High Spectral Resolution Studies of the Atomic Oxygen, Λ630 Nm, Dayglow
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i5, a.11 HIGI{ SPECTRAL RESOLUTION STUDIES OF THE ATOMIC OXYGEN, ),630 nm, DAYGLOI,I A Thesís for the degree of Doctor of Philosophy submítted by TERRY DOUGLAS COCKS, B.Sc. (Hons) THE MAI/üSON INSTITUTE FOR ANTARCTIC RESEARCH UNIVERSTTY OF ADELATDE MARCII, L977. ABSTRACT Using two Fabry-Perot Interfe.rometers (F.P.I") in scrics, the line emission of atomic oxygen at À630nrn tras ireen isolaled from rhe large background of scaEtered sunlight, with a spectral resolutio4 of 2.1 x 10s, permitting estimates Eo be made of the tenperature and wind velocity characteristic of the neutral thermosphere (¡,200 - 250km) during daytime. The data also yr'-e1<1 information on the emission intensity and the Ring effecÈ. The research project was devel-opmental in nature and this report is biased towards a description of the equiprnenL and techniques used. The design and construction of a 1oçø resolution, mechanical-ly scanne<l F.P.I. are described in detail. The coupling of this F.P,I. w:Lth a high resolution seïvo-mechanically conÈrolled F.P.I. is also described. Design criteria are established by the examination of the theory of F.P. spectrometers. A daÈa analysis scheme is described that requires only empirical informaEion to be used. The parameters relating to temperature, intensiLy and wind velocity are estimated by a least squares fitting routine performect in Ëhe Fourier transfonn domain. Ttre re-Liability of the analysis routine and the experimental techníque are established by numerically simulatíng the observational data. Observations r^rere made at Mt. Torrens (34oS, l39oE). Results are presented for the perio<l December, 1975, to }4arch, L976. This period was characterised by days of lorv to moderate magnetic actí.viÈy and occurred near solar ruinimum. The thermospheric temperature \¡ras found Èo vary from abouÈ 800oK duríng the morning t\dilight to a maximum of 1200oK near 1400 hours LMT. r1- ïhe zonal vrinds are predominantly westwarcl ("r,75nt s-l¡ drrring the day wíth a maximum a few trours after Sunrise and reversíng to an eastward direction near 1900 hours LMT. The meridional winds are equatorward duríng the morning twilight with the daytime velocities being little different fTom zeTo. The emission intensity was found to vary consisLently from abou¡ 0.35 kR at a solar zenith angle of 95o to about 2.5 kR at a solar zeníth angle of l0o, The intensity variation had a broad maximum at about 1200 hours Ll{T- This is the first experiment to derive neutral i¡ind velocitj-es fronl observations of the À63Onm emission line duríng the day :rnd is the first ground based experiment to yield data of sufficÍ-ent accuracy ancl reliability to permit a study of the variaEion of the daytime Ehermospheric te-mperature. The successful application of techniques to measure the spectral characteristics of the atomic o)çygen dayglow rro\^7 means that Èhe thermosphere can be directly monitored by a ground based observatory over the full diurnal cycle. lii This thesís cont¿ríns no material which has been accepted for the award of any other degree or dipJ-oma in any UniversiEy, and, to the best of the authorrs knowledge and belief, iÈ contains no material previously published or written by anot-her pe-rson, except when due reference ís made in Ehe texÈ. (T. D. COCKS) iv ACI(I{OI,ILEDGE}MNTS The author is indebted for the support and co-operation received fr<¡m the personnel of the Mawson lrrstitute throughout this project. The author thankfully acknovrledges the encour:agement and guídance provided by his supervisor, Dr. F, Jacka, Director of the Mawson Institute. It was he who inscigated Ëhe use of Fabry-Perot inEerfero- meters i-n nighrglow observations at this laboratory, and proposed Èhe dayglow project. I'fr. D. Creighton made many valuable contributions to the construction of the spectrometer, particularly the electronics. The author wishes to thank Drs. P. tr^Iilksch and A. Bower for providing a high resolution F"P.I. that performed so relíably throughout this work. The rnany discussions held with Dr. P. Wilksch were of immense help. The mechanical constructíon work vras undertalcen with the assístance of Mr. F. Fone and l"Ir. F. Koltai. Their willingness to continually modífy pieces of equipment at a momentrs notíce was much appreciated. The author wishes to express his deep appreciatíon to his wife, Kathy. Her constant supporÈ and encouragement contributed much to the success of f-his projecÈ. For a part of the period spent by the author on this project, he wás supported by a Commonr¿ealth Postgraduate Scholarsl-rip. He ís indebted to his parents and parents-in-law for the willing assistance they provided. The auÈhor is grateful to Mrs. tr^lyaÈt for the typing of this thesis. v CONTENTS AsSTRACT l-l_ STATEMENT l_v ACKNOI,JLEDGEMENTS v I INTRODUCTION I 1.1 Optical Radiation Sensing of Thermospheric Ternperature an<l t'Iind Velocity 1 1.2 Observations of the À630nm [Of] Airglorv 3 1.3 Dayglow Observations 5 1.3.1. Introduction 5 L.3.2. Expected Values of Ternperature, InJind Velocity and Emission Intensi-ty 5 1.3.3. The Background of Scattered Sunlight 6 I.3.4. The Ring Effect 7 1.3.5. Doppler Shifts of the Solar Spectrum B 1.3.6. Absorptj-on by Atmospheríc O2 B I.4 Previous À630nm Dayglow Observations 9 1.5 The Mawson Institute Dayglow ExperimenÈ 1l 1.6 Summary l4 2 TIIE DUAL ETALON FASRY-PEROT INTERFEROMETER THEORY 15 2.L Introduction 15 2.2 The Spectrorneter Transmission Profile, Recorded Spectrum and Transmitted Flux I6 2.3 Spectrometer Selection 20 2.1+ Fabry-Perot Theory : Single Etalon 22 2.4.1. General Princíples 22 2.4.2. Effect of Plate Defects 24 vi 2.4.3. Effect of FiniÈe Field of View 27 2.4 .4. Srrnrnary 30 2.5 Fabry-Perot Theory : Dual EÈalon 31 2.5.L. IntroducÈíon and General Polyetalon Principles 31 2.5,2. Etalon Coupling 3B 2.5.3. Number of Etalons Required 39 2.5.4. Instrumental Profile 4l 2.5.4.I. StatemenË of the Problem 4L 2.5.4.2. Effects of Plate Defects 42 2.5.4.3. Ef fect of a Finite Field of View 44 2.5.5. Srlmmary 45 2.6 Choice of Operating Parameters 47 3. TI]E IIIGIT RESOLUTION FABRY-PEROT INTERFEROI'Í!]TER 52 3.1 Introduction 52 ala Optical Flats/Plates and Reflective Coati-ngs 53 3.3 Parallelism Control 53 3.4 Separation Control 54 3.5 General Structure 56 4 THE LOI^J RESOLUTION FABRY-PEROT INTERFEROMETER 57 4.1 Design and Construction of the Low Resol-uti-on l¡abry-Pero t Interf erometer 57 4.1.1. Intro<1uctÍon 57 4.I .2. Design Concepts 5B 4. r.3. Optical Flats/Plates 60 4.L.4. Mechanical Details 60 4.L.s. Piezoelectric Ceramic MounÈs 62 4.I.6. Temperature Compensation 64 4 .L .7. Plate llountings 65 4.2 Desígn and ConsÈruction of the Etalon Enclosure 66 4.2.L. Origínal Design Concepts 66 vii 4.2.2. General Enclosure Description 66 4.2.3. Temperature Control. 67 4.2.4. Inner Chamber 6B 4.2.5. Oufer Chamber 69 4.3 Scanning and Parallelísm Control 70 4.3.I. Introduction 70 4.3.2. Fringe Viewing System 7L 4.3.3, Electronic Controls 7I 4.4 Operation an.d Performance 73 4.4.1. Scanning the InterferomeÈer 73 4.4.2. Instrument Profile MeasuremenEs 74 4.4.3. Settíng the Order 75 4.4.4. Piezoelectric Ceramic Characteristics 75 4.4.5. Finesse Measurements 76 4.4.6. Parallelism and lfean SeparaEion Srabi-lity 76 5 THE DUAL ETALON FASRY-PEROT SPECTRO}TETER DESIGN, CONSTRUCTION AND OPERATION 7B 5.1 Introductíon 7B <? Mechanical Details 7B 5.3 The Optical System 79 5.3.1. The OptÍcal Configuration 79 5.3.2. Mechanical Details of the Couplíng System B1 5.3.3. Alignrnent Procedure 82 5.3.4. The Interference Filter B3 5.4 The Períscope Bl+ 5.5 Photon Detect,ion 86 5.5.1. The PhoEomultiplíer B6 5.5.2. Digital DetecEion B6 5.5.3. Analogue Detection BE 5.5.4. The Combined DeÈecÈion SysÈern B9 viií 5.5.5. I'fonitoring the Sígnal Levels B9 5..6 Data Accumulatíon and Associated ElectrrrnÍc. Controls 90 5.6.1. Introduction 90 5.6.2. The Scan Generator 90 5.6.3. The }lultichannel Analyser 9T 5.6.4. The Data Acquísition lìoutine 92 5.6.5. The System Configuration 94 5.6.6, Data Handling 95 5.7 Operating Procedures 96 5.7.L. Tuning the Etalons 96 5.7.2. Achj-eving Scan Synchronism 97 5.7.3. InÈensity CalibraLions 99 5.7.4. Instrument Profíle Measurements 99 5.7.5 The Use of Polarízer f-or Background Di-scrimínation l0l TABLE I 103 6. DATA ACCUI'TULATION AND ANAI.YSIS 106 6 .1 RedefÍnition of the Instrument Profile 106 6 .2 Data Accumulation to7 6.2.I. Digítal and Analogue Detectíon IO7 6.2.2. Gaussia¡r Line Profile i11 6 .3 The Dayglow Spectra TL2 6.3.1. Introduction Lr2 6.3.2. Isolation of the Emission Feature 113 6 ..4 Data Analysis : Theory tt7 6.4.I. Statement of the Problem LT7 6.4.2. Analysis Schemes llB 6.4.3. Convolution and Applicatíon of the Discrete Fourier Transform 119 6.4.4. Description of the AnalysÍs Scheme L22 lx 6.4.5. The Least- Squares Fitting Routj-ne L2.4 6.4.6.