Optical Turbulence On The Antarctic Plateau - School of Physics

188 Pages·2004·2.57 MB·English

Optical Turbulence On The
Antarctic Plateau
Tony Travouillon
Submitted in total fulfilment of the requirements
of the degree of Doctor of Philosophy
School of Physics
University of New South Wales
September 2004

Abstract
AtmosphericturbulenceresultstakenontheAntarcticplateauarepresented
in this thesis. Covering two high sites: South Pole and Dome C, this work
describes their seeing and meteorological conditions.
Usinganacousticsoundertostudytheturbulenceprofileofthefirstkilo-
metre of the atmosphere and a Differential Image Motion Monitor (DIMM)
to investigate the integrated seeing we are able to deduce important at-
mospheric parameters such as the Fried parameter (r ) and the isoplanatic
0
angle (θ ). It was found that at the two sites, the free atmosphere (above
0
the first kilometer) was extremely stable and contributed between 0.2(cid:48)(cid:48) and
0.3(cid:48)(cid:48) of the total seeing with no evidence of jet or vortex peaks of strong
turbulence. The boundary layer turbulence is what differentiates 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 difference results in
relatively poor seeing conditions at the South Pole ((cid:39)1.8(cid:48)(cid:48)) and excellent at
Dome C (0.27(cid:48)(cid:48)). 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 3.3(cid:48)(cid:48)
and 5.7(cid:48)(cid:48) for the South Pole and Dome C. This is significantly larger than
at temperate sites where the average isoplanatic angle rarely exceeds 2(cid:48)(cid:48).
This means that wider fields can be corrected without the complication of
conjugation to specific layers. For such purpose the potential is even more
ii
interesting. Weshowthatgroundconjugatedadaptiveopticswoulddecrease
the natural seeing to 0.22(cid:48)(cid:48) for a wide field of 1(cid:48) and 0.47(cid:48)(cid:48) for a field of 10(cid:48)
at the South Pole. At Dome C the results are less impressive due to the
already excellent seeing, but a gain of 0.1(cid:48)(cid:48) can still be achieved over 10(cid:48).
Theseresultsshowthathighangularresolutionobservationscanbedone
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 first thoughts go to my supervisors Michael Burton and John Storey
whohaveobviouslydonethisjobbeforesincetheyhavegivenmetheperfect
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 films. I hope one day to be able to give public speeches of your
standards (but without the help of a table...).
TotheAntarcticteam,thankyouforbeingwhoyouare: MichaelAshley,
I don’t know what we would have done without you. Honestly, how was I
supposed to guess that to fix a corrupted compressed file 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 first 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 first 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
yourgroup. EricAristidi,whodespitehisinabilitytoshaveproperly,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 . . . . . . . . . . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.3.1 Data acquisition . . . . . . . . . . . . . . . . . . . . . 23
2.3.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.3.3 Discussion and conclusion . . . . . . . . . . . . . . . . 32
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 Defining 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 r . . . . . . . . . . . . . . . . . 41
0
3.3.2 The isoplanatic angle θ . . . . . . . . . . . . . . . . . 44
0
3.3.3 The coherence time τ . . . . . . . . . . . . . . . . . . 46
0
3.3.4 The index of scintillation σ2 . . . . . . . . . . . . . . . 47
I
3.4 DIMM Theory . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.5 SODAR Theory . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.6 Turbulence Distribution . . . . . . . . . . . . . . . . . . . . . 52
3.7 Beyond Kolmogorov . . . . . . . . . . . . . . . . . . . . . . . 54
4 Turbulence Profiling : a Review 57
4.1 Optical Seeing Monitors . . . . . . . . . . . . . . . . . . . . . 57
4.1.1 DIMM . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.1.2 GSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2 SCIDAR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.3 MASS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
4.4 SLODAR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.5 Microthermal Sensors and other in situ methods . . . . . . . 69
4.6 SODAR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.7 The Antarctic Campaign . . . . . . . . . . . . . . . . . . . . . 75
5 Boundary Layer Turbulence 79
5.1 Our Instrument . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.2 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.2.1 Integration time . . . . . . . . . . . . . . . . . . . . . 81
5.2.2 Range . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.2.3 Experimental setup. . . . . . . . . . . . . . . . . . . . 83
5.2.4 The wind speed measurements . . . . . . . . . . . . . 88
5.2.5 Calibration coefficient of proportionality . . . . . . . . 89
5.2.6 Detection threshold . . . . . . . . . . . . . . . . . . . 89
5.2.7 Wind speed comparison . . . . . . . . . . . . . . . . . 90
5.3 South Pole Campaign 2001-2002 . . . . . . . . . . . . . . . . 93