motions in the ionosphere

PROCEEDINGS OF THE IRE

Motions in the Ionosphere*C. 0. HINESt

Summary-The ionosphere, even in its "undisturbed" state, isconstantly in motion. Its charged and uncharged constituents maytravel together or independently, while irregularities in the distribu-tion of charge may take a different course again. The pertinent obser-vational data are extensive and, in some cases, conflicting; their in-terpretation is seldom direct. The theoretical factors affecting thatinterpretation are becoming clear, though in some cases their areasof relevance are still subject to divergent opinions.

An attempt is made here to bring into focus the theoretical factorsthemselves, to record their bearing on the observations wherever ithas been established, and to suggest paths of future progress wher-ever it has not. Details of observational results and theoretical de-velopments are largely suppressed, and with them the conflicts anddiscrepancies which, however real, would obscure the presentationof the principles involved.

I. INTRODI-JCTION

a /TOTION was ani essential feature of the iono-, sphere as first postulated, and even- today itprovides maniy of the challenginig aspects of

ionospheric study.'-b A wide variety of motions has nowbeen revealed, anid virtually all of these require examina-tioIn on a world-wide basis to inisure proper initerpreta-tion. The extenisive supply of IGY data, if anialyzed as

assiduously as it has b-een accumulated, may be expectedto resolve maniy of the problems which have so far beenenicounitered.The purpose of the presenit paper is to outlinie the

state of the art at the momenit: to describe, against a

backgrouind of past successes, the poinits oni which clari-ficationi is nmost urgelntly nee(le(d, anid to indicate in somecases the paths by which it may be achieved. No at-tempt will be miade to cover the extenisive literat-ure inits enitirety nior to provide a detailed collatioii ol thefindinigs it conitaiils; onlly the major features and(lbroadaspects of the subject will be trecated.A background of conceptual thougyht oni motionis in

the upper attmosphere untidoLubtedly exists ill us all,stemminig from our direct experience wxvith the lowerlevels in which we dxvell. However, it has beconme well-recogniized in the past two decadles that such ain ex-trapolationi is probably inivalidl wheni extenided iilto ion1o-spheric regionis. Electromagnietic for-ces there begini to

* Originaiil manuiiscript received by the IRE, November 10, 1958.This work was performied unilder project PCC No. D 48-95-11-01.

t Radio Phys. Lab., Defenice Res. Telecoimmun. Elst., DefeniceRes. Board, Ottawa, Canl.

1 S. K. Mitra, "The Upper Atmiiosphere," The Asiatic Society,Calcutta, Inidia, 2Ind ed.; 1952.

2 Internatl. UnlioIn of Geodesy and Geophys. Inifo. Bull., No. 6,pp. 299-336; Ju1ly, 1954.

3 "The Physics of the Ionosphere,' The Physical Society, London,Eng., pp. 88-211; 1955.

4K. WVeekes, ed., "Polar Atmuosphere Symposiumii; Part II, Ionio-spheric Section," Pergamon Press, Lonidon, En-g.; 1957.

b W. J. G. Beynon and G. M. Brown, eds., "The miieasuremenit ofionospheric drifts," Annn. IG Y, vol. 3, pt. 3, Pergamnion Press,London, Eing.; 1957.

dominate over the collisional forces which rule motioniin the denser atmosphere below, and eveni the immediatesources of energy inlput tenid to differ markedly. Indeed,with two importanit exceptionis, there are nio establishedcausal coninectionis betweeni motionls withini the ion1o-sphere and those benieath it. Cautioni has beeni advisfd6againist the acceptanice of this coIclusioni as fiulal, how-ever, and some studies suggest the possibility of closerliniks thani had previously beeni sLupposed.7

Of the two kniowni exceptionls, the first will receiveonlly brief attenitioni. It is the n-iotionl of rotationi aboutthe earth's axis--a motioni which is so commonipla-ce asto be frequenttly overlooked. Some remarks oni this sub-ject are in or(ler if onily for the salke of coniipleteiiess, andthey wxill be preselnted in Sectioni II. lt will be niotedthere that important problemiis in the field still remnaini;but, since these are 1not obviously of direct consequeniceto the balance of the paper, they xvill be treated onilysuperficially.

OnI the other hanid, a fairlyv thorough outliiie nmust begiveni of the seconid exception: the attmospheric tideswhich (laily sweep rouiid the world and cyclically disttirball levels. Not only, are they important in themselves, asglobal events, but evidenice is accUMulllatilng wvhichdemonistrates their direct inifluienice oni more localized ob-servationis of mlotioIn. If a comlposite picture of the latteris to be achieved, theni, it will probably reqluire a properuniderstanidinig of the whole tidal process. SuLch anlunlder-stanidinig nlow atlppears to be niear, and oni somI1e pointsto have beeni reached. The varied history Ln(l presenitStatUS of tidal theory wx ill be ouitlinied in Section Ill.

'I'he observaltionial detectioni of ti(lal motioni is by niomeaniis straightforwNard. But three types of observcationdo exist which, if initerpreted directly or- witlh the aiid ofaccepted theory, yield anI apparently consistent pictureof tides ul) to the 110-120 klm level. PFhat picture will bepresenited inl Sectioni IV. Other pertinienit observationis,anid still others whose pertinienice has beeni guessed butn-ot yet estaiblishecl, will be discussed in Sectioni V.There remailln two types of observed motioni whose

relationi to the attmospheric tides is remuote, and wvhose

6 S. Chapman, "Somie Causes of Wiinds in the loniosphere," Initer-natl. Union of Geodesy and Geophys. Info. Bull., No. 6, pp. 303-311;July, 1954.

7That a continiuouis variation of wiind occurs as the height in-creases has been well established, though the causes of the variationare yet unikniown. Comiipare the resuilts quoted by the followilg:

L. G. H. Huxley, "The measuremiient of ionospheric drifts,"Ann.IGY, vol. 3, pt. 3, Pergamon Press, Lonidoni, Eng.; 1957.

WV. XV. Kellog and G. F. Schilling, "A proposed model of the circu-lationi in the upper stratosphere," J. Mieteorol., vol. 8, pp. 222-230;Auigust, 1951.

R. J. Murgatroyd, "Winds and temperatures between 20 km and100 km-a review," Quart. J. Roy. MIeteorol. Soc., vol. 83, pp. 417-458; October, 1957.

176 February

1959Hines: Motions in the Ionosphere

existence is probably dependent on quite differentprocesses. They will be discussed in Sections VI'and VII.Finally, in Section VIII, the principal conclusions of thepresent paper will be summarized.

II. ATMOSPHERIc ROTATION

It seems to be generally agreed that the upper atmos-phere rotates with the earth,' but the reason for thisbelief is not particularly clear. The usual arguments,based on viscosity, would lead to quite a different con-clusion if extrapolated to heights where the atmosphereinteracts with the interplanietary (coronal) gas. Theatmosphere rotates with the earth (angular velocity Q)at the earth's surface (r=ro), but if it were preventedfrom rotating at some greater distance (r= r1), then itssteady-state velocity under a uniform viscous influencewould be

u = Q X r(r-3 - ri-3)(ro - rr-3)-l. ()

This differs markedly in form from the rigid-rotationresult

u =Q Xr (2)

even when ri-* co, though the difference in magnitude isnot very large if r-ro<<ri.

It may be objected that viscosity loses its formalmeaning at great heights, but this would merely beg thequestion. Moreover, the high conductivity of the outeratmosphere introduces an initense inductive viscositywhich cannot be argued away. The outer atmospherewould be held almost rigidly by the interplanetary gas,if the geomagnetic field should penetrate the latter, andthe effective value of r1 might be reduced considerablyas a result. Appreciable discrepancies between (1) an-d(2) might then be expected even at heights of 100 km.

In the absence of any recogniized indicationi of suchdiscrepanicies, which would appear as a westward windincreasing with height in the ioniosphere, an alterinativedescriptioni must be considered more acceptable. Ac-cording to it,8,9 the geomagiletic field is conifined withinthe earth's outer atmosphere by the effects of strongcurrents induced in a tranisitionial layer. The outer at-mosphere is, theni, dynamically separated from the ex-ternal gas and free to partalke of rigid rotation with theearth and lower atmosphere. The tranisitional layer,which probablv lies within ten earth radii, is expected tobe unstable, and a variety of disturbances may bethought to originate at it. Theoretical and experimentalinvestigationis of the regions conicerned should be pur-sued.One question arises immediately if the foregoing pic-

ture is adopted: why should it, rather than the precedinig

*1 J. WV. Dungey, "Electrodynamics of the Outer Atmosphere,"Ionospheric Res. Lab., Pennsylvania State Univ., State College, Pa.,Sci. Rep., no. 69; September, 1954.

J. W. Dungey, "Electrodynamics of the outer atmosphere", in

'The Physics of the Ionosphere," The Physical Society, London, Eng.,pp. 229-236; 1955.

one, be found in practice? The answer undoubtedly liesin the relative values of some effective cohesive force atthe outer bounidary and at the lower levels where shear-inig would otherwise occur, but the relevant criteria areniot easy to imagine. When established, they shouldcontain new in-formation about the outer reaches of theatmosphere.

It may eveni be that the two descriptions will standtogether. The seconid one, though by nio meanis fullydeveloped, implies the development of "horns" cle-sceindinig from the outer boundary alonig geomagnieticfield lines to auroral regions,8 analogous to the "hornis"which have been pictured in one major sttudy of geo-magnetic storm effects.'0 If the anialogy is complete,then field lines originating at latitudes above the auroralbelt are in fact connected through to the interplanietarygas, anld the upper atmosphere which they traverse mayonce again be held in place by inductive viscosity. Eq.(1) would apply, and the effective ri might be only frac-tionially greater than ro. Shearinig across the auroral beltwould then be expected, as would further effects relatedto the asymmetry of the auroral zonie with respect to theaxis of rotation.

Finlallv, it should be noted that anly motion in thepresenice of the geomagnetic field, including rigid rota-tionI, implies the existence of inductive electromotiveforces (emf's). These emf's can be offset by an appropri-ate polarization field, but an extremely finie balanicingmust be established if residual effects are to be niegligi-ble.1" It is only when such effects can be neglected thatone is justified in ignoring the rotationi uponi adoptinig arotatinig coordinate system in subsequenit studies. It isdifficult to see how the niecessary space-charge distribu-tions canl be mainitained, in view of the large diurnialvariationis which occur in the ionization. densities, with-out the flow of stronig currents and resultailt winids. Ifan asy\mmetric nion-rotating polar region exists the polar-izationl fields established outside of it could have furtherimportanit effects.

XVhile these questionis provide initerestinig miiaterialfor speculation, in their presenit state they canniiot bebrought to bear directlv on- the major problems of thesubsequenit sectionis. Therefore they will be abanidoniedat this poinlt, to be recalled onlY in passing referenice atlater stages.

III. TIDAL MOTIONS

The history of atmospheric tidal studies extenids overa period of two centuries anid more.'2 Early interestcentered on the pressure changes wlhich might be ex-pected, anid on the regular oscillationis which were infact detected in barometric recordings. But modern in-

10 S. Chapman and V. C. A. Ferraro, "A new theory of magneticstorms; part I, the initial phase (continued)," Terrest. Mllag. Atmos.Elec., vol. 36, pp. 171-186; September, 1931.

11 H. Alfven, "Cosmical Electrodynamics," Clarendon Press, Ox-ford, Eng.; 1950.

12 M. V. Wilkes, "Oscillations of the Earth's Atmosphere," Cam-bridge University Press, Cambridge, Eng.; 1949.

-1771959


terest in the tidal phenomenon received its greatest im-petus from a suggestion put forward less than a centuryago,'3 to the effect that certain magnetic variationswhich had been discovered were an indirect manifesta-tion of the tidal motions. The connectinig link was pos-tulated to be an electrically conducting region of theupper atmosphere-a region now recognized as the ioiio-sphere-in which electrical currents would be iinducedby a dynamo process involving the atmospheric tidesand the main geomagnetic field.

This suggestioni was developed into a fairly satisfac-tory mathematical theory some fifty years ago,'4 but itwas subsequentlv confronted with a quantitative dis-crepancy when direct measurements of ionospheric pa-rameters became available. The last few decades haveseen important advances in the study of tidal oscilla-tions and in the understanding of ionospheric behavior,which in combination appear to remove the major diffi-culties of the past. A comprehensive theory nlow seemsto be close at hand.

It will be useful to review briefly the two principaldevelopments which lead to this optimistic view, for theelements they contain are of direct conicern to any fur-ther discussion of motions in the ioniosphere.To begin, it should be remarked that the solar semi-

diurnal component of pressure variation greatly exceedsthe lunar semidiurnal component, although the tide-producing gravitational force of the moon is nearlytwice that of the sun. This finding could be explainedqualitatively if the solar variation were dominated bythermal rather than gravitational effects, but a furtherproblem would then arise: the solar semidiurnal tide(so-called) exceeds the solar diurrnal tide even thoughthe diurnal component of the temperature variation ispredominant.'5 A possible solution to this problem wasproposed some seventy-five years ago'6 and was basedon the postulate that the atmosphere has a natural modeof oscillation whose period is close to 12 hours. A reso-nant response to any exciting agency having that periodcould then be anticipated. Following an initial reverse,"7and a subsequent reorientation of the theoretical ap-proach,1"8,9 a satisfactory "resonance theorv" was finallv

13 B. Stewart, "Terrestrial Magnetism," Encyclopaedia Britan-nica, Chicago, Ill., 9th ed.; 1882.

14 A. Schuster, "The diurnal variation of terrestrial magnetism,'Phil. Trans. Roy. Soc. (London) A, vol. 208, pp. 163-204; April,1908.

15 It is interesting to note that the diurnal thermal input, actingin conjunction with the semidiurnal motion, may actually controlthe period of the earth's rotation. See the following:

E. R. Holmberg, "A suggested explanation of the present valueof the velocity of rotation of the earth," Monthly Notices Roy.Astron. Soc. Geophys. Suppl., vol. 6, pp. 325-330; September, 1952.

16 W. Thomson (Lord Kelvin), "On the thermodynamic accelera-tion of the earth's rotation," Proc. Roy. Soc. Edinburgh, vol. 11, pp.396-405; January, 1882.

17 G. I. Taylor, "Waves and tides in the atmosphere," Proc. Roy.Soc. (London) A, vol. 126, pp. 169-183; December, 1929 (prinited1930).-

18 G. I. Taylor, "The oscillation of the atmosphere," Proc. Roy.Soc. (London) A, vol. 156, pp. 318-326; August, 1937.

19 C. L. Pekeris, "Atmospheric oscillations," Proc. Roy. Soc.(London) A, vol. 158, pp. 650-671; February, 1937.

developed little more than a decade ago.2"The physical picture it provides is this. Tidal energv,

is supplied to the atmosphere in greatest quantity at thelowest, densest regionis. The diurnial componienit propa-gates upwards as a modified pressure wave, unitil itreaches regions of relatively high kinematic, eddy, orinductive viscosity. There, at heights of 100-300 km,the tidal motion is damped to iniappreciable magnitude.The semidiurnial componients (lunar aid solar) propa-gate similarly, but, with their shorter periods, they arestrongly reflected at lower heights (55-100 km) by aregion of diminishinig, low temperature. The reflecteddisturbances return to the ground and suffer further re-flection there. They are, in fact, effectively trapped andmultiply reflected within the lowest 60 or 70 km of theatmosphere. Standing waves are established whose am-plitudes depend markedly on the relative phases at suc-cessive reflections, and a resonant response canl beachieved under suitable circumstances. It appears thatsuch a response does in fact obtain in the case of thesolar semidiurnal variatioin, and that the latter is ac-cordingly magnified greatly in comparisoni with theother solar components and with the nonresonanit lunarsemidiurnal component.

This conclusion, which is based on a detailed anialy-sis,20 establishes with near certainty the validity of theresonance hypothesis. It also reopens the question of theorigin of the solar semidiurnal variation, since the esti-mated amplification factor is comparable to that whichthe observations would demand of a purely gravita-tional theory. A final decision, if one is inideed possible,will most likely be reached by avoiding the complica-tioIns of resonianice entirely-by comparing the gravita-tional and the thermal semidiurnal tides which wouldhave been expected in the absence of resonance, the esti-mates being determinied from the lunar gravitationaltides on the one hanid and from the solar 24-hour (anidperhaps 8-hour) tides onl the other. The coniversioni co-efficienits required for this process would iiot be subjectto the large sources of error which are implicit in theestimatioin of a resonant response. For the present, how-ever, it will be sufficient to note that indirect argumentssuggest stroingly that the part played by thermal effectsis quite significant, whether dominaiit or 1not.2'The development of the resonanice theory, to its pres-

enit state produced ani unexpected seconidary resultwhich is of major conicern to the presenit review. It be-came apparent, contrary to previous belief, that theamplitude of the tidal oscillations would in geineral in-crease with height, at such a rate as to maintaini a niearly,constant flow of eniergy in spite of the decreasinig den-sity.19 20 Amplifications of 50 or 100 (relative to ground)would be attained eveni before the reflectinig regions

20 K. Weekes and M. V. Wilkes, "Atmospheric oscillations and theresonance theory," Proc. Roy. Soc. (London) A, vol. 192, pp. 80-99;December, 1947.

21 S. Chapmani, "The semidiurnal oscillation of the atmosphere,"Quart. J. Roy. Meteorol. Soc., vol. 50, pp. 165-193; july, 1924.

178 l+February

Hines: Motions in the Ionosphere

were reached. The upwards flow of energy could not re-main constant in those regions of course, but it wouldnot diminish totally nor even abruptly. Some leakagewould continue on through, with gradually diminishingstrength, and would emerge finally into the freely propa-gating region above. In contrast to first expectations,the upwards flow of tidal energy there would require adownwards progression of phase,20 22 so the phase angle23decreases with increasing heights. The amplitude of os-cillation would resume its upwards increase, however,and so continue until dissipative effects set in. Theheight variation of amplitude and phase angle for atypical assumed temperature profile would, in the ab-sence of dissipation, have the form depicted in Fig. 1.

It seems necessary to suppose that some type ofdamping does occur in the region below 120 km, at leastin the case of the solar semidiurnal variations, for other-wise the "perturbation" pressure variations would tendtowards 100 per cent. The actual mechanism of dissipa-tion is important, for it may operate selectively againstcertain modes and so alter appreciably the relative mag-nitudes of the various tidal components.But before absorption is discussed any further, it will

be useful to review the second major development whichwas noted above. This is concerned with the absoluteamplitude of the geomagnetic variations, and with thedifficulty of reconciling that amplitude with estimatedtidal velocities and ionospheric conductivities.The magnetic effects were simply too strong. The dis-

crepancy was extreme before the amplification of ve-locity with height was appreciated, and even with thisamplification taken into account it appeared to exceedthe limits of error.24The difficulty can be discussed as an effect of the main

geomagnetic field, which renders the conductivity of theionosphere anisotropic above heights of 80 km or so.'The "longitudinal conductivity" (aO), measured alongthe field, is unchanged by the presence of the field, andit would have been sufficient to provide the requiredcurrents in the lower E region. But much of the currentflow was necessarily oblique and nearly perpendicularto the geomagnetic field, so it appeared that the trans-verse components-the "Pederson conductivity" (u,)and the "Hall conductivity" (0a2)-would be of more di-rect concern. Neither was sufficient.Then it became apparent, in quite a different con-

22 M. L. White, "Gravitational and thermal oscillations in theearth's upper atmosphere," J. Geophys. Res., vol. 61, pp. 489-499;September, 1956.

23 "Phase angle" is to be considered here as the angle 4) which ap-pears in a variation of the form A cos(wt -4)). It is therefore distinctfrom the "phase' proper, wt -o, and its variation with height is op-posite in sense to the variation of the phase. It has the advantage,however, that it gives directly the "phase time," 4)1w, when a maxi-mum value of A cos (wt-o) is attained. The term "phase" has beenused variously in the literature for wt-4, 4, -4, )++7r/2, -)-7r/2,and 01/w, thereby making comparisons difficult. The adoption ofsome clear convention, perhaps that indicated here, would be of greatvalue in further studies.

24 W. G. Baker and D. F. Martyn, "Electric currents in the iono-sphere; part I, the conductivity," Phil. Trans. Roy. Soc. (London) A,vol. 246, pp. 281-294; December, 1953.

Fig. 1-Height variation of amplitude (solid line) and phase angle23(broken lines) in semidiurnal tides.'2'20 The amplitude scaleshown applies to the solar component; the variation for the lunarcomponent is similar, but the absolute value is diminished by afactor of 10 or more. The dotted curve represents the variationwhich would have occurred, had the energy flow been exactlyconstant in height for the assumed atmosphere. Zero phase anglecorresponds to the time of (solar or lunar) transit; maximumpressure is taken to occur at 1000 and 2200 solar hours, 0100 and1300 lunar hours, local time.29

text,25 that intense polarization fields can be establishedby the currents in suitable circumstances, and that thesefields can play a more important part than the originalinduction emf's in determining the ultimate pattern andstrength of current flow. They can, in fact, lead to a sub-stantial increase in the "effective conductivity" whichrelates the composite current to the primary emf, andvalues as high as the "Cowling conductivity" (023 -l+O22/U1) could sometimes be produced. Such values,even if only partially achieved, would be sufficient forthe dyniamo theory. Their relevance to that theory waspostulated a decade ago,26 and established in essence bya number of subsequent investigations.24'27-29

These and related investigations30 differ appreciablyfrom one another in the types of complication they takeinto account, and they reveal a variety of interestingfeatures worthy of further study, but they suffer from acommon defect which also merits examiniation. They alltreat the ionosphere as a thin spherical shell, or perhapsas a series of such shells, and not as the extended plasmawhich it is. The approximations involved are not alwaysconvincing, and those concerning vertical currents andvertical gradients are particularly suspect. It is unlikely

25 T. G. Cowling, "The electrical conductivity of an ionised gas inthe presence of a magnetic field," Monthly Notices Roy. Astron. Soc.,vol. 93, pp. 90-98; November, 1932 (printed 1933).

26 D. F. Martyn, "Electrical conductivity of the ionosphericD-region," Nature, vol. 162, pp. 142-143; July, 1948.

27 M. Hirono, "A theory of diurnal magnetic variations in equa-torial regions and conductivity of the ionosphere E region," J. Geo-magnet. Geoelec., vol. 4, pp. 7-21; April, 1952.

28 W. G. Baker, "Electric currents in the ionosphere; part II, theatmospheric dynamo," Phil. Trans. Roy. Soc. (London) A, vol. 246,pp. 295-305; December, 1953.

29 J. A. Fejer, "Semidiurnal currents and electron drifts in theionosphere," J. Atmos. Terrest. Phys., vol. 4, pp. 184-203; December,1953.

30 An extensive series of papers by Japanese workers should benoted in this connection. Of most immediate interest here is the paperby:

H. Maeda, "Horizontal wind systems in the ionospheric E regiondeduced from the dynamo theory of the geomagnetic Sq variation;part III," J. Geomagnet. Geoelec., vol. 9, no. 2, pp. 86-93; 1957.

1959 179


Oj B. TOTAL CURRENT- DENSITYI lamp/km

v _~

40

t

t /

t

-6 -4 -2 0 2 4 6

NORTHPOLE

EQUATOR -

NORTH _

POLE

EQUATOR .....

C. POLARIZATION

E FIELD ,i 50v/m

-

;80°

60°

Ir40cie0

- Al X . ._

X' #, i A

IC i.

I . . . . . .

T T -T -F I ------T - -J-6 -4 -2 0 2 4 6

D. TOTAL

E FIELD50v/m

. 80".,

a. 600

4

^

0

-6I--- -4 - 2-T 0F- 6----- --6 -4 -2 O 2 4 6

Fig. 2 Patterns of winds, currents, and horizontal electric fields in the dynamo theory of semidiurnal geomiiagnetic variations.29 Thewinds shown, and the other vectors deduced, are based on the winds derived for the solar component at grounld level. 'rhe horizon-tal scale measures hoLurs after the times of maximum pressure-1000 and 2200 solar hours, 0100 and 1300 Iltunar houirs, local time.This scale cain be converted directly into longitude difference fromn the meridian of maxiilmum pressure, uLsing 1 hour = 15°. In this sense,only one quadrant of the globe is depicted; the others may be deduced from symmetry.

that an improved attack would alter the dynamo theoryexcept in its details, but the poinit has been reachedwhere the details are becoming important. A more thor-ough study of vertical variations is now called for.The horizontal polarization fields are of conisiderable

concern in other studies, as will be seen in a subsequentsectioni. They are produced primarily by currents in theE region, but they are carried to other heights by virtueof space-charge accumulationis. The high mobility ofcharge along the geomagnietic field linies tenids to makethe latter equipotential, anid the polarization fields are

effectively mapped by transport along these equipoteni-tials.24 The polarization and other vector fields deducedin one development of the dynamo theory are illustratedin Fig. 2.

If the E region does inideed dominiate the polarizationfields, then it also conitrols the movement of ionizationat greater heights. This point will be taken up againi, butit should be noted here that the premise is based in parton the assumption that the amplitude of the atmos-pheric motion decreases with increasing height throughthe E region. Such a decrease must be attributed to dis-sipative effects as already noted, anid inductive viscosityseems to be the most likely source. This suppositioni ap-

pears the more probable in view of the increased effec-tive conductivities which have now been founid andwhich lead in turn to increased damping. The problemof dissipation is ani exceedingly difficult one to treat indetail, but it must be taken into account from the startif a fully comprehensive theory is to be developed.

IV. DIRECT TIDAL OBSERVATIONS

It is appropriate at this poinlt to brinig experimentalevidence to bear oni the subjects which have beeni dis-cussed. Care must be exercised in selecting the types ofdata to be employed, however, becatise of major compli-cations which arise at ionospheric heights. These com-

plicationis will be discussed more fully in the next sec-

tioln, while an arbitrary choice of "direct" tidal observa-tions will be made for present purposes.

Meteor studies appear to be particularly pertinenit.Atmospheric winids blow upoIn anid distort the originallystraight meteor trails, and the resultanit motions can bedetected both bv optical3" and by radio5'32 meains. Thesemotionis must, over most of the relevant height ranige,

reproduce accuratelyr the atmospheric movemenit whichproduces them.Although prevailing winds, when deduced, are geiier-

ally directed towards the east, there is little agreement

as yet on their magnittLides or seasonial variationis, or on

the diurnal components that are superimposed.5 A coiI-sistent picture of these motions has yet to be developed.The solar semidiurnal component has been more clearlyestablished, however, anid has provided sufficienit clatato warrant a thorough examination. Even in summary,

31 W. Liller and F. L. Whipple, "High- Altitude Winds by Meteor-Train Photography," Internatl. Union of Geodesy and Geophys.Info. Bull., No. 6, pp. 329-330; Julv, 1954.

3 J. S. Greenhow and E. L. Neufeld, "Diurnal and seasonal windvariations in the upper atmosphere," Phil. llIag., vol. 46, pp. 549-562;May, 1955.

60°-

40"-

A. GROUND AIRVELOCITY

I 50cm/sec

--s S 1 4-- -',-

-6 -4 -2 0 2 4 6

180 FeUbru2ary

'.1 _-, - - -

I',,

I "I

- -- I 0

. - . 11 -0

11 4 11 0

. 11 I A *

20°r

O- a. I"0

ti

A

k

IIA

o

0

t

i

I

I

1.

IiA

i . -

0 , -


tLe pattern it reveals32 is most interesting.The velocity vector rotates clockwise in the northern

hemisphere and counter-clockwise in the southern, whenviewed from above, as would be expected. At the 85-kmlevel it is directed eastwards at about 0830 and 2030hours, with an amplitude (wind) of the order 10-30rn/sec. The times become earlier (by 10-25 min/km)and the amplitude greater (at an average rate of2mni/sec/km) as the height increases to 100 km.These results are completely consistent with the reso-

nance theory as set out above. They indicate a velocityat 85 km nearly antiphase to that at the ground,33 andamplified by a factor of 50 relative to it, much as wouldbe predicted from Fig. 1. Even the upwards decrease ofphase angle and increase of amplitude are essentiallythe same as those derived theoretically.Some discrepancies should be noted. Seasonal changes

of phase and amplitude occur, as do random variationsin the ratio of north-south to east-west winds, and bothare unaccounted for in tidal theory as presently de-veloped. One suggested explanation32 for the systematiceffects involves the incorporation of a second compo-nent of the solar semidiurnal tide which has been ig-nored so far in this paper. It has the nature of a standingwave oscillating between the poles and equator, withmaximum amplitude at the higher latitudes. Its rele-vance should be determined by the correlation of meas-urements at separate longitudes, since its phase is de-pendent on universal rather than local time. (This phasedependence has always been difficult to explain, becausesimple considerations of symmetry would argue againstit. A dynamically distinct and asymmetric polar regionsuch as that proposed in Section II might provide anessential clue to the elucidation of this peculiar tidalcomponent.)

Variations in the temperature profile of the atmos-phere, and a possible thermal input at ionosphericheights, have also been cited32 in explanation of theanomalous variations revealed by meteors. The formercould alter the phase angle and amplitude at a givendatum level, while the latter could alter the local rate ofchange of these quantities with height. The question ofhigh altitude thermal effects has been raised independ-ently in other contexts,34'36 and the implications of sucheffects in tidal theory have received some consideration22but probably not enough.The decrease of phase angle is itself of considerable

importance in the development of a complete picture,

3 In the solar semidiurnal tide at ground level, the wind velocitydeduced from pressure variations has an amplitude of about 0.4 m/secat middle latitudes and is directed eastwards (i.e., towards the east)at about 0400 and 1600 hours. See Fejer, loc. cit., or Fig. 2(a).

34 L. R. Rakipova, "Possible effect of dust on vertical air move-ments and isothermy in the stratosphere," Izv. Akad. Nauk SSR,Geograph. and Geophys. Ser., vol. 11, pp. 15-19; 1947. Transla-tion by E. R. Hope, No. T 199 R, Directorate of Sci. Info. Serv.,Defence Res. Board of Can., Ottawa; August, 1956.

"-H. E. LaGow, R. Horowitz, and J. Ainsworth, "Rocket meas-urements of the arctic upper atmosphere," IGY Rocket Rep. Ser.,Natl. Acad. of Sci., Washington, D. C., no. 1; July, 1958.

for it reappears at greater heights in a secoind type ofobservation and provides an urgently required link be-tween the two.36'37 The observations referred to, E-re-gion "drifts," are nlot so certainly understood as are themeteor motions, and will not be examined further untilthe next section. But, if they do represent true atmos-pheric winds, then they become consistent with the tidalmotions revealed by meteors only if the phase shift isincluded. This shift is sufficient to bring the atmosphericwind motion back into phase with the motion at theground, somewhere in the 95-115 km region.That it should be in phase, or nearly so, is a deduction

from yet another type of observation. The geomagneticvariations, if explained by the dynamo theory, imply amaximum eastwards air velocity at about 0400 and1600 hours29 just as at the ground. (One analysis30 indi-cates that the maximum occurs earlier than this in sum-mer and later in winter, a variation which might well beexplained by changes in local heating effects.)

Both the dynamo theory and the E-region drifts indi-cate winds of the order 20-30 m/sec for the solar semi-diurnal component.29 38 40 It is difficult to reconcilethese values with those previously quoted for meteors,when the upwards increase of amplitude is taken intoaccount, unless a rapid damping sets in above 100 km.The damping should be revealed, or the difficulty other-wise resolved, by further close coordination of meteorand drift measurements.The conclusions which have been drawn here can be

summarized by a "harmonic dial" display as in Fig. 3,which is intended to be representative only. Similar in-formation deduced from combined studies at individualstations would be most valuable in consolidating or cor-recting the composite picture it provides. Accurateheight determinations for all detected motions are ob-viously vital to success.The lunar semi-diurnal geomagnetic variations appear

to require a maximum eastwards wind at about 0130and 1330 lunar hours, in phase oppositioni to the motionat ground level.29 They differ from the solar variationsin this respect, and have been attributed to a lower levelin consequence.4' It is possible that diurnial variations ofion density would play some part in causing a distinc-tion of this sort, but the mechanism is not at all clearnor does the distinction itself appear to be required.Tidal theory indicates that the reversed phase angle of

36 I. L. Jones, "The height-variation of horizontal drift velocitiesin the E-region," in "Polar Atmosphere Symposium; Part II,Ionospheric Section," Pergamon Press, London, Eng., pp. 20-22;1957.

37 I. L. Jones, "The height variation of drift in the E region,"J. Atmos. Terrest. Phys., vol. 12, no. 1, pp. 68-76; 1958.

38 G. J. Phillips, "Measurement of winds in the ionosphere,"J. Atmos. Terrest. Phys., vol. 2, no. 3, pp. 141-154; 1952.

39 J. H. Chapman, "A study of winds in the ionosphere by radiomethods," Can. J. Phys., vol. 31, pp. 120-131; January, 1953.

40 B. H. Briggs and M. Spencer, "Horizontal movements in theionosphere," Rep. Prog. Phys., vol. 17, pp. 245-280; 1954.

41 D. F. Martyn, chairman, "Tidal Phenomena in the Ionosphere,"IInternatl. Sci. Radio Union Spec. Rep. No. 2, General Secretariat ofURSI, Brussels, Belgium; publication approved 1950.

Xf!R 7 . 181

PROCEEDINGS OF TIIE IRE

Fig. 3-Tentative represenitationi of height variatioins iin thea1mpli-tLide and phase tinme23 of the observed eastwardswit d compotnenltin the solar semidiutrnial tide.

the lunlar semidiuriial componenit should persist up toheights of 100 or 105 km before any signiificatnt decreasesets in (Fig. 1), so both the solar anid lunar variationismight well originiate in currenit sheets in the 95-110 kmrange. This includes the levels at which maximum effec-tive coniductivities are to be expected,"4 anid at whichintense currenits have already beeni found.42A rapid decrease of phase angle with height is pre-

dicted above 105 km (Fig. 1). This could provide coII-sistency with E-regioni drift measurements of the luniarsemidiurnial tide, which generally require eastwardswinds at about 0600 anid 1800 lunar hours,","' atnd itsuggests anl explaniationi for wide variations which havebeeni observed in such stuidies.4''

V. INDIR(ECT TIDnAL OBSERVAxTIONS

As has beeni indicated already, care must be exercisedin the interpretation of apparenit motioins at ioniosphericheights. It should be nioted as a basic point that virtuallyall indicationis of motioni depend on the existence of in-homogeneities in the medium observed, and that themotioni revealed is really some composite of the motionand deformationi of the inhomogeneities. It may be thatthe latter move with the atmosphere, as in the case oftnoctilucent and self-luminous clouds, but even these are

complicated by growth and decay at the edges anid some-times by wave-like irregularities passing through. Themotioni of more complex structures, such as auroralforms, mav be dominated by precipitation or excitationfrom regionis entirely external to those observed.

e2S. F. Singer, E. Maple, and WV. A. Bowen, "Evidence for iono-spheric currents from rocket experiments near the geomagneticequator," J. Geophys. Res., vol. 56, pp. 265-281; juine, 1951.

T'he difficulty is compouLiided iii radio studies, whereelectronis alonie are important. Eveni when the movementof irregularities in the electroni distribution cani be in-terpreted in terms of the actualmotioil of electronis, thelatter motion may be quite differenit from that of thesurrounidinig atmosphere. Electric fields are importanit,anid at least three types ma- be distiniguishel. T'he firsttAwo are the inlducedl anid polarizationl fields establishedoni a very large scale by tidal actioni or by some othermajor process. These are beconing uiniderstood, as hasbeeii seeni. The third conisists of the local polarizationifields established at the irregularities. Their scale is thatof the irregularities, while their fornm depenids oni theform of the irregularities and oni the patterni of motioni ofthe charged particles. A feedback mechaniism exists: thefield and the motion must be determinied simultanieouslyin anv theoretical approach. This reniders the interpreta-tioin exceediniglv difficult, an-id only onie rigorous anialysishas succeeded as yet.43 Its importatnce in the case ittreats can scarcelyt be overemphasized, and its value asa more genieral guide is conisiderable.A gross example of the various problems is provided

by tidal oscillationis in the heights of ioniospheric layers.The solar variationis are badlv masked by variations inlthe rate of produ(tioll of ionizationi, and indirect meth-ods must be emiployed to separate out the small coni-tributioni of the tidal motionl itself.44 This difficulty isniot so severe in the case of the lunar variatioiis, particui-larlk, for the F laxvers, but ani extenisive reductioni of datamust be completed even for them.4 Various character-istics have beeni deduced for the differenit layers, anid allhave fountid at least partial explanlationl onl a theoreticalbasis.41

TIhe direct vertical oscillationi of the atmosplhereplay.s only a minior part in the movemenit of the layers.The vertical velocities of the charged particles conisider-abl\ exceed those of the atmosphere itself, in the E re-gionI anid above, beiiig dlominiated by the iniduced and(ipolarizationi fields established bv the tide. Although ver-tical curren-its mar, be suippressed, a vertical imiotioni of

43 P. C. Clemmow, M. A. Johnson, anid K. \Veekes, "A note onthe motion of a cylitndrical irregularity ini an ioniized n3editini," in"The Physics of the Ionosphere," The Physical Society, LonIdIoI,Eng., pp. 136-139; 1955. rhe expressions for V, and V, in (3) of thispaper should be interchaniged; at preseint they conflict with the illus-trations, which are correctly marked (Clemmow, private communlica-tion).

44D. F. Martyn, "Atmospheric tides in the ionlosphere; part IV;studies of the solar tide, and the location of the regions producing thediurnal magnetic variations," Proc. Rov. Soc. (London) A, vol. 194,pp. 445-463; November, 1948.

45 The initial difficulty is one of extracting a cyclic variation fromdata which contain a wide scatter. This has been done successfullyby a number of workers now. However, there remains the difficultyof establishing that a variation having the lunar semidiurnal periodis in fact due to the influence of the moon. Seasonal or irregular varia-tions in the dominant solar semidiurnal component could easilyproduce Fourier components at the lunar semidiurnal period. Thissame objection has been raised (Briggs and Spence, loc. cit.) in con-nection with drift measurements. The most satisfactory resolutiotnof the difficulty would be provided by a complete Fourier analysisover periods from 11.5 to 13 solar hours, but this is seldom practicable.Certain apparent discrepancies between results for so-called lunartides muist remain suspect, however, until some stuch an analysis isapplied.

182 Fvebruary


the whole charge complex does persist, with a magnitudeapproximating that appropriate to the heavy ions alone.If recombination occurs only slowly, this motion carriesthe layer vertically with it; if recombination is rapid, thelayer is merely distorted in the direction of motion. Inthe latter case the displacement of the layer is in phasewith the vertical velocity of the charge, while in theformer it is in phase quadrature. The motion of the layermay bring it into regions of the atmosphere where re-combination times are sufficiently altered to producedistortion, and height variations of the motion mayfurther complicate the net effect.While various combinations of these details can be

adduced to explain the observed tidal changes in thedifferent layers,4" a fully deductive theory has yet to bedeveloped. Such a theory will probably emerge as ourunderstanding of the horizontal motions and polariza-tion fields at the lower levels improves.The study of the horizontal motions will likely pro-

ceed along lines such as those followed in the previoussection. Included there were results from E-region driftmeasurements.38-40 These are studied as systematic mo-tions in a changing diffraction pattern. The pattern isdetected by the diversity reception of a fixed-frequencyionospheric sounder, and is imposed on the radio wavesby irregularities in the E-region distribution of ioniza-tion.The extraction of a meaningful systematic motion is

sometimes difficult.46-48 Its interpretation as a drift ofE-region irregularities is fairly direct, but the interpreta-tion of that drift is once again difficult and complicatedby the effects discussed above. The assumption adoptedin the preceding section, that the drifts reveal directlythe atmospheric motion, has no sure theoretical ground-ing at the moment. It has the purely empirical supportthat it seems to provide agreement with both the meteorand the dynamo studies.The drifts reveal a further interesting feature which

has yet to be explained in any detail. Their speeds areindependent of magnetic activity for a K index less than5, but increase with the K index at higher values.39 Itseems reasonable to suppose that the atmospheric mo-tion does indeed dominate the drifts in normal circum-stances, and that increased polarization fields becomemore important and finally control as the magnetic ac-tivity rises.

This supposition is consistent with the behavior ofF-region drifts, which are detected in the same way. Theatmospheric motion is believed to be less than in theE region, whereas the tidal polarization fields are of thesame order. The latter may be expected therefore to

46 J. A. Ratcliffe, "The analysis of fading records from spaced re-ceivers," J. Atmos. Terrest. Phys., vol. 5, pp. 173-181; July, 1954.

47 G. J. Phillips and M. Spencer, "The effects of anisometric am-plitude patternis in the measurement of ionospheric drifts," Proc.Phys. Soc. (London) B, vol. 68, pp. 481-492; August, 1955.

48 I. L. Jones, "Theoretical views on drift measurements," in"Polar Atmosphere Symposium; Part II, Ionospheric Section,"Pergamon Press, London, Eng., pp. 3-11; 1957.

come into prominence at lower values of the K index,and the F-region drift speed does in fact increase withmagnetic activity even when the activity is low.39The same behavior is found for the ionization irregu-

larities which lead to radio star scintillations.49 Theseappear to be in the F region, and to be extended ascolumns along lines of the geomagnetic field.50 Theirmotion appears to be essentially east-west, at least onthe average, even after spurious effects due to theirelongated shape have been taken into account.". Thereason for this is by no means obvious at the presentstage of development, but the geometry of these irregu-larities is essentially the same as that which has beentreated successfully by theory, and some elucidation ofthe motion may therefore be near.The irregularities generally move westwards in the

evening and eastwards in the morning hours.48 This is aproperty they share with auroral structures,52-54 and thetwo types of inhomogeneity may simply represent dif-ferent degrees of the same phenomenon. The suggestion54is certainly quite reasonable, on the basis of what is nowknown, that the high speeds frequentlv attributed toauroral forms represent the upper extreme in an in-herently continuous and magnetically-dominated spec-trum.These last few topics have been included under the

heading of "indirect tidal effects" purely on conjecture.It seems probable that the higher velocities are indeedproduced by electric fields, and polarization fields es-tablished by diurnal or semidiurnal tides provide alikely source. Certainly the reversal of direction nearmidnight, if it is an ionospheric effect, demands some-thing on the scale of the tides as its causal agency. Theconfirmation of this conjecture or equally its invalida-tionl would mark a further major step in the study ofioniospheric motionls.

VI. TURBULENCE

Nothing has yet been said as to the nature of the ir-regularities detected in drift and scintillation measure-ments. Had their motion been attributed to wave propa-gation-and there are some arguments in favor of thisinterpretation-the irregularities would have been ex-plained directly as the perturbation involved. But if, ashere, the motions are attributed to (tidal) forces ex-tending over very great spacial scales, then some smal-

49 A. Maxwell, "Investigation of F region drift movements byobservation of radio star fading," in "The Physics of the Iono-sphere," The Physical Society, London, Eng., pp. 166-171; 1955.

50 M. Spencer, "The shape of irregularities in the upper iono-sphere," Proc. Phys. Soc. (London) B, vol. 68, pp. 493-503; August;1955.

51 G. C. Reid, "The variation with sidereal time of radio starscintillation rates," Can. J. Phys., vol. 35, pp. 1004-1016; September,1957.

52 A. B. Meinel and D. H. Schulte, "A note on auroral motions,"Astrophys. J., vol. 117, pp. 454-455; May, 1953.

5' K. Bullough and T. R. Kaiser, "Radio reflections from aurorae-II," J. Atmos. Terrest. Phys., vol. 6, pp. 198-214; April, 1955.

54 B. Nichols, "Drift Motions of Auroral Ionization," Geophys.Inst., Univ. of Alaska, College, Alaska, Sci. Rep. No. 1, ContractAF 19(604)-1859; July, 1957.

1959 183


ler-scale agency must be assumed. Turbulence has re-ceived the greatest attention.55-57

Turbulence, too, has been cited often as the cause ofrandom motions in both drift and meteor studies. Thelatter undoubtedly reveal the existence of a strong verti-cal shearing of horizontal velocities,"5 of an amountwhich has beeni considered sufficient to produce turbu-lence at the heights concerned.56 On this basis a theoryof turbulenit diffusion anid deformationi of meteor trailshas beeni developed, and indeed extrapolated upwardsto heights appropriate to E anid F-region drifts andradio star scintillations.56 The details and application58of the development have been criticized59 and reaf-firmed,60 anid the presenit status of the subject is verymuch a matter of opinion.

In any event, a study of turbulenice at ionosphericheights now appears to be well launched, and early im-provements in the theory may be anticipated. One modi-fication is urgently required-the incorporationi of aniso-tropies. The necessity for this at lower heights is bornieout by the meteor observationis themselves, for meteorsreveal no vertical motions comparable in magnitude tothe horizontal speeds anid certainly no vertically-circu-latinig isotropic eddies of observable scale size (>1 km).Indeed, the wind shears on which predictions of turbu-lence have been based could be explained entirely on thebasis of a ranidom spectrum of atmospheric waves,59which, on the pertinent scales, can be showni to propa-gate of necessity nearly vertically anid to oscillate nearlyhorizointally. But the amplitudes inivolved (whichwould be the "winds" observed) are appreciable frac-tionis of the phase speeds deduced, so the oscillationiwould at best be nionlinear. Some form of turbulenicemight be expected as a result, but probably niot of anisotropic nature. It is certainly not clear that the usualcriteria for turbulence would apply.

Anisotropies associated with the geomagnetic fieldwill become more important at greater heights in radiostudies. It should be noted, for example, that a simplevertical shear in a purely laminar horizontal motion ofthe atmosphere can produce ionization irregularitiesjust as if the atmosphere itself were turbulent.6' Shear-

55 A. Maxwell, "Turbulence in the upper ionosphere," Phil. Mag.,vol. 45, pp. 1247-1254; December, 1954.

56 H. G. Booker, "Turbulence in the ionosphere with applicationsto meteor trails, radio star scintillation, auroral radar echoes, andother phenomena," in "Polar Atmosphere Symposium, Part II,Ionospheric Section," Pergamon Press, London, Eng., pp. 52-81;1957. Repeated in J. Geophys. Res., vol. 61, pp. 673-705; December,1956.

57 H. G. Booker, "The use of radio stars to study irregular refrac-tionl of radio waves in the ionosphere," PROC. IRE, vol. 46, pp. 298-314; Janiuary, 1958.

58 H. G. Booker and R. Cohen, "A theory of long-duration meteor-echoes based oni atmospheric turbulence with experimental confirma-tioni," J. Geophys. Res., vol. 61, pp. 707-733; December, 1956.

59 L. A. Manninlg and V. R. Eshleman, "Discussion of the Bookerand Cohen paper, 'A theory of long-duration meteor echoes based onatmospheric turbulence with experimental confirmation'," J. Geophys.Res., vol. 62, pp. 367-371; September, 1957.

60 H. G. Booker, "Concerning ionospheric turbulence at themeteoric level," J. Geophys. Res., vol. 63, pp. 97-107; March, 1958.

61 J. WV. Dungev, "The influence of the geomagnetic field on turbu-lence in the ionosphere," J. Atmos. Terrest. Phys., vol. 8, pp. 39-42;Februiary, 1956.

inig of this type might therefore provide an explanationfor short-lived layer effects anid even for the "scatter"transmission of VHF waves, both of which have beensuggested in the past as manifestations of isotropic (ornearly isotropic) turbulence.56The geomagnetic anisotropy would appear to domi-

nate in the F region, at least in the productioni of scintil-lationi irregularities. It has been suggested that the con-trol exists only on1 the charged particles, with the aniiso-tropy of their irregularities beiiig determiiied more orless by the aniisotropy of their mobilities;56 a niumber ofionizatioIn irregularities might theni be horizontallyspaced within the single atmospheric eddy which pro-duces them. Such an assumption appears unwarranited,and a converse viewpoint might well be advaniced-thatvariations of denisity in directioins perpendicular to thefield would be difficult to genierate, and that the rela-tively small scale-sizes detected for those directionswould argue against isotropy in the atmospheric turbu-lence. The required anisotropy might result from shear-inlg at the geomagnetic field lines which rise from theauroral belt, as suggested in Section I I, or it may be dueto effects, such as precipitation from the outer atmos-phere or interplanetary gas, which have nlothing to dowith F-region turbulence.The future of turbulenice theory in ionospheric studies

can onily be guessed at the moment. But, whatever theguess, the immense potential of the phenomenon cani-not be ignored.

VII. LARGE-SCALE TRAVELING DISTURBANCESThe final type of motioni to be discussed is that re-

vealed by very extensive irregularities in the F regioni,stretching over fronts hundreds of kilometers in lengthand progressing horizontally over similar distances.62Various techniques have been used for their study, butthe simplest in principle is the correlation of verticalincidence ionograms obtained at separated stations.Typically, each stationi detects the appearaiice of aiiirregularity at the top of the F trace, and follows it downithrough the F and possibly into the E layer, but thetimes of occurrence at the different stations differ be-cause of the horizontal componient of motion. The ir-regularities can be of considerable amplitude, reachinig10 per cent or more in the fractional variation of elec-tron density.

It has been argued on occasioni that the horizontalmovement revealed in these studies is basically that ofan atmospheric wind, perhaps associated with a cellularwave,63 or alternatively that of an ionization drift in-duced by an electric field.64 It is difficult to reconcile

62 G. H. Munro, "Travelling disturbances in the ionosphere,"Proc. Roy. Soc. (London) A, vol. 202, pp. 208-223; July, 1950.

63 D. F. Martyn, "Cellular atmospheric waves in the ionosphereand troposphere," Proc. Roy. Soc. (London) A, vol. 201, pp. 216-234; March, 1950.

64 D. F. Martyn, "Interpretation of observed F2 'winds' as ioniza-tion drifts associated with the magnetic variations," in "The Physicsof the Ionosphere," The Physical Society, London, Eng., pp. 161-165; 1955.

184 Fe-bruary


these views with other features however. An atmos-pheric wind would tend to produce a vertical shift inthe layer as a whole, amounting to some tens of kilome-ters in the course of a disturbance. None is seen. Anirregularity would tend to diffuse rapidly away alonggeomagnetic field lines if a uniform wind or externalfield were acting, whereas in fact the observation of asingle disturbance may last for more than an hour.Furthermore, the horizontal speeds deduced from near-by stations differ markedly from those deduced by com-binations of distant results,65 in contrast to the expecta-tions of either a wind or a field process.These features are, however, all consistent with the

assumption of a propagating atmospheric wave as thesource of the disturbance, and such a source is the onenow most commonly accepted. The wavelength, on thisbasis, is typically of the order 100-200 km, and theperiod of the order 15-20 min. Atmospheric waves inthis range would be strongly affected by gravitationalforces as well as the restorative pressure gradients, whileinertial forces associated with the oscillation would berelatively small. The phase speed would exhibit strongdispersion.An oscillation of this type could propagate as a per-

turbation, leaving the height of the layer as a whole es-sentially unchanged. Moreover, such an oscillation cre-ates its own inhomogeneities as it progresses, and soavoids the difficulty which would be inherent if diffusionwere important. And finally, the speed detected by sta-tions spaced over distances comparable to a wavelenigthwould be the phase speed, while that detected from morewidely-spaced stations would be the group speed; thetwo should not be the same.

Several aspects of the study of waves might be con-templated, anid some of these have already been at-tacked. The energy source of the wave is an obvioustopic to pursue, and various suggestions have beenmade: thermal effects within the atmosphere,66 inter-actions between the interplanetary gas and the earth'souter atmosphere,8 and shock waves emitted by thesun.67 Only the last of these has received detailed theo-retical analysis, anid its possible correlation with the ob-served occurrences has yet to be effected.

In searching for likely energy sources, some weighthas undoubtedly beeni given to the observed fact thatthe vertical componient of propagation is invariablydownwards. But this feature, and others which appearto be characteristic, may have only a relatively inciden-tal relation- to the actual source. For it should be notedthat our only means of detection is depenldent on the

" R. E. Price, "Travelinig disturbances in the ionosphere," in"The Physics of the Ionosphere," The Physical Society, London, pp.181-190; 1955.

6 J. A. Pierce and H. R. Mimno, "The reception of radio echoesfrom distant ionospheric irregularities," Phys. Rev., vol. 57, pp. 95-105; January, 1940.

67 S. Askasof i, "Dispersion relationi of magneto-hydrodynamicwaves in the ionosphere and its application to the shock wave," Sci.Reps. of Tohoku University,japan, Ser. 5, Geophys., vol. 8, pp. 24-40;November, 1956.

ability of the disturbance to produce irregularities inthe electron distribution, so an observational selectionis implicit in the measurements. A disturbance which at-tempted to compress the electron plasma by differentialmotion along geomagnetic field lines, for example,would be more likely to succeed than one which re-quired compression across those lines. The possibilityexists, then, that waves propagating in many directionswith many characteristics are actually generated, butonly those having certain characteristics are observed.

This possibility has been explored along two linesonly, both involving the concept of resonance-first inthe electromagnetic field engendered by the movingcharge,68 and secondly in the oscillation of the electronplasma itself.69 The theoretical development of the latterwas faulty, and it will have to be converted in effect to astudy of resonance in the ion plasma before further de-tailed comparisons with the observations can be pur-sued.70 The principles involved are clear, however.Oscillations of the plasma alone are governed by a dis-persion relation between the frequency and the (three-dimensional) spacial scales. For given spacial scales, anatural period of oscillation of the plasma may be de-duced. The spacial scales may be considered as beinigset by the atmospheric disturbance, and that disturb-ance too has a specific associated period of oscillationdetermined by its own dispersion relationi. The forcedoscillation of the plasma will have an amplitude propor-tionial to that of the forcing atmospheric oscillationi, anidthe factor of proportionality will depenid oni the degreeof matchinig that exists between the two periods. Largeresonant motions can result if the two periods are equal.Mathematically, the relevanit conidition is that the twodispersion equations should be satisfied simultaneously.

Powerful selection principles can be deduced by thisapproach, and subsidiary conditionis can be added. Thelatter might include, for example, a requirement thatthe disturbance not be damped out too rapidly by vis-cous or inductive effects.The further elucidationi of large-scale traveling dis-

turbances must be left to the future. But it should benoted that, if they are produced by waves, their inter-pretation will probably be facilitated by noting theirvarious characteristics (period, speed, directioni, verticaltilt of wave front, etc.) separately for individual events.The more usual procedure in presentations at the mo-ment is, instead, to summarize a large number of ob-servations of isolated characteristics, thereby losingvital information on inter-relations. And further, theobservation of these disturbances in depth, as they de-scend through the F layer, is probably as important totheir understanding as is the detection of their hori-

68 C. 0. Hines, "Hydromagnetic resonance in ionospheric waves,"J. Atmos. Terrest. Phys., vol. 7, pp. 14-30; August, 1955.

69 C. 0. Hines, "Electron resonance in ionospheric waves,"J. Atmos. Terrest. Phys., vol. 9, pp. 56-70; July, 1956.

70 The conversion will be presented elsewhere shortly, but it maybe noted here that the orders of magnitude involved in the phe-nomenon remain essentially the same, and these are certainly perti-nent to the observed disturbances.

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PROCEEDINGS OF TI-IE IRE

zontal motions; sweep frequenicy soundings are thereforeto be encouraged in conjunction with any study of thehorizontal motiotn.

VIII. CONCLUSIONPerhaps the most immediate conclusion that cani be

drawn from all these remarks is that a very great dealhas yet to be learned about motions in the ionosphere.It must also be clear, however, that much groundworklhas already been laid on which to build.

Elementary problems associated with the earth's ro-tation have yet to be investigated in any detail. Theseiniclude purely terrestrial effects of geomagnetic inter-actioin with a rotating ionosphere, and coupling effectsbetween the ionosphere and the interplanetary or co-ronal gas. Neither subject has been more thatn touchedoni insofar as "undisturbed" conditions are concerned.

Iidal theory has been more fortunate, both in thestudy given it and in the results achieved. Further ob-servational effort is required, aimed primarily at the col-lationi of measurements obtained by a succession oftechniiques leadinig, each in turn, to successively higher

levels. As many levels as possible should be inivestigatedat anyr one stationi, in preference to a scatteritng of iso-lated investigations, but a distribution of such stationisin both latitude and longitude is essetntial to the develop-menit of a finial composite aCCOuInt. As the observationisbecome available, the theory will undoubtedly advanceto incorporate them.

In studies of the more localized pheniomenia, inicludinigdrifts, scintillation-s, auroral forms, turbulence andlarge-scale traveling disturbanices, the theory seems tolag well behind the observations. Unitil theoretical ad-vances are made, leadinig to the deductioni of crucialtests of conflicting hypotheses, the observationis mustproceed with little guidanice. It is to be hoped that thisphase will not continue lonig.

ACKNOWLEDGMENTThe author would like to ackniowledge the value of

discussions with his colleagues at the Defence ResearchTelecommunications Establishment during the prepara-tion of this paper, and the assistanice of Miss C. A. Maywho computed the vector patterns of Fig. 2.

Meteors in the Ionosphere*L. A. MANNINGt, SENIOR MEMBER, IRE, AND V. R. ESHLEMANt, SENIOR MEMBER, IRE

Summary-When meteors enter the lower E region of the iono-sphere, they produce trails of ionization. Sensitive radio systems atfrequencies of 3 to 300 mc can record echoes from these ionized trailsat rates of thousands per hour. Study of these echoes has benefitedastronomy, the physics of the upper atmosphere, and radio communi-cation. This paper presents a review of the nature of meteoric echoes,and describes the principal uses of meteors as research -tools. Someof the directions in which further meteoric research may proveprofitable are suggested as well. When the results of the IGY pro-grams are correlated, we may expect the knowledge gained from thestudy of meteors to play an important role.

INTRODUCTIONSINCE the second world war, the radio study of

meteors has received a vast amount of attention-perhaps more thani has any other branch of ioIno-

spheric physics. At first little more was known than thatcertain transitory radio echoes were correlated in theiroccurrence with the sighting of visual meteors. How-ever, as the outgrowth of vigorous research programs ata number of institutions, meteoric echoes are now im-portant and well-understood tools in astronomical andupper atmospheric research; they are also the basis for

* Original manuscript received by the IRE, December 15, 1958.,t Radio Propagation Lab., Stanford University, Stanford, Calif.

new techniques of radio communication. The knowledgegained from the study of meteors must be considered invirtually every other investigationi of the lower iono-sphere. Using meteors, detailed measurements of atmos-pheric density and temperature, scale height, diffusioncoefficient, wind velocity, and turbulence have beenmade. Meteors are a source of sodium anid other diluteatmospheric constituents, a tool for the study of tidalmotions, and a means for determining the height of low-lying layers of absorption. This paper will review brieflythe nature of meteoric radio echoes and some of theirmany applications. From the IGY programs, furtherresults may be expected, notablv from the coordinatedobservation of meteors by visual, photographic, andradio techniques.

THE NATURE OF METEORS

A meteoric particle is a small solid body that normallyrevolves about the sun1 as a member of our solar system.Such particles describe elliptical orbits and travel withheliocentric velocities less than the solar escape velocity.Upon interception by the earth's atmosphere, the mete-oric particle is heated by collision with air molecules,and produces phenomena of light and ionization. Occa-

186 .February

motions in the ionosphere

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