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Measurements of Optical Turbulence on the Antarctic Plateau and Their Impact on Astronomical Observations

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Measurements of Optical Turbulence on the Antarctic Plateau and Their Impact on Astronomical Observations Measurements of Optical Turbulence on the Antarctic Plateau and their Impact on Astronomical Observations. Tony Travouillon Submitted in total ful¯lment of the requirements of the degree of Doctor of Philosophy School of Physics University of New South Wales September 2004 Abstract Atmospheric turbulence results taken on the Antarctic plateau are presented in this thesis. Covering two high sites: South Pole and Dome C, this work describes their seeing and meteorological conditions. Using an acoustic sounder to study the turbulence pro¯le of the ¯rst kilo- metre of the atmosphere and a Di®erential Image Motion Monitor (DIMM) to investigate the integrated seeing we are able to deduce important at- mospheric parameters such as the Fried parameter (r0) and the isoplanatic angle (θ0). It was found that at the two sites, the free atmosphere (above the ¯rst kilometer) was extremely stable and contributed between 0.200 and 0.300 of the total seeing with no evidence of jet or vortex peaks of strong turbulence. The boundary layer turbulence is what di®erentiates the two sites. Located on the Western flank of the plateau, the South Pole is prone to katabatic winds. Dome C on the other hand is on a local maximum of the plateau and the wind conditions are amongst the calmest in the world. Also linked to the topography is the vertical extent of the temperature in- version that is required to create optical turbulence. At the South Pole the inversion reaches 300 m and only 30 m at Dome C. This di®erence results in relatively poor seeing conditions at the South Pole ('1.800) and excellent at Dome C (0.2700). The strong correlation between the seeing and the ground layer meteorological conditions indicates that even better seeing could be found at Dome A, the highest point of the plateau. Having most of the turbulence near the ground is also incredibly ad- vantageous for adaptive optics. The isoplanatic angle is respectively 3.300 and 5.700 for the South Pole and Dome C. This is signi¯cantly larger than at temperate sites where the average isoplanatic angle rarely exceeds 200. This means that wider ¯elds can be corrected without the complication of conjugation to speci¯c layers. For such purpose the potential is even more ii interesting. We show that ground conjugated adaptive optics would decrease the natural seeing to 0.2200 for a wide ¯eld of 10 and 0.4700 for a ¯eld of 100 at the South Pole. At Dome C the results are less impressive due to the already excellent seeing, but a gain of 0.100 can still be achieved over 100. These results show that high angular resolution observations can be done better on the Antarctic plateau than any other known site. Declaration iv Acknowledgements I never thought I'd say that, but this PhD went too fast... Time flies when you have fun and when you like what you do and the team you work with: My ¯rst thoughts go to my supervisors Michael Burton and John Storey who have obviously done this job before since they have given me the perfect balance of autonomy and support. I want to thank them for teaching me rigor, patience, and a whole lot of grammar. In particular, Michael, thank you for the teaching skills that I hope to have picked up along with your enthusiasm to promote science to a larger audience. John, thank you for all you have done to help me develop myself professionally and your exquisite taste in ¯lms. I hope one day to be able to give public speeches of your standards (but without the help of a table...). To the Antarctic team, thank you for being who you are: Michael Ashley, I don't know what we would have done without you. Honestly, how was I supposed to guess that to ¯x a corrupted compressed ¯le all I had to do is type: \dd if=hdb BS=102983 skip=3"? To me, it makes just as much sense as a movie from David Lynch. Jon Lawrence, a great companion on every trip to Antarctica. An ideas man, that Jon: I still don't know how you got the siderostat pointing so well, but I won't forget when the star showed up on the eye piece on the ¯rst try. Paolo Calisse, the only other person in the group who understands the importance of cheese in the life of a human being. Jon Everett, a great man to work with despite his love for Alpha Romeo cars. Jessica Dempsey with whom I share the same fascination for Antarctica and who has been on that road with me since ¯rst year physics. Finally, Suzanne Kenyon who has joined us recently and who will write a sequel to this thesis. To my colleagues and friends from the University of Nice who have been strongly involved in this work: Eric Fossat thank you for adopting me in vi your group. Eric Aristidi, who despite his inability to shave properly, quickly became a good friend. Jean Vernin, Max Azouit and Karim Agabi for the fun time we had at the OHP and Alex for the best Genepi ever... Cormac, Steve, Stevo, Vincent, Matt, Ra, Julian, thank you for creating a multitude of distractions and I hope, one day, you will understand that you only need to dress like pirates, sheiks, sperms, etc...on Halloween night. Most importantly to my family, thank you for clearing the path to my ambitions. Contents 1 Introduction 1 1.1 Motivations . 1 1.2 Characteristics . 2 1.2.1 Atmospheric extinction . 3 1.2.2 Atmospheric emission . 4 1.2.3 Atmospheric turbulence . 4 1.2.4 Further criteria . 5 1.3 Why Antarctica? . 6 1.3.1 The near space atmosphere . 6 1.3.2 Brief history of Antarctic Astronomy . 7 1.3.3 The sites . 8 1.4 The AASTO: A Site Testing Philosophy . 8 1.5 The Site Testing Instruments . 9 2 Meteorology of Antarctica 13 2.1 Generalities . 13 2.1.1 The Topography of the Antarctic Continent . 13 2.1.2 Temperature . 15 2.1.3 Surface Winds . 16 2.1.4 High Altitude Winds: The Polar Vortex . 19 2.2 South Pole . 20 2.3 Dome C . 22 2.3.1 Data acquisition . 23 2.3.2 Results . 24 2.3.3 Discussion and conclusion . 31 viii CONTENTS 3 Theory of Turbulence 35 3.1 Formation of Optical Turbulence . 36 3.2 Kolmogorov's Statistical Theory of Turbulence . 37 3.2.1 De¯ning optical turbulence through energy cascade . 37 3.2.2 Wave propagation through the atmosphere . 40 3.3 Figures of Merit . 41 3.3.1 The Fried parameter r0 . 41 3.3.2 The isoplanatic angle θ0 . 44 3.3.3 The coherence time ¿0 . 46 2 3.3.4 The index of scintillation σI . 47 3.4 Turbulence Distribution . 49 3.5 Beyond Kolmogorov . 51 4 Turbulence Pro¯ling : a Review 53 4.1 Optical Seeing Monitors . 53 4.1.1 DIMM . 53 4.1.2 GSM . 57 4.2 SCIDAR . 60 4.3 MASS . 63 4.4 SLODAR . 65 4.5 Microthermal Sensors and other in situ methods . 66 4.6 SODAR . 69 4.7 The Antarctic Campaign . 73 5 Boundary Layer Turbulence 77 5.1 Our Instrument . 77 5.2 Calibration . 78 5.2.1 Integration time . 78 5.2.2 Range . 79 5.2.3 Experimental setup . 80 5.2.4 The wind speed measurements . 82 5.2.5 Calibration coe±cient of proportionality . 86 5.2.6 Detection threshold . 87 5.2.7 Wind speed comparison . 88 5.3 South Pole Campaign 2001-2002 . 90 5.3.1 Turbulence and boundary layer evolution . 90 5.3.2 Meteorological parameters . 91 CONTENTS ix 5.3.3 Seeing . 97 5.3.4 A telescope on a tower . 98 5.3.5 Other astronomical parameters . 99 5.4 Dome C Campaign 2003-2004 . 102 5.4.1 Long and short scale temporal variations . 103 5.4.2 Boundary layer seeing . 106 6 Integrated Seeing 109 6.1 South Pole Campaign 2001-2002 . 109 6.1.1 System description . 110 6.1.2 Data processing . 112 6.1.3 Results . 114 6.1.4 Temporal variations . 115 6.2 Dome C Campaign 2003-2004 . 119 6.2.1 Daytime DIMM results . 119 6.2.2 Preliminary MASS results . 124 7 Adaptive Optics Performance 127 7.1 General Considerations . 127 7.2 Ground Layer Adaptive Optics . 128 7.2.1 The Simulation and GLAO principles . 130 7.3 The South Pole Simulation . 132 2 7.3.1 The CN pro¯les . 132 7.3.2 South Pole Results and Discussion . 133 7.4 Dome C Simulation . 139 7.4.1 The pro¯les . 139 7.4.2 Dome C Results and Discussion . 141 7.4.3 Simulation conclusions . 143 8 Conclusions 147 x CONTENTS List of Figures 1.1 ATRAN (Lord (1993)) simulation of the sky transmission be- tween 1 micron and 10 cm at Mauna Kea (Burton et al. (1994)). 4 2.1 Topographic map of the Antarctic continent showing the ma- jor region of interest.(Taken from Parish and Cassano (2002)) 14 2.2 Katabatic flows on the Antarctic continent (Taken from Parish and Bromwich (1987)). 18 2.3 Monthly average temperatures at South Pole (26 year aver- age) and Dome C (5 year average) . 21 2.4 Typical winter temperature pro¯le at South Pole taken with a balloon borne radiosonde. 22 2.5 Mean monthly wind speed at the South Pole (1977 to 1998).
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