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RADIO Meteor Astronomy - a REVIE'ií

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RADIO Meteor Astronomy - a REVIE'ií RADTO METEOR ASTRONOMY by G" Gartrel-r, B.Sc. (Hons" ) A thesis presented for the degree of DOCTOR OF PHILOSOPHY in the UNIVERS]TY OF ADELATDE (Physics Departrnent ) May l-971 TO MY'WTFE LESLYE CONTE{TS SUMT{ARY PREFACE CHAPTER T CITAPTER IÏ RADIO METEOR ASTRONOI\4Y - A REVTET\I CHAPTER T]] - RA.DIO REFLECTTON FROM METEOR TRATLS 3.1 Introduction 21 3.2 Trail Formation 2T 3.3 Reflection of Rad.io 'nlaves by Meteor Trails 23 3. 3.1 Phase Angles 2l+ 3.3.2 The Cornu Spiral- 26 3.3.3 The Effect of Wind. and. llind. Shear DO 3. 3.4 Irregular lonization 32 3.3.5 Diffusion 33 CHAPTER IV THE A.DEI,AIDE RADIO METEOR SYSTEM 3)+ )+.r Introduction 3\ \.2 Equipnent 3' \.2.t Transmitters 35 )+.2.2 Transraitting Aerials 36 )+.2 .3 Main Station Receiving Aerials 3T )+.e.)+ 27 Î,¡ftlz Receivers 38 )+.2 ,5 The Outstations and. Telemetry Links 39 \.2.6 Main Station Reeord.ing Equipment )+3 \.3 Film Record.s )+T \.\ Grour¡d Geometry and. Reflection Point Determination l+8 l+. \. r Location of Outstations 48 )+. \.2 Location of Reflection Points )+9 CHAPTER V DATA REDUCTTON 5rI Fil-rn Read.ing 52 5.2 Deternination of Velocity a¡rd. Position ,6 5.3 Aceuracy of Resrrlts 6o CHAPTER VT ATMOSPHERTC RETARDATTON 6T 6 l_ Introd.uction 67 6 2 Derivation and. Solution of the Drag Equation 6B 6 3 Aecuracy of Results 75 6 4 Measured. Val-ues of Surface Area/Mass Ratio 76 6 5 Caleul-atlon of Unretarded Geocentric Velocity 92 6 6 Sunna^:ry 95 CHAPTER VII - METEOR STREAT4S 97 7.r fntroduction 97 7.2 Assoeiation Tests for Strea.m Meteors 98 7.3 The Significance of Small Groups 10\ 7.\ Association Significance and. Salrple ,Size 108 7.5 A Systenatic Strea¡n Search 109 7.6 Discussion of Particular StreFms a.nd. Some Possible Conetary Assocíations 110 7 .6.t Ihe Geninid ltitreteor Strenm 110 7.6.2 The Virginids ar6 7 .6.3 The Arietid.s 118 7 .6.)1 Hy¡rerbolic Stre¡ms 420 7.6.j The Monocerotid. Strea.m ]t23 7.6.6 The Southern Taurid.s a¡rd. Ç-Perseid.s 1z)1 T .6.7 The Andronedid (siefj.a) Strean L26 7.6.8 Strea¡r Rad.iants Possibly Associated. w"ith Comet Lexell 1770 I Lzg T.T Toroid.aJ- Meteor Stre¡ms 131 7.8 A Conprehensive Southern Hemisphere Search for Cometary Assoeiations with }4eteor gf,¡q¡ms L39 CHAPTER VIII - DISTRTBUTTONS 1\8 8.r Introduction 1\8 8.2 Selection Effects 150 Page No. 8.2.t Astronomi cal Sele ct Íon 150 8.2.2 Atmospheric Interaction Seleetion ]-r2 8.2.3 Fragmentation Lr6. 8.3 Dístribution with Reflection Point Height l,57 8.s.r A Possible Velocity-Ifeight Selection Effect L5T 8.s.2 Und.erd.ense and. Persistent Echoes 158 8. 3.3 H¡ryerbolíc Meteors anci a Check on the Atmospheri c Retard.ation Correction ]-59 8.\ Rad.iant Distributions' rr9 8.5 Velocity Distributions L63 8.6 Orbital Elenent Distributions 166 CHAPTER TX SOME CONCLUSIO}TS FROM THE PRESENT SIIR\TEY Æ\ÏD Æ\Ï E"XPI,AT{ATION OF SOME APPARENÎ ANOMALIES ]N TERMS OF A HTPOTTTETICAL ORIGIN TOR COMETS r82 9.1, Recapitulation r82 9.1.1 Atmospherí c Deceleration ltde asurements 182 9.1.2 Meteor St?eâns ancl Cometary Assoeiations 183 9 .1.3 Orbit Distributions 18)+ 9.2 The Relation of Meteors in Orbits of High ïnclination to the System of T,ong-Period Comets 186 9.3 An Intra-Solar System Origin for the Systen of Long-Period. Comets 493 9.3.a Meteoroid. Density 193 9.3.2 Conetary Cosmogony a9 )+ 9.3.3 Concl-usions 198 9. \ Future llork 200 APPENDTX 203 REFERENCES 2LL SUMMARY This thesis describes an investigation of the orbits of meteors cletected at Adelaide (tatitu¿e ¡5oS) using a combined. multi-stati.on e.w. and pulse rad.ar system. The orbit survey deseribed. is the only rad.io survey conducted. in the Southern Hemisphere to measure the orbits of indivÍd,ual- meteors d.own to a liniting rad.io magnitud.e of + 8. The equip- ment ¡¡as operated. for one rreek eaeh month from.,Deeember 1968 to Jr.me 1969 with a further rr:r¡ during October f969, To date ]1667 orbj-ts have been deterrnined from six months of d.ata. Reflections from a mininum of three and. a maximr:m of five points along the trail ¡¡ere used. to d.eternine the rad.iant and. velocity. A long- ôistance outstation enabled. occasional measu¡ements of refleetion point separations as large as l-6 kms., although the najority of reliably determined separations were.of the ord.er of 6 kns. or t"s=. The equip- ment was simultaneously use,d tc measure meteor region wind.s and wind shears, enabl-ing account to be taken of these factors in the d.etermina- tion of the rad.iant and. veJ-ocity d.ata. Distributions of the various geoeentrie and. helioeentrie quantities shol¡ broad- agreement with those from other surveys cond.ucted. along similar lines, althougþ the present clata eontain a significantly larger proportion of high velocity meteors than other surveys to a cornparable liniting magnitud.e. This ís attributeè to a selection effect in d.ata reduction as well as the use of the exbremely clear pre-specular reflec- tion point d.iffraction waveforr. available in a coherent phase system for the d.etermination of velocity. Measurements of d.eceleration have been carried. out for each meteor of the sì.rrvey. Althougþ a wicle range of d.ecelerations ar¡d. even accelera- tions was observed., no d.oubt in part d.ue to velocity measurement errors combined. with small. refleetion:point separations, the longer reflection point separations afford.ed. by the d,istant outstations have enabl-ed. com- putation of sr¡¡face areafmass ratios from measu¡ed. mean d.eeel-erations. Est'imates of the average mass, size and density of the meteors d.etected. have been maile, depend.ent uBon assì,uq)tions regarding the ablation energ¡, the ionizing effieiency and. the magnitude. The action of a height selection effect acting on meteor d.ensity is apparent. Conparison of the present d.ensity estinates with other d-ata indi- cates that average meteor bulk densities increase r¡ith d.ecreasing mass. This gives support for an aggloneration mod.el of meteor fornation, , although fragmentatíon does not seem to be pronouneeÈI anongst the present rad.io meteor popr:lation. In ad.d.ition to the ¡rreII-knovn helion, antihelion and. apex concen- trations of apparent meteor radlants this surve¡r confirms the existence of a lesser concentration eentred. on the longitud.e of the apex at ecl-iptic latitud.e - 6o0. Both this and the dpex concêntration are virLually removed. r¡hen aceount is taken of observational and. astronomi- eal seleetion factors although a strong exbend.ed. high latitud.e concen- tration remains. Comparison of the rad.iant d.istributi-ons with sinil-ar d.istributions for Northern Hemisphere data shows marked. symmetry ahout the ecliptic. The d.ata has been systematically seareÏ¡ed. for strea.n meteors an¿ the significa.nce of minor associations has been re-appraised.. Altogether )+O.\/" of the orbíts were found to be associated. with at least one other orbit wÍthin the linits of resolution of the neasulement technique. This figure drops to 2).8/o when associations of pairs only are exclud.ed.. Nunerous ninor streans with high inclination and. low eccentricity have been fountl at d.eep Southern d.eclinatÍons from Dece¡lber to March, T,r"ith littIe activity in this quarber during Jrxre and October. In actd.ition to several previously establ-ished. cometary associations a couprehensive search has inòicated. that 34 of the meteor associations found. may be related to 17 comets. Associations between several long- period. comets and 1ow eecentrieity high inelination streams appear in- ctisputable and confirm trre origin of the 'toroidal- group' meteoïs, previously a matter of some doubt. Prefaee Ilris thesis contains no naterial whicl¡ has been accepted. for the awarcl of any other degree or diploma in arry University. To the best of the authorls kno¡¡leclge and. belief , this thesis contains no material previously published or wrÍtten by another person, except where due reference,is matle in the te:ct of the thesis; (Cra¡rt GartrelL) oû1e nts Ttre project deseribed. in this thesis was carried. out within the Department of Physics of the University of Adelaid.e r:nder the super- vision of Dr. l[. G. E1ford.. The author would like to thank Dr. Elford. for suggesting the project, for helpfirl d.iscussions and. parbicularly for his reassurance and. encouragement througþout the course of the work. The author would also like to thank Dr. C., S. Nilsson for suggestions d.uring the latter stages of the project. The buil-d.ing, operation a¡d. maintenance of the rad.io meteor system Ìras very mueh a cooperative coneern. Without, the assistance of a number of people the work outlined in this thesis r¡ou1d have been impossible. The author is ind-ebted. to his colleague Dr. B. J. McAvaney, who, in collaboration with Mr. J. Irl. Srûith, designed. some of the outstation and tape d.eIay system equipment. Dr. MeAvaney also shared. with the author the responsibility for operation and maintenance of the nulti-station system. Thanks are al-so due to Mr. N. Bro¡rn for assisting with the equipnent operation.
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