Dome C

Where is the best site on Earth?
Domes A, B, C and F,
and Ridges A and B
Will Saunders 1,2 , Jon Lawrence 1,2,3 , John Storey 1 , Michael
Ashley 1 , Seiji Kato 4 , Pat Minnis 4 , David Winker 4 , Guiping Liu 5
1 University of New South Wales. 2 Anglo-Australian Australian Observatory.
3 Macquarie University. 4 NASA Langley Research Center.
5 Space Sciences Lab, University of California Berkeley.
PASP, almost in press

Site comparison includes
Boundary layer thickness
Cloud cover
Auroral emission
Airglow
Atmospheric thermal backgrounds
Precipitable water vapour (PWV)
Telescope thermal backgrounds
Free atmospheric seeing
Doesn't include
Non-winter use
Sky coverage
Dark time
Infrastructure
Non-astronomical uses
F
A
C
B

•F
•A
•G
B
•C
Site Latitude Longitude Elevation Λ(2010)
Dome A 80.37S 77.53E 4083m -84.9°
Dome C 75.06S 123.23E 3233m -84.1°
Dome F 77.19S 39.42E 3810m -77.0°
Ridge B 76-79°S 97-93°E 3700-3809m -85.4° -87.1°
South Pole 90°S 0°E 2800m -80.0°

BOUNDARY LAYER
Swain & Gallee 2006a, 2006b;
van Lipzig 2004, Parish & Bromwich 2007
• Thinnest along
'katabatic ridge'
Dome A – Ridge B
• + Dome F, Dome C
• Dome F is best
• Dome A is not quite
on katabatic ridge

CLOUD COVER
Aqua/MODIS satellite data, validated against GLAS, also CALIPSO
Spring
Summer
Autumn
Winter
• Best weather
along Dome F –
Dome A – Ridge
B – Dome C
• Best weather
misses Dome A
• Dome C has best
weather?

AURORAE
Dempsey et al 2005 results for Dome C incorrect
Used Hardy 1991 models, + South Pole
measurements from Dempsey et al 2005
Site Λ (2010)
South Pole -80°
Dome A -84.9°
Dome C -84.1°
Dome F -77.0°
Ridge B -85.4° -
87.1°
• Dome A, Dome C,
Ridge B very similar
• Not much dependence
on solar activity level,
but brightest at solar
minimum
• 20-30% increase at V,
100% at B
• Dome F much worse

AIRGLOW
From models of Liu et al 2008
OI + OH
Validated only for |latitude| < 40°
Not much dependence on local time
OI
OH
• OI much worse ( x 5) than
temperate sites
• OH ~25% worse
• Spring OH hole,
only Dome C can use

FREE ATMOSPERIC SEEING
• Data from NCAR/NCEP reanalysis, May-Aug, 1995 - 2008
• Use standard relations to get
C n
2
= 3.62 x 10 -5 (P g) -2/3 (1/θ dθ/dz |dV/dz| ) 4/3
• Assume |dV/dz| ∝ V
(i.e. profiles are self-similar)
• Use Trinquet et al (2008)
profile as template
⇒ C n2 profile at all positions

-60°
-75°
Seeing
FREE ATMOSPHERIC SEEING cont.
• Best between SP and DA
• Dome F very good
• large factors (x10!) in 'coherence volume'
• Predicts South Pole better than Dome C ?
• Very crude model, can do better!
Isoplanatic angle
Site ε 0
(") θ 0
(") τ 0
(ms)
South Pole 0.19 5.43 18.8
Dome A 0.22 5.57 16.0
Ridge A 0.21 6.29 17.6
Dome C 0.26 3.39 8.44
Dome F 0.21 5.17 15.4
Coherence time
Ridge B 0.23 4.07 11.1

ATMOSPHERIC THERMAL EMISSION
Temperature data from NCAR/NCEP reanalysis
Colder site ⇒ wider K dark
Coldest between South Pole and
Dome A
Only measurement of K dark
is at SP
Origin of K dark
background unknown
Factors of 1.5-2 in thermal IR
background between sites
(Lawrence 2004)
10000
1000
100
Sky Brightness (Jy.arcsec -2 )
10
1
0.1
0.01
1E-3
1E-4
1E-5
1E-6
3 4 5 6 7 8 9 10 11 12 13 14 15
W avelength (μm)

PRECIPITABLE WATER VAPOUR
Prediction from Swain and Gallee 2006b
Should have real data from MHS sensor
on NOAA-18 satellite.
• Factor of 2 variations
• Best between South Pole
and Dome A
• Dome F almost as good
• Dome C not so good

SURFACE TEMPERATURE
Prediction from Swain and Gallee 2006b
Measurement from MODIS on Aqua satellite
Variations quite small (few °)
Dome A 204.1
Dome C 204.9
Dome F 204.9
Ridge B 203.6
Dome B 206.9
Ridge A 203.5

DOME A versus 'RIDGE A'
• Boundary layer, weather, free atmospheric seeing, atmospheric
thermal emission, PWV, surface temperature ALL favour a site
nearer South Pole
• Obvious candidate is at the end of 'Ridge A' at (81.5°S 73.5°E 4053m)
•RA
•DA

'SUMMARY TABLE'
SP DA RB DC DF RA
Cloud Cover × √ √√ √√√ √√ √√
Boundary Layer × √ √√ √ √√√ √√
Aurorae × √ √ √ ×× √
Free seeing √√√ √√ √ √ √√√ √√√
IR sky brightness/PWV √ √√√ √√ √ √√√ √√√
Surface temperature × √√√ √√√ √√ √√ √√√

• The coldest surface temperatures are found along Dome F – Dome A – Ridge B
– Dome C. But offset from topographic ridgeline.
• The best winter weather is found along the same line, with same offset. Dome C
may have the best weather.
• Thinnest boundary layer is found at Dome F, Dome A, along Ridge B, and at
Dome C, in that order. Again there is an offset from the topographic ridge.
• Best expected free seeing, isoplanatic angle, coherence time is found between
Dome A, Dome F and the South Pole.
• The lowest wintertime atmospheric thermal emission, and the lowest
precipitable water vapour, is also found between Dome A, Dome F and the
South Pole.
• Domes A and C and Ridge B are all similar from an auroral point of view, with
significant contribution to the optical sky brightness. Dome F is much worse.
• Airglow is significantly brighter than for temperate sites, except possibly in the
spring, when there is an 'OH hole'. Dome C is the only site able to use this
opportunity.
• Dome A is clearly the best existing site. ’Ridge A’ (~150km southwest of Dome
A ), is likely to be even better.
• Dome F is a superb site, with the exception of auroral activity.
• The peak of Ridge B is a very good site but without any unique advantages.
• Dome C may be almost as good as any other site for 2.5m K
use.