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Spectral Characteristics UHF Radar Aurora Spectral Characteristics UHF Radar Aurora Brian J. JackeI Graduate Program in Physics Submi tted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Faculty of Graduate Studies The University of Western Ontario London, Ontario October 1997 @ Brian J. Jackel 1997 National Library Bibliothèque nationale 1*1 dC-da du Canada Acquisitions and Acquisitions et Bibliographie Services services bibliographiques 395 Wellington Street 395. rue Wellington OrtawaON K1AON4 Ottawa ON KIA ON4 canada canada The author has granted a non- L'auteur a accordé une licence non exclusive licence allowing the exclusive permettant à la National Lhmy of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or seil reproduire, prêter, distribuer ou copies of this thesis in microfom, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfiche/film, de reproduction sur papier ou sur format électronique. The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fiom it Ni la thèse ni des extraits substantiels may be p~tedor othemïse de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. A bstract Bistatic radar observations at 440 and 933 Megahertz ha .ve been used to study the spectral characteris t ics of CHF scat ter from the auroral E-region. Scattered power spectra were prirnarily composed of a singie peak? and were distributed asymrnet- rically about the peak with a sharper cut-off at higher speeds. Using an approach not previously applied to auroral echoes, spectral moments were estimated directly from the autocorrelation functions. avoiding problems caused by truncation in the lag domain. ACFSwere also characterized in terms of t heir "correlation time- (related to the spectral width) and -decay exponent" (related to the spectral shape). Spectral shapes were neither Gaussian nor Lorentzian, but had an intermediate form that was well characterized by a simple model. Mean Doppler shifts ranged from O to 600 m/s. with a significant number of low speed observations. Large aspect angle echoes were wider than those from srnall magnetic aspect angles, although the range of Doppler shifts was similar at al1 aspect angles. Results for 16 and 34 centimetre irregularity wavelengths also showed the same range of speeds. as well as correlation times (at smali aspect angle). .A small number of 'narrow" (long-lived) echoes were observed at bot h frequencies: these had correlation t imes of nearly 1000 microseconds. more than twice the "typical" values. Keywords: auroral, radar. spec t ra. moment es tirnation Acknowledgements Many thanks to Don Moorcroft for his advice and support. Thanks also to the rest of the Space and Atmospheric research group for their important contributions. Contents Certificate of Examination IL.. Abstract 111... Table of Contents v List of Figures VI~I... List of Tables ix 1 Introduction 1 1.1 Feat ures of the radar aurora . 1 1.2 VHF and CHF . .I- 1.3 Bistatic radar notes . 4 1.1 Review of previous experimerits . 6 - 1.4.1 Millstone Hi11 (1393 LIhz) . 1 1.4.2 Homer . - . 8 1.4.3 EISCATand COSCAT. 9 1.4.4 Millstone Hill (140 Mhz) . 11 1.4.5 Summary . 12 1.5 Structure of this thesis . 12 2 Spectral Characterization 14 2.1 From LPMSto ACFSto spectra ..................... 15 3.2 Selected Observations ........................... 20 2.3 Spectral Moments ............................. 21 2.3.1 Estimation in the frequency domain ............... 25 2.3.2 EstimationfromA~~s...................... 27 3.4 Models of the .4 CF magnitude ...................... 2s 2.5 Otherfeatures ............................... 32 2.5.1 Peak frequency .......................... 32 2-52 Full-width half-maximum (FWHM)............... 33 2.53 Double peaks ........................... 31 2.6 Summary ................................. 36 3 Results at 34cm: MIDASC 37 3.1 MIDASC .................................. 39 3.1.1 Algonquin ............................. 12 3.2 Some results ................................ 34 3.9.1 Double peaked spectra ...................... 11 3-22 Very strong echoes ........................ 37 3.2.3 Results frorn London ....................... 39 .3.3.4 Xarrow echoes ........................... 53 3.2 ..5 Monostatic satellites ....................... 51 3.3 Discussion ................................. 56 4 Results at 16cm: EISCATand COSCAT 58 4.1 Data from 1989 .............................. 59 4.2 Datafrom1992 .............................. 63 4.2.1 Large aspect angle ........................ 66 4.2.2 Srnailaspectangle ........................ 67 4.3 Discussion ................................. 69 5 Discussion 73 5 .1 Doppler Speed ............................... 74 5.2 Spectral width or correlation time .................... 76 5.2.1 Xarrow spectra .......................... 79 a.3 Skewness .................................. 81 5 Spectral Shapes .............................. 85 rr 3.3 New analysis methods .......................... 8'; 5.6 Futureexperiments ............................ 91 5.7 Summary ................................. 92 A Mat hematical Details 94 4.1 Fourier transform relations ........................ 94 .A.? Moments in the frequency and lag domains ............... 95 A.3 Derivat ives of the auto-correlat ion function ............... 97 4.3.1 Determining deripatives from discrete data ........... 97 A.4 Standard line shapes ........................... 99 A.5 Full width half maximum ......................... 101 A.6 Standard deviation as a width estimator ................ 104 4.7 Logarithmic ACF magnitude ....................... 106 A.8 Locating a single peak .......................... 108 A.9 Twin peaks ................................ 109 A . 10 Spikiness or RMS Spectral Power .................... 110 References 112 Vita 117 vii List of Figures Lag Product Matrix examples ...................... 1; Display of -4~~sand power spectra ................... 21 Exampies of coherent ACFS and their power spectra .......... 23 Example XCF magnitudes on a logarithmic scale ............ 31 Cornparison of .4 CF magnitudes to the exponential mode1 ....... 32 Oscillating ACF with single spectral peak ................ 35 Location of MIDASCradar sites ..................... 30 E-region aspect angles .......................... 41 Complex spectral shapes from multiple scat tering regions ....... 47 Experiment geometry for an F-region target .............. AS Coherent echoes from Algonquin April 7 1995 ............. 19 Comparison of Millstone Hill and Algonquin results .......... 50 Coherent echoes from London !darch 8 1994 .............. 51 Example of a satellite echo ........................ 55 Cornparison of moments for the remote EISCAT1989 data ...... 61 Other spectral parameters for the EISCAT1989 data .......... 63 Decay exponent vs . SNR and correlation time ............. 66 Correlation tirne. decay exponent and SKEWB vs . velocity ....... 68 Spectral parameters at small aspect angle ............... 70 Superposition of two Gaussian spectra ................. 84 Standard line-shapes ........................... 102 Cornparison of three spectral width parameters ............ 104 Polynomialfit to the ACF magnitude .................. 107 List of Tables 1.1 Summary of previous experiments .................... 13 2.1 Example Observations ............................')3 2.2 Moments of example data ........................ 29 2.3 Correlation times and decay exponents ................. 30 2.1 Peak velocity ............................... 33 3.1 -UIDASCtarget parameters ........................ 45 3.2 Spectral characteristics for !vIarch 8 1994 ................ 52 3.3 Narrow echo characteristics ....................... 53 4.1 EISCAT1989 experiment details ..................... 60 4.2 Summary of EISCAT1989 spectral characteristics ........... 62 1.3 E1sc.4~1992 pointing information .................... 65 4.4 Narrow spectra from June 28 1992 ................... 71 5.1 Correlation times ............................. -"r r First detected by Harang and Stoffregen [1938], the radar aurora is a phenornenon in which enhanced radar echoes are received from regions of the earth's ionosphere where visible aurora are often observed. In subsequent research it has been found that these anomalous echoes are due to structure in the distribution of free electrons. which can result from a variety of plasma instabilities. As a consequence, so cailed "coherent- returns can be observed at a range of altitudes and locations not necessarily associated wi th the optical aurora. Hoirwer. the high latitudes where visible aurora generally occur are also regions where high electric fields and intense fluxes of charged particles provide considerable energy for plasma instabilit ies. As well, the eart h's dipole-like magnetic field geometry is mostly vertical at auroral latitudes. which affects not only the plasma physics which lead to the echoes. but also the ways in which they can be observed. For these and other reasons. observations of the high latitude radar aurora are sufficiently unlike lower latitude results that their study has developed into a separate enterprise. 1.1 Features of the radar aurora Data have been collected with a wide variety of experimental configurations, and although they share a common subject, the goals of individual experiments have varied considerably. Generally speaking, these
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