of several newmodes of wave propagation.

In all the items enumerated, the major contributionshave come from the use of rockets and satellites supple-mented, in some instances, by the incoherent backscattersounder, which represents the one important new ground-based tool for obtaining data to altitudes of several thou-sand kilometers at the very few locations where the requisiteelaborate inst allat ions exist.

Both Soviet Russia and the United States have extensiveresearch programs in ionospheric and radio physics ; n theUnited States, rocket and satellite investigations have beenpursued by DOD and NASA. While the U.S.S.R.launched the first satellite and obtained the first high-altitude measurements from rockets, relatively little of thiswork has received timely publication in any detail. Majorcontributions have been made by DOD in pioneering workon the solar input during quiet and disturbed conditionsand on the absorption of solar radiation as a function ofaltitude and wavelength. DOD has also made significantcontributions in other areas, such as ion composition andwind shear. A substantial portion of the remaining workhas been performed through NASA; some of it in-house,some supported by grants and contracts at a number of in-stitutions, and some under international cooperative pro-grams.

This summary was compiled and written by E. R.Schmerling, Physics and Astronomy Programs, Office ofSpace Science and Applications.


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Contentschapter1 INTRODUCTION ..................................2 THE IONOSPHERIC REGIONS .....................The Collision-Dominated Region ................

The Photoequilibrium Region ...................The Diffusion-Dominated (Topside) Region ......The Protonosphere and VLF Phenomena ........

3 DYNAMIC EFFECTS................................Electrodynamic Drifts ...........................Sporadic E .......................................Irregularities and Winds ........................4 CONCLUSIONS AND OUTLOOK..................

REFERENCES .......................................

Page1558

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chapter 1Introdzcction

HE EXISTENCE O F IONIZED REGIONS in the upper at-T osphere was first postulated in the middle of the19th century, notably by Gauss ( 1839, ref. 1 and BalfourStewart (1878, ref. 2 ) , to account for the small fluctua-tions in the Earths magnetic field. This hypothesis wasindependently revived by Kennelly (1902, ref. 3 ) andHeaviside (1902, ref. 4 ) to explain Marconis feat in 1901of transmitting radio signals across the Atlantic despite theEarths curvature. Direct proof of the downward reflec-tion of radio waves from the ionosphere was not obtaineduntil 1925 by Appleton and Barnett (ref. 5 ) . It is inter-esting to note that about a quarter century elapsed betweenthe electromagnetic field theory of Clerk Maxwell and theestablishment of transatlantic radio communications byMarconi, and another quarter century between thisachievement and the direct proof of ionospheric reflectionby Appleton and Barnett. It is implicit in this report thatconsiderable progress has been made in recent years inreducing substantially the time between major milestones.Because of the ionospheres importance for radio com-munications and its inaccessibility to more direct methodsof investigation, indirect exploration by radio wavesdominated ionospheric research for about a quarter of acentury after Appleton and Barnett.

The period from 1946 to 1957 marked the beginnings ofrocket exploration. During this time basic techniques weredeveloped that were later combined into increasinglysophisticated payloads. Notable during this period wasthe realization, largely through the work of Jackson and


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..IONOSPHERES AND RADIO PHYSICS 'Seddon ( 1958, ref. 6 ) that the traditional ionosphericlayers are not, in general, well separated as had been sup-posed, but merge gradually into one another, forming asingle profile of ionization density with only shallowvalleys and gentle peaks.

Another new era began with the launching of Sputnik Iin October 1957 and the creation of NASA in 1958. Thisperiod marked not only the dawn of the satellite era butalso the coming of age of rocket exploration. The experi-ence of small groups of experimenters was pooled, andlarger teams, with increasing support, began to put togetherincreasingly complex payloads for the simultaneous meas-urement of different, but physically related, quantities. Alist of the NASA-sponsored satellites and space probes im-portant in ionospheres and radio physics research is givenin table I. The year 1958 also saw the beginning of NASA'suniversity and international programs, which made a broadbase of technology available to scientists with expert knowl-edge of their own fields but with limited technologicalresources.

This report attempts to outline the major areas of prog-ress in ionospheric research over the years 1958 to 1964. Itdeals primarily with the plasma properties of the iono-sphere, since the neutral atmosphere, the chemistry of theatmosphere, and the Sun as a source of ionizing radiationare discussed in separate reports. Particular emphasis hasbeen given to the topside of the F-region, about which littlewas known before 1958. No attempt has been made tocover storm and other disturbance phenomena, since rela-tively little progress has been made in these areas. Drasticselection was necessary in a report of this large scope butsmall size, and topics and references which might otherwisehave been included had to be omitted.

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chapter 2T h e Ionospheric Regions

THE COLLISION-DOMINATED REGIONHE D-REGION IS USUALLY CONSIDERED the lowest por-T ion of the ionosphere, and ranges in altitude fromabout 50 to 85 kilometers. The lower part of this range issometimes called the C-region because of its cosmic ray

origin. Below 50 kilometers is a region where ions exist inthe form of clusters. In the D-region the motion of elec-trons is dominated by collisions with neutral particles, thuscausing most of the radio-wave absorption to occur here.This absorption provides a powerful tool for making meas-urements. Too high for balloons and too low for satellites,the D-region is investigated by the transmission of radiowaves through it, from the ground or from above, and bydirect-measurement devices carried on rockets. The lowdensity of charged particles, the high neutral density, andthe high probability of negative ion formation account forthe difficulties experienced in both theoretical and experi-mental investigations of this region.

Electron density measurements may be made from theground by a number of techniques. The partial-reflectionmethod of Gardner and Pawsey first reported in 1953 (ref.7 ) has been used recently by some workers, but requiresgreat care in its execution and interpretation. The wave-interaction method depends on both electron density andcollision frequency. By using pulses, Fejer and Vice (ref.8) succeeded in improving the height resolution; and bymeasuring both the phase and amplitude of the inter-action, Weisbrod, Ferraro, and Lee (ref. 9 ) can, in prin-

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. ,IONOSPHE RE S A N D R A D IO PHYSIC Sciple, obtain collision frequency and electron densityindependently. The height resolution is, however, anorder of magnitude poorer than can be readily obtainedfrom rockets. The riometer technique of Little andLeinbach (ref. 10) measures the absorption of incomingcosmic noise and is capable of providing coarse electron-density profiles over a limited height range. It is par-ticularly useful during disturbed conditions (ref. 11) .The more traditional radio methods based on measuringsignals received from low-frequency transmitters all sufferfrom the disadvantage that they provide integrated infor-mation over the transmission path which is difficult to in-terpret, but these methods can be useful for checkingprofiles and for monitoring.

The use of sounding rockets brought a tremendous ad-vance in observational techniques, since the integratedproperties measurable by radio-propagation methods canproduce accurate profiles when these integrals are obtainedas functions of known rocket altitudes. The Faradayrotation and differential absorption methods, first de-scribed in 1958 by Jackson and Seddon (ref. 6 ) and Sed-don (ref. 12), are particularly suitable for investigatingthis region and have provided the most reliable results.An interesting refinement of these techniques has rccentlybeen described by Knoebel and Skaperdas (ref. 13). Inthis version, a receiver in the rocket is used in a servo loopto control the power of the extraordinary wave emitted bya transmitter on the ground so that the rocket always seesa constant ratio of ordinary to extraordinary signal. Thismethod insures a large dynamic range and places the sim-plest portion of the experiment in the rocket.

Apart from the discovery that the traditional ionosphericlayers are not well separated, two other important featuresemerged from the early rocket experiments on radio propa-gation. Jackson and Seddon proved experimentally that6


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. ,IONOSPHERES AND RADIO PHYSICSsolar flares which result in radio-communication blackouts(sudden ionospheric disturbances (SID ) in the sunlithemisphere in medium latitudes. The Lyman-alpha fluxremains relatively constant within the limits 3 to 6 ergs/cm2/sec, while the X-ray flux can vary by several ordersof magnitude, according to data from a number of rocketflights and the Greb, Ariel, and OS0 satellites. (See, forexample, refs. 25 and 26.)

Other D-region disturbances are characterized by anunusually high absorption of radio waves. I t has beenshown by Bailey (ref. 2 7 ) and Webber (ref. 28) that polarcap absorption events (PCA) are due to solar protons.Some disturbances are also associated with aurorae, anddays of high absorption are occasionally found in winterat medium latitudes. Aikin, Kane, and Troim (ref. 22)have suggested that the latter may be related to changesof mesospheric pressure, which affects the collision fre-quency. The evidence found by Bossolasco and Elena(ref. 29) and Aikin, Kane, and Troim (ref. 22) for sucha variation of collision frequency with mesospheric pres-sure requires further verification; however, this effect ap-pears to be an important new result that must be consideredin the interpretation of radio measurements currentlybased on constant-collision-frequency models.

THE PHOTOEQUILIBRIUM REGIONDuring the day, there is in this region an approximate

equilibrium between the process of ion production andloss. This region extends from about 85 kilometers tosomewhat above 200 kilometers, including the E and F1,as well as the lower part of the F2, layers. The densityof the neutral atmosphere here is sufficiently low to reducethe collision frequency between electrons and neutral par-ticles to such an extent that there is little absorption ofradio energy; thus refraction and reflection are the domi-nant effects on radio waves.8


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..IONOSPHERIC REGIONS

' This region may be explored by means of ionosondes, andthe work of Budden (ref. 30) and Jackson (ref. 31 ) led theway for modern methods of obtaining electron-densityprofiles from ionograms. Although useful for the daytimeF-region, these methods do not always show enough detailin the E-region and can produce considerable errors atnight from underlying ionization. The incoherent back-scatter technique, first suggested by Gordon in 1958 (ref.32) and first implemented by Bowles (ref. 33), can givedata from about 100 kilometers upward when using 50Mc/sec near the magnetic equator, and also at lower alti-tudes when using higher frequencies at other latitudes.The amplitude and spectral distribution of the returnedsignal can provide information on electron densities, elec-tron and ion temperatures, and ionic composition. Theseparameters are often difficult to separate, but Moorcroft(ref. 34) discusses how this may be done.

Rocket- and satellite-borne techniques for measuringelectron and ion densities and temperatures include cwpropagation (Faraday rotation and dispersive Dopplermodes (ref. 6 ) ), and a variety of Langmuir probes, iontraps, and antenna impedance devices.

The daytime E-layer has a peak electron density near110 kilometers (Robinson, 1959, ref. 35) and is the mostregular of the ionospheric layers, being only slightly per-It is produced by the action ofLyman-beta (1026 A) radiation on 0 2 and, to a lesserextent, by soft X-rays in the 10-A to 170-A range, actingon all the neutral constituents (refs. 36 and 3 7 ) . Massspectrometer measurements show that the main ionic con-stituents below 200 kilometers are NO + and 0:. Athigher altitudes, the F-region is formed by radiation in the170- to 900-A range acting principally on atomic oxygen(refs. 36 and 37 ) ,and the main ionic constituent becomes

. turbed by tidal effects.

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IONOS PHERES A N D RA DIO PHYS ICS0'. The conclusion reached in 1956 by Ratcliffe',Schmerling, Setty, and Thomas (ref. 38), on indirect evi-dence, that the electron density peak is well above theelectron production peak has been strikingly confirmed byexperiment. Figure 1 shows the electron densities meas-ured in 1961 by Bauer and Jackson (ref. 39) and thephotoionization rates obtained by Hinteregger and Wata-nabe (ref. 37). Although not taken at the same time, thesemeasurements were taken under comparable conditionsand show clearly a rising electron density above 200 kilo-meters caused by a recombination rate that decreases morerapidly with height than the photoionization rate. Rat-cliffe, Schmerling, Setty, and Thomas (ref. 3 8 ) averagedover seasons and obtained an exponentially decreasingelectron loss rate with a value of 1x 10-'/sec at 300 kilo-

Electron denrlty, electro ns /cm3

I I IIC? IO'Photoionization rate, ion polrs/cm'/ sac

Figure 1 -Electron density (ref. 39) and photoionization rate(ref. 37) as functions of altitude.

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meters. The loss rate at night has recently been reexam-ined by Nisbet and Quinn (refs. 40 and 41 ) by consideringthe loss of electrons within a tube of force. They find aloss rate which varies with temperature and with seasonbetween 0.2 and 2.0x lO-/sec at 300 kilometers. Theirresults imply a seasonal change ofcomposition which mighthave an important bearing on the seasonal anomaly in theF-region.

One of the more striking recent discoveries is the markeddifference between electron, ion, and neutral temperatures.Measurements of this disparity from rockets have been re-ported by Aono, Hirao, and Miyazaki (ref. 42) , usingresonance methods and Langmuir probes, and by Brace,Spencer, and Carignan (ref. 43) and Nagy, Brace, Carig-nan, and Kana1 (ref. 44), using Langmuir probes. Similarresults from backscatter and satellite experiments havebeen obtained but are more pertinent to higher altitudes.Figure 2 shows the measured electron temperature ( T , )for a quiet day at midlatitudes in midafternoon comparedwith the neutral gas temperature (T, ) according to Harrisand Priester, and measured values of nitrogen temperature(7). The first theoretical attempt to compute electrontemperatures by examining energy loss mechanisms forphotoelectrons was made in 1961 by Hanson and Johnson(ref. 45 ) . Revisions made by Hanson (ref. 46) and Dal-garno, McElroy, and Moffet (ref. 47) seem to be in gen-eral accord with the observations below 350 kilometers.

The diurnal, seasonal, and latitudinal changes of elec-tron density in the E-layer (near 100 kilometers) followquite closely the dependence on solar zenith angle first de-duced for a simple ionospheric layer in 1931 by Chapman(refs. 48 and 49). At higher altitudes, however, markeddeviations from this simple behavior occur, and numeroussuch anomalies have been named. Figures 3 and 4show typical sets of curves illustrating the average diurnalvariations of electron density at fixed heights over Huan-

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Temperature, O KFigure 2.-Measured electron temperature (T , ) ,measured N ,

temperature (TN2) ,nd neutral temperature (70 computedfrom the Harris and Priester model. Wallops Island, April 18,1963, 1604 e.s.t., x =a",ndisturbed conditions.

cayo, Peru, and Washington, D.C., for April 1958(Schmerling, ref. 50). A rather large maximum occurs12


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Figure 3.-Average diurnal variations of electron density at h e daltitudes for Huancayo, Peru, April 1958.

well before noon, despite the fact that a simple isothermaltheory necessarily predicts a maximum after noon. Thehigher electron densities over Huancayo (on the magneticequator) and the more exaggerated form of the anomalywhen compared with Washington (at medium latitude)are also to be noted.

There is currently no comprehensive theory of the F-region which can account satisfactorily for the diurnal,

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IONOSPHERES AND RADIO PHYSICS

I 1 , 1 , 1 , 1 , 1 , 1 , 1 I 1 1 1 1 1 1 1 1 1 1 100 02 M 08 01 IO P 14 m II 20 2 2 00I t m e . hr

Figure ?.-Average diurnal variations of electron density at fixedaltitudes for Washington, D.C., April 1958.

seasonal, and latitudinal changes observed. Only quiterecently have attempts been made to incorporate the knowndiurnal temperature changes of the neutral atmosphereinto theoretical models. Norton and Van Zandt (ref. 51) ,restricting themselves to the daytime equatorial F-layer,were able to generate curves similar to figure 3 ;and Rish-beth (ref. 5 2 ) obtained curves similar to figure 4 or me-dium latitudes. Discrepancies still exist between labora-tory measurements of reaction rates and those required bythe above theories, as well as the values needed to accountfor day and night behavior.

Another striking feature of the electron density distribu-tion is the geomagnetic control, which increases with alti-tude to the F2 peak. This geomagnetic control has beenexamined in detail by Croom, Robbins, and Thomas (ref.5 3 ) . The nearly symmetrical behavior at equinox noonis shown in figure 5. Although attempts have been made14


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IONOSPHERIC REGIONS. >----.- . . . .

Figure 5.-Nmn values of electron density at fixed altitudes asfunctions of magnetic dip, September 1957. Curves are dis-placed for clarity. The zero point for each curve is indicatedby the figure in the left margin. The scale is the same for allcurves.

to account for this distribution by the action of ambipolardiffusion and/or electromagnetic drifts, none of these hasyet been free of arbitrary assumptions.

Above the photoequilibrium region, the neutral densitydecreases sufficiently for plasma diffusion to become im-portant. The F2 electron peak marks the transition be-tween the photoequilibrium region below and the diffusionregion above. This transition represents a delicate balancebetween the competing processes of electron production,recombination, and diffusion; and many effects perturbmarkedly the height and electron density of the F2 peak.

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. ,tIONOSPH ERES A N D RAD IO PH YSICSAlthough many attempts have been made to account theo-retically for the diurnal, seasonal, and geographical varia-tions of electron density around the F2 peak, no reallysatisfactory theory is currently available.

THE DIFFUSION-DOMINATED (TOPSIDE) REGIONAbove the F2 electron peak, the vertical distribution

of ionization is influenced mainly by diffusion processesrather than electron production and loss, and the verticalgradient of electron density is more easily accounted forthan its magnitude, since the former is given approximatelyby the hydrostatic-electrostatic equation while the latteris the solution of a complicated boundary-value problem.

The first accepted measurement in this region was madeby Gringauz from a Russian stabilized rocket (ref. 54)using a cw propagation technique. Since then, there havebeen many rocket flights with propagation experimentsand a variety of direct measurement techniques based onLangmuir probes and antenna impedance measurements.The first satellite measurements were made on the re-fraction of radio signals (radio rise and radio set) and onFaraday rotation and Doppler dispersion. These tech-niques are discussed in some detail by Garriott and Brace-well (ref. 55). A more accurate combined Faraday-Dop-pler (hybrid) method has bken devised by Garriott anddeMendonqa (ref. 5 6 ), and second-order refraction cor-rections have been discussed by Ross (ref. 57 ) . Basically,these observations provide the integrated electron density(electron content) between the ground and satellite andcan give information on horizontal gradients and iono-spheric irregularities. This type of experiment has twointeresting variations. In the first, the transmitter ismounted on a spacecraft with a highly eccentric orbit sothat a nearly vertical altitude scan is obtained over part ofthe orbit, as is being done with OGO I (formerly OGO A)by Lawrence and Garriott (as reported by Schmerling16


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..IONOSPHERIC REGZONS

(iri ref. 58). In the second, a synchronous (geostationary)satellite is used so that the diurnal variation may be ob-served in detail without the drastic averaging needed withother orbits (ref. 59). Important differences have alreadybeen found between the electron content variations atdifferent latitudes, thus suggesting a latitudinal change inthe ratio of ionizing to recombining constituents. '4 moreelaborate multifrequency experiment capable of measur-ing group and phase-path differences as well as Faradayrotation would enable the F-region electron content to beseparated from the protonospheric electron content by thedifferent (magnetic) weighting in the electron contentsobtained by these methods. Such an experiment iscurrently under consideration.

The incoherent backscatter technique, which was dis-cussed in connection with the photoequilibrium region, is apowerful new tool capable of making nearly continuousob-servations, but rather elaborate facilities are needed.Recent developments in spectral analysis of the returnedsignal and combined use of backscatter measurements withtopside sounder and rocket profiles will probably lead tosignificant advances in separating the effects of electrondensity, electron and ion temperatures, and ionic mass.

The plasma resonances, first reported in 1961 on a rocketflight by Knecht, Van Zandt, and Russell (ref. 60) andsince observed regularly by Alouette I and Explorer XX,have revived interest in the physics of plasma oscillationsand have provided a valuable tool for measuring both themagnetic field and the electron density around satellites.Three types of resonance are found: at multiples of theelectron gyrofrequency, f R ; at the electron plasmafrequency, f N ; and at the upper hybrid frequency,f ~ = d / f ~ ' + f x ' . These resonances are excited by energyfrom the transmitter and are detected by a receiver.Fejer and Calvert (ref. 61) have explained them in termsof electrostatic oscillations and can account for the main

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resbnance. This lies between the electron and protongyrofrequencies, defines a cutoff frequency for propaga-tion transverse to the Earth's magnetic field, and is ob-served as a sharp lower edge on a band of noise seen by apassive receiver in the 2- to lO-kc/sec range. Unlike theother resonances, separate excitation from a local transmit-ter does not seem to be needed. This resonance is likely toprove important because the frequency dependson an eff ec-tive ion mass in the vicinity of the satellite; thus an in-dependently measured parameter is available.

By far the most prolific source of electron density infor-mation to date has been the swept-frequency topsidesounder of the Canadian-U.S. satellite Alouette I , launchedin September 1962. Not only has this sounder producedover lo 6 electron density profile records from above theF2 electron peak, where only a handful had previouslybeen available, but it has actually provided much moredetail on the topside than has been obtained on the bottomside from all experiments put together. The sounder sweepsfrom 0.5 to 11.5 Mc/sec every 18 seconds at an altitudeof 1000 kilometers, and a profile can be obtained every 120kilometers on its trajectory, or approximately every degreeof latitude. The ionograms are similar to ground-basedionograms, and their reduction to electron density profilesis also similar, with two important differences. First, theplasma frequency at the equipment is finite and must bedetermined. Fortunately, the plasma resonances afforda solution to this problem. Second, allowance must bemade for the variation of the Earth's magnetic field withaltitude. This only requires a refinement in the computa-tions, but must be done with care to avoid significanterrors in the deduced vertical gradients used for evaluatingthe important physical parameters of the topside. Thereare still some unsolved problems in deducing these profilesfrom the ionograms, as on a number of occasions the bot-tomside and topside profiles do not match properly (e.g.,

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- .IONOSPH ERES AND RADIO PH YSICS

ref. 6 3 ) . However, on the few more elaborately instru-mented tests, using simultaneous rocket and backscattermeasurements, no serious discrepancies have emerged (ref.6 3 ) . Figure 7 shows such a comparison of topside electrondensity profiles deduced from each of the following: Alou-ette I, a backscatter sounder, and a rocket probe whichcontained two independent experiments and was fired closeto the satellite. It may be seen that on this occasion goodagreement was obtained, but further work is needed to ac-count for the discrepancies observed at other times betweentopside and bottomside soundings.IM0I00

0 Alouette satellite roundmar (0922ertl

0 I I I I I 1 1 1 1 1 I I I I ! I I l l 1I0 IO8 10

Figure :.--Comparison of topside electron density profiles obtainedfrom: ( 1 ) Rocket flight two-frequency cw propagation and iontrap; (2 ) Alouette I satellite soundings; and (3 ) incoherentbackscatter.The fixed-frequency sounder on Explorer XX, sampling

every 100 milliseconds, at six spot frequencies, providesmuch better horizontal resolution of irregularities andplasma resonant structure but much coarser vertical profiledata (ref.6 4 ) .Topside Morphology

As revealed by Alouette I, the topside can be roughlydivided into three regions: ( 1 ) from the magnetic Equator20

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. . IONOSPHERIC REGIONSto a dip angle of roughly 30, where geomagnetic controlis very strong; ( 2 ) from about 30" to 70" of dip, where thespatial variation is smooth and regular; and (3) aboveabout 70" of magnetic dip, where the spatial distributionshows marked irregularities. The strong geomagnetic con-trol is illustrated in figure 8, which shows electron densityas a function of magnetic dip for a number of fixed alti-tudes on October 3, 1962, taken somewhat before localnoon over Singapore. The tendency for the maxima tofollow a magnetic field line is easily seen. The develop-ment of this structure for different local times along the75" W meridian is shown in more detail by Lockwood andNelms (ref. 65). An example of a nighttime distributionon February 17, 1963, is given in figure 9. No adequatetreatment of the time-dependent behavior of this distribu-

Field line calculated12-XIO5 from magnetic dip

c

0\u)

rr ) 10-E

? 8 -c0alaJ-._ 6 -u)c0 )-a

L I I I I I I I I 1 1 1 1 1 150" 40" 30" 20' IO" 0 Io"South Nor thMagnetic dip

Figure &-Topside electron densities at fixed altitudes taken inthe late morning on October 3,1962.

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_ .IONOSPHERES A N D RADI O PHYSICSI I I I I I I l l 1

f-fLaUII0)

\

ii 208z00Qz0s:z02

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IONOSPHERIC REGIONS1 .I tion exists, but quite an extensive literature on the steady-

state distribution is available. The effect of diffusion inmodifying the equatorial vertical profiles into a reasonableapproximation of the observed distribution has beendiscussed by Goldberg, Kendall, and Schmerling (ref.

l SS), and computations which include the effects of, electrodynamic drift terms have been presented most

recently by Bramley and Peart (ref. 67) and Baxter(ref. 68). Unfortunately, although the effects of an as-sumed electrodynamic drift can be computed, probablyquite accurately, there is at present neither a satisfactorytheory for predicting what this drift should be nor a satis-factory method for measuring it experimentally.

Field-alined strata (Sayers, ref. 69) and ledges in theelectron density profiles (King, et al., ref. 70) are commonfeatures. Figure 10 shows an Alouette I ionogram, withcusps in both ordinary and extraordinary traces near 2.5Mc/sec, indicative of a ledge. The corresponding elec-tron density profile is shown in figure 11. Since the ledgeis less prominent on the profile, it may be more preciselydefined by taking as coordinates the plasma frequency ofthe cusps and the true height corresponding to this plasmafrequency. Figure 12 shows the alinement of these ledgecoordinates, represented by dots, along a magnetic fieldline.The existence of these field-alined structures and theducted propagation effects seen on Explorer X X can beconsidered evidence of the magnetic guidance that permitsionization to move along, but not across, field lines. At-tempting to interpret the slopes of the profiles, King (ref.70) suggested that they should be measured along field linesrather than vertically. Preliminary results obtained in thisway seem to be more nearly in accord with independentlyobtained temperature and composition data.

Although the latitude variation is currently quite wellestablished (for the minimum phaseof the solar cycle), the

23

I

208-480 - 6 3


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IONOSPHERES ANU KAULU rti I XLJ

24


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.-IONOS PHERlC REGIONS

m-

800-

5--p 400-crP200-

I I I I I I I I IPlasma frequency, Mc/sec

Figure I I.-Profile corresponding to ionogram in figure 10.

IicU% 6-

5 8-mL

u-a

IO-

Fieldline

Heiaht. km

-550

500475

b 'Magnetic equatorII I 1 I I00 -100 -200

Geographic latitudeFigure 12.-Plasma frequency contours with dots to show ledge

coordinates. November 2, 1962, 1800 I.m.t., 88.7" W.diurnal variation is not known nearly as well, since the slowprecession of the Alouette I orbit necessitates averagingover several months, and the F-region exhibits considerable

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1 1 1 1 1 1 1 1 1 1 ~ ~ 1Height, km -400

tL o c a l m e a n time, hr

Figure 13.-Average diurnal variation of electron density at fixedaltitudes: 35" N to 40" N, October-December 1962.

Topside CompositionImmediately above the F2 electron peak, the chief ionicThis has been generally accepted ononstituent is 0'.

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IONOSPHERlC REGlONS1 1 1 1 1 1 1 1 1 1 1 1 1

Height, km

I l l l l l l l l l l l l0 4 0 12 16 2 0 24

L o c a l mean t ime, hrFigure 14.-Average diurnal variation of electron density at fixed

altitudes: 40" N to 45" N, October-December 1962.theoretical grounds for many years, but the first direct proofcame from a mass spectrometer measurement made in1958 on Sputnik I11 by Istomin et al. (ref. 7 2 ) . Atgreater heights, 0 was thought to be gradually replacedby H' as principal constituent, via a charge-exchange re-action. In 1961, however, Nicolet (ref. 7 3 ) suggested ontheoretical grounds that helium was likely to be an impor-tant constituent near 1000 kilometers, forming an interme-diate layer. The experimental discovery of He+ as amajor constituent was first reported in 1962 by Bour-

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. IIONOSPHERES AND RAD IO P HY SI CSdeau, Whipple, Donley, and Bauer (ref. 74), using an iontrap on Explorer VIII. Independently, Hanson (ref. 75)inferred the same result from a reinterpretation of an ionprofile obtained by Hale, on the grounds that a fourfolddecrease of mass with altitude was more likely than a four-fold increase in temperature near 1000 kilometers.

Further work has provided some indications of the vari-ability of the altitudes of transition between regions whereO', He+, and H' are the major constituents. By using thediffusive-electrostatic equilibrium conditions of Mange(ref . 76), together with the appropriate boundary condi-tions, Bauer (ref. 77) deduced the variation of these transi-tions with neutral temperature shown in figure 15. Noteespecially that at low temperatures the helium layer is toothin to be detected from the slopes of electron density pro-files. Measurements taken on a number of rocket flightsand the Ariel I ion spectrometer seem to agree in generalwith this theory at medium latitudes, but Bowen, Boyd,Raitt, and Willmore (ref. 78) also find a longitudinal vari-ation (alined with the geomagnetic equator) and an in-crease in the 0' concentration with latitude. Their re-sults suggest that the ion composition depends more onmagnetic than geographic latitude.Electron Temperatures

Strong experimental evidence indicates that the electrontemperature exceeds the neutral (and ion) temperature bya considerable amount, even at high altitudes, during theday, and by a lesser, but still significant, amount at night.This evidence has come from the direct measurement de-vices on Explorer VI11 (ref. 79), on Ariel I (ref. 80),onExplorer XVII (ref. 81 ) ,on Discoverer satellites (ref. 82),and on a number of sounding rockets (ref. 83) and back-scatter soundings (ref. 84) . There is also some indicationthat the magnitude of this temperature excess varies con-28


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. . IONOSPHERIC REGIONS

H+ dominanf

EY

L0E08

Figure 15.-Transition altitudes between regions where O', He+,and H are dominant constituents, as functions of neutral tem-perature (theoretical).

siderably with the sunspot cycle. From the same sources,it also appears that the electron temperatures increase withlatitude toward the poles and the electron temperature ata fixed altitude shows a rapid rise after sunset, with a maxi-mum before noon and a fairly flat portion after noon.Figure 16 shows some preliminary, smoothed, diurnal

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. .IONOSPH ERES AND RADIO PHYSICS3500 1 1 I 1 I I

o 2500

15002000

1000

Y

0 4 8 12 16 20 24Local time, hr

Figure 16.-Diurnal variations of electron temperature for northlatitudes of lo", 40,and 60" and altitudes in the 260- to 400-kilometer range. Data from Explorer XVII.

temperature variations for three latitudes from ExplorerXVII.The theoretical treatment of charged-particle tempera-

tures above about 300 kilometers is rather difficult, sinceconsideration must be given to all the mechanisms bywhich photoelectrons can lose their energy, including theenergy lost far from the region of the original photoioniza-tion. Hanson (ref. 46) and Dalgarno, McElroy, andMoffett (ref. 4 7 ) concluded that the electron temperatureshould approach the gas temperature at high altitudes.Both Bourdeau (ref. 85) and Geisler and Bowhill (ref. 86)have pointed out that these calculations were made for par-ticular conditions of high electron density near sunspotmaximum when the electron heat conduction is small.Near sunspot minimum, with lower electron densities,

thermal runaway" is possible, leaving the previously negli-gible electron heat conduction term the controlling factorin the electron temperature distribution and permittingthe electron temperature to exceed the gas temperature athigh altitudes. The elevated electron temperatures in themorning and at higher latitudes can be qualitatively under-

( 6

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. Il O N O S P H E R E S A N D R A D I O PHYSICSthe history and modern theory of whistlers and other VLFphenomena has been given by Helliwell (ref.89).

The impulsive noise generated by a lightning flash entersthe protonosphere and is dispersed in frequency. This fre-quency-dispersed signal, in the form of a gliding tone, orwhistler, either on the ground or in space, can provide anintegrated measurement of the properties of the mediumbetween the transmitter and receiver. Since the traveltime is weighed inversely as the magnetic field in the inte-gral, the dispersive properties are more sensitive to condi-tions near the top of the path. As discussed by Carpenterand Smith (ref. 90),careful measurement of whistlers ob-served on the ground enables the vertical electron densityprofile to be estimated in the equatorial plane, subject toproper identification of the whistler path from the nosefrequency and assumptions about the approximate sym-metry of the distribution when inverting the dispersionintegral. A composite of such results for a variety of con-ditions is shown in figure 17. Each curve has been obtainedas an average of a number of different observations, sinceone profile cannot be obtained from a single observation.During magnetic storms these electron densities may bereduced by as much as an order of magnitude (Carpenter(ref.91 ) and Helliwell, private communication). Thereis also evidence of a knee, or rapid decrease in the elec-tron density, at distances of several Earth radii (ref. 90).Direct measurements by ExpIorer X V III ( I M P I ) haveconfirmed this for the equatorial plane (ref.9 2 ) ,and meas-urements on Lunik I, 11, and 111 indicate that the kneemoves in close to the Earth at high latitudes (ref. 9 3 ) andduring magnetic disturbances.

According to Liemohn and Scarf (ref. 9 4 ) , he high-fre-quency cutoff of whistlers is below the value expected onsimple theory for the electron gyrofrequency at the top ofthe whistler trajectory, but can be accounted for by Lan-32


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, - IONOSPHERIC REGIONS

Q)-W I 0 lI 2 3 4 5 6Geocentric distance, Earth radii

Figure 17.-Experimental equatorial electron density profiles inthe protonosphere: (a) Storey, (b) Allcock, (c) Carpenter andAngerami, (d) Smith, (e ) Pope, ( f) Bowles, (g) Schmelovsky,and (h) Schoute-Vanneck and Muir.

dau damping from the high-energy tail of a non-Maxwel-lian electron velocity distribution. According to F. S.Johnson (private communication), this damping might bemore naturally considered as arising from the low-energytail of the trapped-particle population.

The theory of the electron distribution in the exospherehas recently been reexamined by Angerami and Thomas(ref. 95) . They assumed diff usive equilibrium along fieldlines, took account of centrifugal force, and used the ob-served electron density at 1000 kilometers (from AlouetteI ) as a lower boundary condition. Some results of thesecomputations are shown in figure 18. Good agreementcould be obtained with the observations shown in figure 17

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.- lONOSPHERlC REGlONSInjun 111,Lofti I, and OGO I. Observations from satel-lites enable the whistler path to be more accuratelydetermined and usually have much better signal-to-noiseratios than those made from the ground.A new phenomenon, the subprotonospheric whistler, hasbeen discovered in a rocket flight and the Alouette I (VLF)records. As described by Carpenter, Dunckel, and Walk-up (ref. 97), this type of whistler is characterized by re-peated reflection of energy between about 100 and 1000kilometers. The lower reflection mechanism has long beenfamiliar, but the upper has been explained by Smith (ref.98) as caused by refraction through a region of transversepropagation by the action of hydrogen ions and a horizon-tal density gradient.

Another new type of whistler, the proton whistler, hasbeen discovered in Alouette I and Injun I11 records (ref.99) immediately following a fractional-hop whistler. Theproton whistler consists of an ascending tone that starts ata very low frequency and approaches the proton gyrofre-quency asymptotically. The explanation appears to bethat a whistler originating at the ground can pass througha coupling region where this additional signal is generated,with a circular polarization of opposite sense to the usualwhistler and a longer travel time.

A considerable body of data has been collected on avariety of emissions with dispersion characteristics differentfrom whistlers. Possible mechanisms for generating thesefrom charged-particle interactions have been summarizedby Brice (ref.100). Helliwell (ref. 101) showed that peri-odic VLF emissions could be triggered by whistlers andthat the data are incompatible with a generation mecha-nism based on particle bunches oscillating between mirrorpoints.

The mechanism for the passage of whistler energy intothe ionosphere through the collision-dominated region is

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. IIONOSPHERES AND RADIO PHYSICS 'not yet fully understood, and more work on this problemis needed. Recent attempts to measure the amplitude anddirection of propagation of VLF signals from rockets haveled to results not fully in accord with theory (Storey, pri-vate communication). The distribution of VLF energyin the protonosphere itself poses interesting problems.Ground-based evidence and, in particular, the multicom-ponent whistlers examined by the Stanford group haveled to the current ideas on propagation along field-alinedducts. Although such ducts are seen at higher frequenciesby Alouette I and Explorer XX, there is as yet no directin situ evidence for ducted propagation at VLF. On thecontrary, the reception of manmade signals from satellitesindicates that propagation is not limited to such ducts, sincesuch signals can be received over very large areas (ref.102). Direct evidence for the existence of diffuse whis-tlers not associated with ducts has also been obtained fromrocket flights by Cartwright (ref. 103).

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chapter 3Dyaamic Effects

ELECTRODYNAMIC DRIFTST HAS FREQUENTLY been suggested that electrodynamicI forces play an important role in controlling theundisturbed ionosphere. The gravitational (tidal) andthermal motions of the neutral atmosphere sweep electrons

and ions across the Earths magnetic field, generating a cur-rent by dynamo action. This current, flowing at about 100kilometers, is responsible for the magnetic fluctuations seenon the ground. It is then supposed that the electric fieldsgenerated in this region can in turn cause the F-regionionization to move by a motor action. These ideas havebeen summarized by Ratcliffe and Weekes (ref. 104),whoprovide an extensive bibliography.As discussed in previous chapters, a number of thephenomena for which electrodynamic explanations hadbeen devised have now received alternative explanations,and no direct evidence has been found for the motortheory. And yet it seems clear that there should be somesuch effect. The success of Baker and Martyn (ref. 105)in explaining the high currents observed in the equatorialelectrojet by a polarization field which inhibits the flow ofHall current is certainly strong evidence for the existenceof polarization fields at heights of the order of 100 kilo-meters over the magnetic equator. The question still tobe resolved is whether the motor effect of such fields onthe F-region is a dominant factor or a small perturbation.Measurements of the currents and electric fields are badlyneeded, but are very difficult to make.

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. .I ON O SP H E RE S A N D RAD10 P H Y S l C S .The current at the magnetic equator is much more in-

tense than at higher latitudes and has been measured withrocket-borne magnetometers by Singer, Maple, and Bowen( 1951, ref. 106) and Cahill (1959, ref. 107). The resultsof these and some more recent flights do not entirely agreewith theory, as they sometimes indicate a double currentsheet and at other times a thick current sheet extendinginto the lower F-region. The experiment becomes moredifficult at midlatitudes, partly because of the smallercurrent and partly because of the much greater directionaldiscrimination needed. Measurements were made in1964 at Woomera by Burrows and Hall (ref. 108), whofound a current sheet slightly lower and thinner than ex-pected, and by J. N. Davis at Wallops Island (privatecommunication) with somewhat similar results. Moreexperimental work is needed to map the current system,since neither the vertical current distribution nor the mag-netic field at zero current can be obtained from ground-based magnetometers.While the exact nature and magnitude of the contribu-tion of electrodynamic effects to the quiet midlatitudeionosphere are still an open question, there are high-lati-tude phenomena, particularly those associated with mag-netic storms, that more clearly indicate magnetic fieldeffects. Fejer (ref. 109) has shown how auroral electrojetcurrents similar to those inferred from magnetic measure-ments may be formed by convective motions in the mag-netosphere in which trapped particles participate. Di-rect verification of this theory awaits simultaneousobservations of the trapped particles, the distorted geo-magnetic field, and the electric field.

SPORADIC ESporadic E is a thin layer of ionization or irregularitiesfound at altitudes near 100kilometers, with a complicated

morphology (ref. 110 ). Quite probably a number of dif-38


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. I1ONOSPHERES A N D RADIO PHYSICS

"P4 0 jl' O r0

Figure 21.-Wind direction from trimethyl aluminum release(same flight asfigs. 19 and 20).

in the collision-dominated region, there is no evidence forfield alinement, and at intermediate altitudes there is evi-dence that blobs of irregularities are elongated in thedirection of field lines.

The origin of ionospheric irregularities is still uncertain,although several mechanisms have been postulated and anumber of different phenomena appear to be involved.The theory of internal atmospheric gravity waves has beendeveloped by Hines (refs. 119 and 120). These are gen-erated by tidal and wind forces in the lower (neutral)atmosphere, grow in amplitude with height due to the de-creasing density, and perturb the plasma in the process.Although this theory seems to account for a number ofthe observed features, Heisler and Whitehead (ref. 121)42


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. *D Y N A M I C EFFECTS

report measurements of group and phase velocity on trav-eling disturbances which disagree with this theory. Inaddition, the theory apparently cannot account for thelatitudinal distribution of irregularities, although thismight be explained by a development which includesmagnetospheric convection as discussed by Axford andHines (ref. 122). It has already been mentioned thatequatorial sporadic E is thought to arise from a two-streaminstability associated with the electrojet, and midlatitudesporadic E from wind shears. Other mechanisms, involv-ing turbulence and hydromagnetic waves, have also beeninvoked; but thus far theories based on them do not seemfirmly grounded.

Irregularities and associated movements may be in-vestigated by observing released chemicals and meteortrails. Radio methods include observation of the scintilla-tion of radio stars and comparison of fading patterns atspaced receivers. These are discussed in detail by Briggsand Spencer (ref. 123). The spaced-receiver method suf-fers from the disadvantage that the height at which themovements are measured is not clearly established; in thelower ionosphere, where the winds are known to vary rap-idly with altitude, this disadvantage can be quiteimportant.

Rocket and satellite techniques have produced a wealthof new data. By observing the fluctuations of satellite sig-nals at several locations, information is obtained about theintensity, size, and altitude of the irregularities in theF-region. Yeh and Swenson (ref. 124) have summarizedobservations over the past 5 years and provide an extensivebibliography. They report strong diurnal, seasonal, solar-cycle, and latitudinal effects on the occurrence rate of thescintillations. The irregularities were seen at all altitudes,with a peak in the rate of occurrence at about 350 kilo-meters, at night. The diurnal variation of the altitude

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a .

IONOSPH ERES A N D RA DIO PH YSICSdistribution at temperate latitudes has been given in mofedetail by McClure (ref. 125) using Transit 4A observa-tions, and is shown in figure 22. Another manifestation ofirregularities is known as Spread F, after the spreadingof ionogram traces in the F-region. Calvert and Schmid(ref. 126) have recently examined the Alouette I sounderrecords for Spread F and report three distinct types. Acommon form is due to aspect-sensitive scattering fromfield-alined irregularities. The frequency with which thisform was seen on records for the 1962/63 northern winteris shown in figure 23 as a function of both time and geo-magnetic latitude. At present, there are no adequatetheories to account for these observations.

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.-

J=aD7In

D Y N A M I C EFFECTS

Q &cIncu

0D 0: 0 O l S


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- 1

IONOSPH ERES AND RADIO PH YSICS

I I I I I

m

I I I I I8 9 8 8 08 0 3 8 2

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. *

chapter 4Conclzlsions and Ozltlook

REAT ADVANCES H A V E BEEN MADE in understandingG he ionosphere, largely as a result of the informationobtained from rockets and satellites. The ground-basedincoherent-backscatter technique has also made some con-tributions and will probably be of increasing value in deter-mining electron and ion temperatures and ion composition,particularly when used with rocket and satellite data.

Data have been obtained from regions previously inac-cessible, notably the D-region and the upper part of theF-region, for which a sunspot-minimum morphology is nowavailable in greater detail and with better coverage thanfor the bottomside. I t is hoped to continue this work atleast through the next solar maximum. Topside sounderscan, in principle, make possible a better use of communi-cations channels by permitting the optimum choice of fre-quencies based on current conditions measured over largeportions of the globe.

Throughout much of the ionosphere, the solar spectralregions responsible for producing ionization have been de-termined. Less progress has been achieved in determiningthe mechanisms of electron loss, since many reaction ratesare imperfectly known and the role of minor constituentsis still uncertain, particularly in the lower ionosphere.More laboratory work is needed on the reaction rates, andimproved mass spectrometers are needed to obtain dataat the lower altitudes.I t has been shown that electrons in the F-region are pro-duced by a wide range of wavelengths, and that the F2region is formed where the electron production rates de-

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. .IONOSPH ERES AND RADIO PHYSICS

crease with altitude in a manner more complicated thancan be described by a single Chapman function.

New measurements have established that the electrontemperature above the E-region generally exceeds the neu-tral gas temperature, even at night. This temperatureexcess is dependent on altitude, latitude, and time of day,but has yet to be observed over a solar cycle. I t is believedthat these results can be accounted for by the known en-ergy-exchange mechanisms between photoelectrons, ions,and neutral molecules.

The detailed diurnal and seasonal variations of electrondensity at fixed altitudes in the F-region are not properlyunderstood, although some progress has been made in ac-counting for the morning maximum by incorporating intothe theory the diurnal temperature variation of the neutralatmosphere.

Helium was discovered to be an important ionosphericconstituent, and theories of diffusive equilibrium have ex-plained the variation with temperature of the altitudes atwhich oxygen, helium, and hydrogen become the majoratmospheric constituents. These theories seem to accountfor a number of day and night measurements at middlelatitudes during different parts of the solar cycle, but notfor the recently reported latitudinal variation of heliumconcentration.

Plasma resonances discovered on topside sounder recordshave been shown to arise from longitudinal oscillations atthe electron gyrofrequency and its harmonics, at the elec-tron plasma frequency, and at an rms combination ofthese frequencies. Neither the detailed excitation mech-anism nor the structures observed have yet been explained.Other VLF resonances have been discovered which arisefrom multi-ion effects, and new types of whistlers have beenobserved involving the proton gyrofrequency resonance anda subprotonosphere reflection effect.48


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,-C ON C LU SIO N S A N D O U T L O O K

Work on irregularities has revealed the statistics of oc-currence and the altitude distribution as a function of time,and it has provided some indications of the size of theirregularities. Field-alined ducts have been observed bythe topside sounders and shown to be effective in guidingradio energy in the megacycle range.It has been shown that, at the magnetic equator, spo-radic E arises from an acoustic wave generated by a two-stream instability associated with the electrojet. Atmiddle latitudes, sporadic E is a very thin, intense layer ofionization associated with wind shears, but in a more com-plicated way than described by current theories whichoccasionally predict the wrong sign for the wind shears.

The maintenance of the polar ionosphere in the ab-sence of solar illumination is not understood. Both ioniza-tion by particles and transport of ions from elsewherehave been suggested, but neither possibility has yet beenadequately evaluated. Further work is needed on thepossible contributions by particles to ionization at highaltitudes and high latitudes. In addition, the dynamiceffects in the ionosphere, beginning with motions of theneutral atmosphere and continuing with dynamo andmotor effects, should be thoroughly examined.

The processes determining the height of the turbopause,where atmospheric mixing ceases and diffusive separationstarts, have to be clearly identified, since the turbopauseheight determines the ratio of constituents at greateraltitudes.A start has been made in measuring the current respon-sible for quiet-day magnetic variations (the Sqcurrent) atmiddle latitudes. More measurements are needed to mapthis current system in detail to relate it to ground-basedmagnetometer measurements and to electric fields in theionosphere. Despite some missing details, more is knownabout the origins and nature of the small short-term varia-

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- .

References

1. GAUSS,C. F. : AllgemeineTheorie des Erdmagnetismus. Gaussand Weber (Leipzig), 1839.2. STEWART, . : Hypothetical Views Regarding the ConnectionBetween the State of the Sun and Terrestrial Magnetism.Encyclopaedia Britannica. Vol. XVI, ninth ed., 1878, p. 181.

3. KENNELLY,. E.: On the Elevation of Electrically ConductingStrata of the Earths Atmosphere. Elec. World Engr., vol. 39,1902, p. 473.

4. HEAVISIDE,. Telegraphy-Theory. Encyclopaedia Britannica.Vol. IX. Tenth ed., 1902, p. 213.

5. APPLETON, . V.; A N D BARNETT,M. A. F.: On Some DirectEvidence for Downward Atmospheric Reflection of ElectricRays. Proc. Roy. Soc.,London, Ser. A,vo l . 109, 1925, p. 621.

6. JACKSON,. E.; AND SEDDON,. C.: Ionosphere Electron-DensityMeasurements With the Navy Aerobee-Hi Rocket. J.Geophys. Res., vol. 63, 1958, p. 197.7. GARDNER,. F.; AND PAWSEY,. L.: Study of the IonosphericD-Region Using Partial Reflections. J. Atmospheric Terrest.Phys., vol. 3, 1953, p. 321.

8. FE ER, J. A. ;AND VICE,R. W. : An Investigation of the Iono-spheric D-Region. J. Atmospheric Terrest. Phys., vol. 16,1959, p. 291.

9. WEISBROD,.; FERRARO, . J.; AND LEE, H. S.: Investigationof Phase Interaction as a Means of Studying the Lower Iono-sphere. J. Geophys. Res., vol. 69, 1964, p. 2337.

10. LITTLE, C. G.; AND LEINBACH,.: The Riometer-A Devicefor the Continuous Measurement of Ionospheric Absorption.Proc. IRE, vol. 47, 1959, p. 315.

11. PARTHASARATHY,.; LERFALD, . M.; AND LITTLE,C. G.:Derivation of Electron-Density Profiles in the Lower Iono-sphere Using Radio Absorption Measurements at MultipleFrequencies. J. Geophys. Res., vol. 68, 1963, p. 3581.

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.*IONOSPHERES AND RAD IO PHYSICS 12. SEDDON,. C.: Differential Absorption in the D and Lqwer

E-Regions.13. KNOEBEL, . S.; AND SKAPERDAS,.: A Radio Propagation

Experiment Using a Rocket-Borne Receiver. Paper 3-5-6presented at Fall Meeting, URSI (Urbana, Ill.), Oct. 1964.14. KANE, . A. : Re-Evaluation of Ionospheric Electron Densitiesand Collision Frequencies Derived From Rocket Measure-ments of Refractive Index and Attenuation. J. Atmos.Terrest. Phys., vol. 23, 1961, p. 338.

15. SEN,H. K. ;AND WYLLER, . A. : Generalization of the Appleton-Hartree Magneto-Ionic Formula. Phys. Rev. Letters, v01. 4,1960, p. 355.

16. BOURDEAU,. E.; WHIPPLE,E. C., JR.; AND CLARK, . F.:Analytic and Experimental Electrical Conductivity Betweenthe Stratosphere and the Ionosphere. J. Geophys. Res., VO~.64, 1959, p. 1363.

17. KRASNUSKIN,. E.; AND KOLESNIKOV,. L.: Investigation ofthe Lower Ionosphere by Means of Long Radiowaves andLow Frequency Radiosondes Placed on a Rocket: Detectionof a New Ionosphere Layer. Dokl. Akad. Nauk SSSR, vol.146, 1962, p. 596.

18. MLODNOSKY,. F.; A N D GARRIOTT, . K.: The V.L.F. Ad-mittance of a Dipole in the Lower Ionosphere. Proc. Intern.Conf. on the Ionosphere, London, 1962. Phys. SOC. London),1963, p. 484.

19. SMITH,L. G.: Rocket Measurements of Electron Density andTemperature in Nighttime Ionosphere. J. Geophys. Res., vol.67, 1962, p.. 168.

20. SAGALYN, R. D. ; AND SMIDDY, .: Rocket Investigation of theElectrical Structure of the Lower Ionosphere. In: SpaceResearch IV (P. Muller, ed.), North-Holland Pub. Co.(Amsterdam), 1964, p. 371.

21. NICOLET, . ; ND AIKIN,A. C. : The Formation of the D Regionof the Ionosphere. J. Geophys. Res., vol. 65, 1960, p. 1469.22. AIKIN,A. C.; KANE, . A.; AND TROIM,.: Some Results ofRocket Experiments in the Quiet D Region. J. Geophys.Res., vol. 69, 1964, p. 4621.

23. BARTH,C. A.: Rocket Measurement of the Nitric Oxide Day-glow. J. Geophys. Res., vol. 69, 1964, p. 3301.

J. Geophys. Res., vol. 63, 1958, p. 209.

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REFERENCES24.. POPPOFF, . G.;WHITTEN,R. C.; AND EDMONDS,. S .: The

..

25.26.

27.28.

29.

30.

31.

32.

33.

34.

35.

Role of Nonflare X Radiation on the D-Region. J. Geophys.Res., vol. 69, 1964,p. 4081.

FRIEDMAN, . : Solar Radiation. Astronautics, vol. 7, 1962,p. 14.

BOWEN,P. J.; NORMAN, . ; POUNDS, . A.; SANFORD,. W.;AND WILLMORE,. P.: Measurements of the Solar Spectrumin the Wavelength Band 4 o 14A. Proc. Roy. Soc., London,Ser. A, vol. 281, 1964,p. 538.

BAILEY,D. K. : Polar-Cap Absorption. Planetary Space Sci.,vol. 12, 1964, p. 495.

WEBBER,W.: The Production of Free Electrons in the Iono-spheric D Layer by Solar and Galactic Cosmic Rays and theResultant Absorption of Radio Waves. J. Geophys. Res., vol.67, 1962, p. 5091.

BOSSOLASCO,. ; AND ELENA, .: On observe une correlationentre labsorption ionospherique anormale dhiver et la tem-perature moyenne a 10 mbar. Compt. Rend., vol. 256, 1963,p. 4491.

BUDDEN, . G.: A Method for Determining the Variation ofElectron Density With Height ( N ( z ) Curves From Curves ofEquivalent Height Against Frequency (h , f) Curves). Proc.Conf. Physics of the Ionosphere, Cambridge, 1954. Phys.SOC. (London), 1955,p. 332.

JACKSON,. E. : A New Method for Obtaining Electron-DensityProfiles From P-2 Records. J. Geophys. Res., vol. 61, 1956,p. 107.

GORDON, . E.: Incoherent Scattering of Radio Waves by FreeElectrons With Applications to Space Exploration by Radar.Proc. IRE, vol. 46, 1958,p. 1824.

BOWLES, . L. : Observation of Vertical-Incidence Scatter Fromthe Ionosphere at 41 Mc/sec. Phys. Rev. Letters, vol. 1,1958, p. 454.

MOORCROFT,. R.: On the Determination of Temperatureand Ionic Composition by Electron Backscattering From theIonosphere and Magnetosphere. J. Geophys. Res., vol. 69,1964, p. 955.E-Layer. Rept. Progr. Phys., vol. 22, 1959,p. 241.ROBINSON,. J. : Experimental Investigations of the Ionospheri

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..I ON O S PH E RE S A N D R A D IO PHYSICS -36. WATANABE,. ; AND HINTEREGGER,. E. : Photoionization R@esJ. Geophys. Res., vel. 67, 1962,37. HINTEREGGER,. E.;AND WATANABE,. : Photoionization Rates

in the E and F Regions. J. Geophys. Res., VO~.7,1962, p. 3373.38. RATCLIFFE,. A.; SCHMERLING,. R.; SETTY,C. S. G . K.;NDTHOMAS,. 0.:The Rates of Production and LOSSf Elec-

trons in the F Region of the Ionosphere. Phil. Trans., ROY.Soc.,London, Ser. A, vol. 248, 1956, p. 621.

39. BAUER,. J.; AND JACKSON,. E.: Rocket Measurement of theElectron-Density Distribution in the Topside Ionosphere.J. Geophys. Res., vol. 67, 1962, p. 1675.

40. NISBET,J. S.; AND QUINN, T. P.: The Daytime EquatorialF-Layer.41. QUINN,. P.; AND NISBET,. S.: Recombination and Transport

in the Nighttime F Layer of the Ionosphere. J. Geophys.Res., vol. 70, 1965, p. 113.

42. AONO,Y . HIRAO, .; AND MIYAZAKI,. : Rocket Observationof Ion Density, Electron Density and Electron Temperaturein the Ionosphere. J. Radio Res. Labs. (Japan) , vol. 9, 1962,p. 407.43. BRACE, . H . ; SPENCER, . W.; A N D CARIGNAN,. R.: Iono-sphere Electron Temperature Measurements and Their Im-plications.

44. NAGY, . F.; BRACE, . H.; CARIGNAN,. R.; AND KANAL,M .:Direct Measurements Bearing on the Extent of Thermal Non-equilibrium in the Ionosphere. J. Geophys. Res., vol. 68,1963, p. 6401.

45. HANSON,W. B.; AND JOHNSON, F. S.: Electron Temperaturesin the Ionosphere. Mem. SOC.Roy. Sci. Licge, vol. 4, 1961,p. 390.

46. HANSON,W. B:: Electron Temperatures in the Upper Atmos-phere. In: Space Research I11 (W. Priester, ed.), North-Holland Pub. Co. (Amsterdam), 1963, p. 282.

47. DALGARNO,.;MCELROY,M. B.; AND MOFFETT, . J. : ElectronTemperatures in the Ionosphere. Planetary Space Sci., vol.11, 1963, p. 463.

48. CHAPMAN,.: The Absorption and Dissociative or IonizingEffect of Monochromatic Radiation in an Atmosphere on a

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102. LEIPHART,. P.; ZEEK,R. W.; BEARCE, . S.; AND TOTH,.:Penetration of the Ionosphere by Very Low Frequency RadioSignals.

103. CARTWRIGHT,. G.: Rocket Observations of Very Low Fre-quency Radio Noise at Night. Planetary Space Sci., vol. 12,1964,p. 11.

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Significant Achievements in Ionospheres and Radio Physics, 1958 - 1964

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