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Design and Construction of a Potassium Faraday Filter for Potassium Lidar System Daytime Operation at Arecibo Observatory

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2002:373 CIV EXAMENSARBETE Design and Construction of a Potassium Faraday Filter for Potassium Lidar System Daytime Operation at Arecibo Observatory JONAS HEDIN MASTER OF SCIENCE PROGRAMME in Space Engineering Luleå University of Technology Department of Space Science, Kiruna 2002:373 CIV • ISSN: 1402 - 1617 • ISRN: LTU - EX - - 02/373 - - SE Abstract Presented here in this thesis is the project work performed for the Master of Science degree in Space Engineering from Luleå University of technology, Sweden. This project work was conducted at the Arecibo Observatory in Arecibo, Puerto Rico, where I, during 6 months, designed and built a Potassium (K) -vapor Faraday filter to make daytime lidar observations of the mesopause region (80-105 km altitude) in the atmosphere a reality. The K Doppler-resonance lidar at the Arecibo Observatory is used to probe the temperature structure of the mesopause region. The climatology of this region is of vital importance in the understanding of earth’s climate system. The effects of global change are expected to be reflected strongly in the mesopause region, where tropospheric global warming will cause substantial cooling. This thesis will talk more about the background for making lidar observations of the mesopause region and the need for extending this observation to, not just nighttime, but also to daytime by making a filter that blocks broadband background skylight and only lets the ultra-narrow band signal pass. The thesis will present and explain the physical effects applied in the filter, such as anomalous dispersion and absorption at resonance frequency, the Zeeman effect and Faraday rotation. The filter characteristics will be given and the function of the filter will be explained. A computer program to determine the filter parameters (magnetic field strength, operating temperature and K-vapor cell length) is explained briefly and attached as an appendix, and the design and integration of the filter will be shown. The design and construction is based on a magnetic field strength of 1.3 kG, a temperature of 114˚C and a K-vapor cell length of 40 mm, which will give a full width half maximum (FWHM) of 5 pm for the filter. This is wide enough for a 99% transmission (for already polarized incident light and without reflection in the optical surfaces in the filter) of the three frequencies used in the observations. i Acknowledgements First I would like to thank my project advisor Dr. Jonathan S. Friedman, Department of Space and Atmospheric Sciences, Cornell University, Arecibo Observatory, for presenting me with a challenging project and for introducing me to the field of lidar based observations of the middle atmosphere. Secondly I would like to thank Raúl Garcia for all the help and guidance in the construction of the parts of the filter. I would also like to express my thanks to the Electronics Department staff for advice and help when this was needed, to Dr. Biff Williams of the Colorado State University lidar group for letting me use and modify his program for determining the filter parameters and answering questions about its operation, and to Dr. Josef Höffner at the Institute of Atmospheric Physics (IAP) in Kühlungsborn, Germany, for information regarding filter construction. Many thanks to my family and to a very special person for their support and for believing in me. And finally big thanks to everyone else who made my work and stay at the observatory so very pleasant. The work presented in this thesis has been performed at the Arecibo Observatory in Puerto Rico. The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the National Science Foundation. ii Table of contents Abstract....................................................................................................................................... i Acknowledgements.................................................................................................................... ii Chapter 1. Introduction ...............................................................................................................1 Chapter 2. Physical Effects of the Filter .....................................................................................5 2.1 Dispersion .........................................................................................................................5 2.1.1 Normal dispersion......................................................................................................5 2.1.2 Anomalous dispersion................................................................................................6 2.2 Zeeman Effect...................................................................................................................8 2.3 Faraday Rotation.............................................................................................................12 2.3.1 Classical theory........................................................................................................12 2.3.2 Electromagnetic theory ............................................................................................13 Chapter 3. Filter Characteristics ...............................................................................................15 3.1 Filter transmission...........................................................................................................15 3.2 Filter description .............................................................................................................20 Chapter 4. Filter Design and Construction ...............................................................................22 4.1 Determining the filter parameters ...................................................................................22 4.2 Designing and constructing the filter..............................................................................24 4.3 Integration.......................................................................................................................30 Chapter 5. Conclusion and Discussion .....................................................................................31 5.1 Tests ................................................................................................................................31 5.2 Future improvements ......................................................................................................32 Appendix A...............................................................................................................................33 A.1 Main Program ................................................................................................................33 A.2 The Hjerting Function....................................................................................................40 A.3 The Rachkovsky Function .............................................................................................42 A.4 Dialog Select..................................................................................................................44 List of References .....................................................................................................................46 iii Chapter 1. Introduction The mesopause region of the atmosphere is a 25 km thick layer at an altitude from 80 to 105 km and is the boundary between the ionosphere and the neutral Atmosphere. This region has many properties that make it both interesting and difficult to study [1]. At and below this region, molecules, neutrals and neutral-ion chemistry are predominant and the atmospheric density is such that turbulence still occurs. Above, there is no turbulence and the atoms and atomic ions drift almost freely only driven by daily heating and cooling cycles and perturbed by braking gravity waves. The mesopause is the coldest region of the atmosphere with temperature minima in the range of 170–200 K (see Figure 1.1), and also the region where meteors deposit much of their material as they “burn up” when entering the atmosphere, creating metallic layers (the E-layer). The temperature here is colder in the summer than in the winter. It has been observed that this temperature difference is bigger at high latitudes and smaller at low latitudes and should become practically zero at the equator. Figure 1.1. Temperature profile of the atmosphere. [2] The climatology of the mesopause region is of vital importance in the understanding of the climate system of the earth. The effects of global change are expected to be reflected strongly in the mesopause region, where tropospheric global warming will cause significant cooling. There are different ways to study the mesopause. One is by optical observations of airglow emission from OH, O 2 and O. This gives some information of the chemistry and the winds but uncertain and incomplete altitude coverage. Another way is by Incoherent Scatter Radar (ISR) studies of the region, but mostly at daytime hours when the electron density is higher. At night ISR will only see the narrow sporadic E layers or descending tidal ions. Satellites are only good for higher altitude and cannot dip down that low in their orbit to do in-situ measurements. Of course, rockets can make in-situ measurements, but only occasional spot observations and at great expense. The best and easiest way to study the mesopause region is by utilizing lidars. The existence of the metallic layer (see Figure 1.2) makes it possible
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