Microthermal Measurements of Optical Turbulence - Toulouse, March 1993

Rodney Marks, School of Physics, University of New South Wales

The general aim of this experiment, a Franco - Australian
collaboration, is to investigate the seeing conditions at the South
Pole Station, the only easily accessible site with comparable (but
inferior) atmospheric conditions to Dome A, in order to determine the
extent of the improvements in seeing we can expect to obtain in
Antarctica. Atmospheric seeing is due to wavefront perturbations caused
by turbulence occurring in thin horizontal layers, on a scale of
several cm to ~100m. Turbulent mixing creates temperature and
refractive index inhomogeneities on the same scale as the eddy sizes.
The seeing can be determined by measuring these minute temperature
fluctuations, and integrating over all turbulent layers in the
atmosphere. This is the problem presently being studied by Jean Vernin,
of the University of Nice, with the aim of perfecting the microthermal
measurement techniques, and applying them to the testing of potential
observatory sites in Antarctica, and elsewhere in the world.

The experiment involves launching meteorological ballons, with a gondola
containing the temperature sensors, consisting of the filaments of a
small light globe, the resistance of which are extremely sensitive to
temperature.  Also attached are the necessary electronics to amplify
and process the signal from the sensor circuitry, and a standard
meteorological "radiosonde", which provides information on temperature,
pressure, wind velocity, and altitude. With the various balsa wood
attachments for the sensors, and to prevent the gondola from tilting
and spinning, the overall contraption can end up looking somewhat like
a Wright Brothers aircraft!  The balloons are inflated so as to ascend
at about 3-4 m/s, to a height of 20-30 km, at which point they explode
(due to the low external pressure) and the gondola descends by
parachute.

Both the radiosonde and the microthermal circuitry are tracked by a
Digicora station, which sends the data to a computer every 1-2s, where
it is stored for late later analysis. The raw microthermal data is
converted to a temperature structure function Ct^2 at each point
in the ascent, which gives the corresponding refractive index structure
function Cn^2, with which we can plot a vertical "map" of the optical
turbulence encountered at different altitudes.

The current experiment is being conducted at Meteo France in Toulouse,
the central weather station for France. Here, we have easy access to
the necessary Digicora equipment, as well as a convenient launching
facility.  The experiment is not directly concerned with optical
seeing, but has two other aims. The first is to iron out some of the
technical problems, and generally improve the operation of the
experiment. Problems have been encountered with the stability of the
temperature sensors at equilibrium, and in maintaining the radiosonde
at an adequqte temperature (>0 C) for the duration of the flight,
around 2-3 hrs. The external temperature drops to around -50/60 C at
10km, and it is necessary to heat the internal circuitry of the
radiosonde for it to continue functioning. We are currently working on
variations to provide better thermal stability for longer periods.

The second part of the experiment is a detailed "Kolmogorov" (the
founder of theoretical modelling of turbulent media) study. Based on
his theory, the power spectrum of the turbulence is expected to follow
a -5/3 power law.  The accuracy of this has not been tested
conclusively to date, and it is a side project of Vernin,s to do so,
using pairs of sensors at different sep separations on the same
gondola.

Following Toulouse, we are now working on improvements to the
simplicity and reliability of the experiment, and will possibly carry
out another test launch at Toulouse in April. After that, we will begin
working with the Americans running South Pole scientific operations,
with the aim of setting up microthermal experiments (using both
balloons and the South Pole mast) to be conducted during the 1994
winter...