Arctic astronomy
Liviu Ivanescu, ESO (livanesc@eso.org)
Studies
about the quality of the poles for astronomical observations are very recent
and only at the South Pole. There, the quality of the seeing at only 100-200
meters elevations was found to be much better than anywhere else. More, there
are new infrared windows to the sky. In Antarctica there are no peaks, only domes and the bad seeing may eventually go up
if there are weather instabilities around. At the North Pole there is an ocean
and the little information about the quality of the sky comes from the ground
or ocean level only. Most of the information is for the summers that are
characterized by damp and foggy weather and weak cyclones with rain or snow. However,
due to the freezing of the ocean, the wintertime is very different. The most
significant feature of the polar night is a massive radiation inversion which
allows a cold and very stable weather with very clear skies. Absolute humidity
tends to be quite low and the precipitations are lower than that in Sahara
or Chile. More, on the ocean border there are some interesting
high peaks (more than 2000 m) and quite close to the pole. The fact that winds
are never very strong is a good point not only for seeing reasons but in the
case of building there a large facility for astronomical purpose too.
Introduction
The quality of a particular site
for astronomical purpose may be affected by access facility or by different
interests to economically develop some remote areas. However, it focuses mostly
on the environment quality: meteorological and earthquakes parameters. The last
ones are not a big issue as long they are in normal range. The most important
are the atmosphere characteristics which affect the quality of astronomical
images by the transparency of the sky (less transparency, more noisy images)
and by the turbulence (more turbulence, less angular resolution). Some of the
most important meteorological parameters laying in the first category are:
cloud amounts; cloud opacity; clouds altitude (to know what happens at
different elevations); absolute humidity; visibility; aerosols (haze);
precipitations; precipitable water vapor (this changes the observational
parameter known “air mass”); aurora. For the second category one may note: wind
speed; gust speed; wind speed variations; wind direction variations; wind,
pressure and temperature altitude profiles; temperature variations. One should
note that all this parameters are most significant close to the ground level.
The easiest way to get out of them is to put astronomical observatories on the
top of the mountains. For this reason one starts with some topographic
considerations and then a meteorological analysis.
Topography
The
atmosphere is thicker at the equator and thinner at the poles. One may expect
that the ground problematic layers of the atmosphere are even closer to the
ground too. In this way, even relatively low elevations near the poles become
interesting. The Figure 1 shows a map of the surrounding topography of the
North Pole. One may see that the northern Canada
and Greenland (which are the closest to the North Pole)
present unexpected high peaks for the ones thinking that at the North Pole it’s
just an ocean. The highest point in that area is Barbeau peak of 2616 m (8544
feet) at 81o56’N, 75o00’W, in the Canadian island
Ellesmere. Actually this northernmost island has some of the highest peaks in Canada
(Figure 2). There are there other peaks of more than 2000 m but
no one as close to the pole as Barbeau. However local weather conditions may
make them eventually good astronomical sites as well and should be investigated
further. In the Figure 3 one may see some of these interesting tops spotted
out in red. The Figure 4 shows a view of the Barbeau
Peak, while Figure 5 and Figure
6 show surrounding views from there.
Figure 1 North Pole topography
Figure 2 Relief of Canada
Figure 3 Ellesmere Island
Figure
4 Barbeau Peak.
Photo by Eric Phillips (icetrek.com)
Figure 5 View from Barbeau
Peak (I). Photo by Eric Phillips
(icetrek.com)
Figure 6 View from Barbeau
Peak (II). Photo by Eric Phillips
(icetrek.com)
In the
northern Greenland (Peary Land)
there is the northernmost mountain range on Earth, the Roosevelt
Range. The highest peak there is
Helvetia Tinde (Swiss Peak) of 1929 m (6300 feet) at 83o20’N, 34o40’W.
One may see it in the Figure
7 spotted out in red, as well as another slightly lower
peak of 1898 m (6200 feet). A picture of this second one as well as a
surrounding view to the north may be seen in the Figure 8.
Figure 7 Northern Greenland
(Peary Land)
Figure 8 Second highest peak
(1898 m) in Peary Land, Northern Greenland
Accessibility
Ellesmere
Island is far the easiest to access and has the easiest logistical
support (Figure 9). First of all there are already there two important
facilities active 12 moths a year: the Eureka
weather station (79o59’N, 85o59’W) and the Alert military
station (82o30’N, 62o30’W) with hundreds of people all
around the year and with Hercules airplane supplies few times a year. Satellite
communications are already in place as well. The closest facility to Barbeau
Peak is the Hazen Camp weather
station (on the border of Hazen Lake
- Figure 3) which is open in summer time only. Secondly,
Canadian research programs developed already a network of supply bases to support
accessibility and logistics to the North (airplanes, helicopters etc.). These
programs support financially as well arctic research projects, as the Canadian
government is interested to develop activities in the North. Among these
programs are PCSP (Polar Continental Shelf Project), DIAND, AINA, RCGS etc. Both,
Ellesmere Island and Peary Land,
can be easily accessed in summer time by the open water ocean.
Figure 9 Accessibility
to Ellesmere Island
Figure 10 Camps and Supply stations
near Grand Land
Mountains (where is the Barbeau
Peak)
Transparency of the sky
The most
significant feature of the northern polar night, which is important for
astronomy, is a massive radiation inversion. This is a stable and deep feature
which calms down most the meteorological activity. At the Eureka station, as experienced during the LIDAR measurements during the
winter, there is very little cloud cover, and so useful observations are
possible on about 80% of the nights.
The sites of
interest here are close to the magnetic North Pole, in the center of the auroral
oval (Figure 11). The brightness of the aurora is therefore a minor
observational issue.
More, over
the winter the ice in the Arctic produce a significant
albedo which reflects back to space about 90% of the incoming solar energy in
late falls to early springs. This means that the Earth will irradiate much less
energy in the infrared. In the same time, the albedo
drops down the ground and atmosphere temperature of the polar nights.
The arctic
haze, which is an important amount of aerosols coming from Eurasian industrial
areas, is clearly seen in this clean environment. It has an optical depth of
0.1 at most (referenced to vertical, at 500 nm) and it’s observed especially in
late winter, early spring months. The wavelength dependence is about 1 over
lambda. The haze is primarily below 1 km altitude, though one finds
occasionally bands up to 5 km. Moreover, this is present mostly along the
costal borders. The Barbeau peak should not experience any such contamination.
In general
poor visibility occurs more often in summer because of the more frequent
occurrences of the fog at low elevations. Poor visibility commonly occurs at
higher elevations just prior to the beginning of the melt. This is due to the
blowing of the uncompacted snow available in that period. To avoid such thing
it may be possible to remove the small quantity of multi-secular snow over the
peak.
Near the
North Pole one finds too a diminished ozone concentration, by 10 to 40% in some
areas. This may eventually allow ultra-violet astronomical observations.
The
atmospheric water vapor makes important absorptions of the light, mostly in
infrared. As this parameter is not very significant in the Artic winter, new
infrared windows are opened for astronomy. Extremely, this water vapor allows
clouds formation too, associated obviously with a very important opacity
effect. A first indirect indication of this basic parameter and its effects is
getting from the precipitations statistics.
The Astronomical Polar Night is a period of the arctic
winter north of 84°33'N, when horizon during astronomical twilight. The Nautical Polar Night
is a period of the arctic winter north of 78°33'N, when the most light is a
faint glow in the southern sky, but it is impossible for an observer to make
out any horizon. Alert and Eureka
stations experience Nautical Polar Night.
Precipitations
The Arctic
is a cold desert and the northern areas are among the driest on Earth with less
than 100 mm total precipitations per year, as in Ellesmere Island
or Peary Land. Most of that is received in the summer due
to the increase in moisture capacity of the warmer air and an increased area of
open water in the Artic Ocean.
For example, not far from the Barbeau
Peak, Eureka
station (Figure 9), at an altitude of 10 m, has an average of only 18.6
mm for the 6 month period November to April. For the same period, at Alert, not
very far from Barbeau either (Figure 3, Figure
9), one has 45 mm precipitations. This is somehow
more because of the influence of the Arctic Ocean, but
still very dry. Measurements well inside Pearly
Land at 82o10’N, 30o30’W
shows 14 mm of precipitations for the same period.
It’s well
known that good astronomical sites are at high elevations, slightly inside the
land (as the Chilean border) or inside islands (Hawaii).
One needs high rising peaks to access high atmospheric layers without changing
them very much, but not exactly on the sea border because there is too much
moisture. The Roosevelt Range
in Peary Land seems to be more exposed to the moisture
of the Arctic Ocean. Going more inside Greenland
is definitely drier but there one finds too the high and wide ice plateau which
changes the stability of the atmosphere (the top of this Plateau is a very
windy place). However this should be investigated further. On the other hand, the
Barbeau Peak
in Ellesmere Island has all the characteristics of a good
site. Actually, as a proof of very stable and dry weather conditions inside the
Ellesmere Island is Hazen
Lake, very close to Barbeau
Peak (Figure 3). The annual precipitations there are 25 mm, 2.7
times less than Eureka. So, one may
expect for the winter period a total of less than 7 mm. This means that at the
Barbeau elevation the precipitations should be close to 0 mm for the winter
nighttime, which makes it the driest astronomical site known. By comparison, at
Paranal peak in Chile at 2635 m altitude, considered to be the actual best site,
on has around 10 mm/year, so about 5 mm per annual nighttime.
Form a large
scale statistical point of view, the northern Canadian islands, including the Ellesmere
Island, is the driest area in the Artic (Figure 12). On some parts of the land area, there are a lot of
interpolations because of the small number of weather stations.
Figure 11 Auroral oval
Figure 12 Mean arctic precipitations in
February (from Arctic Meteorology and Climate Atlas)
Satellite data analysis
The raw data was recorded with AVHRR
(Advanced Very High Resolution Radiometer) satellite sensor carried on NOAA
polar-orbiting satellites, at 04:00
and 14:00 Local Time (LT), from 1982
to 1999. There are three different data sets, at 1.25, 5 and 25 Km resolutions.
The atmospheric parameters where then retrieved by post-processing (CASPR
program) and they are available as monthly means with 25 Km resolution at:
http://stratus.ssec.wisc.edu/products/appx/.
The algorithms used to obtain the results are explained there as well.
On a 25 Km
grid, the atmospheric parameters for sharp high peaks are averaged out. On a
future step, I will compute clear sky chart at 1.25 Km resolution as well. For
the present study I considered only the data at 04:00
LT, for January (winter) and July (summer).
The “clear
sky” means percentage of time with no clouds of any kind, including cirrus.
This is what we call a “photometric sky”. The “mm precipitable water” is a
vertical column of precipitable water vapor present in the atmosphere,
expressed in millimeters. The spatial resolution here is lower. The “surface
temperature C” is expressed in degrees Celsius (C).
As a comparison, in Paranal there
are actually 76% photometric nights (averaged all over the year), while the
precipitable water vapor is 1.5 mm in the winter (in the summer is more).
Figure 13 Winter atmospheric conditions
in the Arctic
Figure 14 Summer atmospheric conditions
in the Arctic
