| I. Review Paper: MECHANISM OF FORMATION OF OPTICAL TURBULENCE Jean Vernin |
Optical turbulence arises when two conditions are fulfilled: Dynamical turbulence must be present in a medium in which the field of refractive index is stratified. The first condition is described by the so-called Kolmogorov
turbulence, when the energy cascades from large eddies to
smaller ones. The second condition is encountered in an
atmosphere where the vertical gradient of potential
temperature, or density, is not zero.
If one assumes that temperature is a passive and
conservative additive, like did Yaglom and Obukhov, 1949,
its spectral law is identical to that of the velocity
filed, i.e k^(-5/3). But we proved that temperature is not a passive additive and that viscosity, at small eddy size,
is very efficient and homogenizes very soon the temperature
field. This gives rise to two thin laminae of optical
turbulence at the boundary of a thick dynamical turbulent layer.
With this new phenomenological description one can
explain why the outer scale of optical turbulence is
so small, when compared to the outer scale of dynamical
turbulence. Other authors simulate such a stratified medium,
leading to the same conclusion. |
| I.1 CHARACTERISTICS OF THE ATMOSPHERIC TURBULENCE IN THE NATIONAL ASTRONOMICAL OBSERVATORY OF BOGOTA William Cepeda |
The National Observatory pretends to determine the characteristic of the atmospheric turbulence in the National Astronomical Observatory of Bogota.
A net of the meteorological stations in Bogota City register hourly the wind velocity and temperature in two highs. These meteorological components are important in the study of the atmospheric turbulence.
The turbulence is complex phenomenon that occurs in a flow. In the turbulence atmospheric is necessary to introduce the Reynolds equations. The solutions of these equations are very complex. In this paper it's apply The Similarity Theory of Monin-Obukhov. The principal parameter is the high of the atmospheric Boundary Layer. In the contemporaneous models of dispersion is necessary to know the high of the mixer layer zi, the Monin-Obukhov scale, L, temperature scale T* and friction velocity of the wind u*. The knowledge of the Characteristic of The Atmospheric Turbulence in The National Astronomical Observatory of Bogota, allow to predict the atmospheric optical quality.
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| I.2 ATMOSPHERIC TURBULENCE PARAMETERS MEASUREMENT Vladimir Lukin |
In this paper I would like to present some approaches for set-ups and results for atmospheric turbulence parameters
measuring. I have about 25 years expireance for working under problem of atmospheric remote sensing. I am presenting my results for spectral density, inner scale, outer scale and anisotropical parameters measurements with optical wave propagating through a layer of atmosphere.
Using the experimental data I would suggested to estigate
behavior of spectral density of atmospheric turbulence in the region of large spatial scales. Special efforts will be done to detect the variability of large optical inhomogeneities as manifestation of the influence of thermodynamic instability of the atmosphere. In practical calculations of fluctuations of the optical waves various models are used to describe the spectrum in the region of large scales: von Karman, Greenwood-Tarazino, and Russian models. These models have already had two parameters, one of which was so-called outer scale of turbulence.
I have had final analysis of data on the astroclimatic
characteristics obtained in the region of the Elbrus mountain. In our analysis of the experimental data I assume that the atmosphere is atratified and its inhomogeneities have the shape of elongated ellipsoids of revolution. This implies that: (1) in the case of vertical propagation (precisely along the zenith direction) the image jitter must be practically isotropic, (2) the anisotropy of the jitter of an optical source image must be maximum for the case of horizontal propagation and will be determined by the atmospheric instability. I have investigated effective outer scale of turbulence for imaging through atmosphere with different models of vertical evolution of structure parametrs and outer scale of turbulence.
Additionally, I am creating some new approaches for tip-tilt correction under knowledge of models of atmospheric turbulence, structure parameters and outer scale of turbulence.
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| I.3 OBSERVING AT OUKAIMEDEN THROUGH THE EXTENDED WAVE SYSTEM OF THE UPPER ATMOSPHERE EMISSIVE LAYER G. Moreels, D. Pautet, P. Rousselot, C. Reyli & J. Clairemidi |
A cloudless sky observed from a high-altitude site presents in the near-infrared part of the spectrum
an aspect that is completely different from the one seen in the visible. Observations conducted at the
Pic de Chbteaurenard (Altitude : 2989m, 44042'N, 6054'E) in the I band at 800 nm shows that the atmospheric
emission resulting of chemical reactions around the 85 km level appears as an extended wave system.
A panorama of the sky shows wide archs extending from a point in the W-NW horizon to the diameter
opposite E-SE point. When the perspective is inverted, the image appears as a satellite view of the
emission over a 1100 km radius circular area showing an extended wave system. The emission, mainly due to
the OH radical is intense and varies during the night by as much as 50%. A detailed knowledge of this
emission is essential to conduct precise photometric observations and spectroscopic identifications. In
addition, the wave system induces intensity variations that are about 20% of the emission.
Measured periods of these waves are 16, 8.9 and 5.7 minutes, with an horizontal speed equal to 42 m/s.
In addition, the OH spectrum consists of numerous lines that may be used to calibrate spectra between
700 and 3000 nm. Given the wide field that is covered from an observing station, we propose to conduct
measurements of the atmospheric emissive layer from the Oukaimeden site, given all the advantages of the
site : altitude, climate, quality of the sky. We also propose to conduct simultaneous measurements at
Oukaimeden and Chbteaurenard in order to obtain a coverage of the atmospheric layer extending in latitude
from 210 to 550. The object of this programme is to precisely evaluate the contribution of the atmospheric
emission to intensity of deep-sky objects.
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| I.4 ESTIMATION OF THE SPATIAL COHERENCE OUTER SCALE FOR DAYTIME OBSERVATIONS Nassim Seghouani |
The theoretical expression of the transverse covariance of angle-of-arrival fluctuations of the
incoming wave-fronts integrated all over the telescope pupil has been established in the case of one-layer
model and using the Von Karman theory. This model depends on three atmospheric parameters: the spatial
coherence outer scale L0, the Fried parameter r0 and the size of isoplanatic patch I0. We use this model
to analyse the fluctuations observed on the solar limb, which finally consist on the estimation of the
three parameters L0, r0 and I0. The value of the spatial coherence outer scale L0 for day-time observations
is not known. Its estimation is particularly important in the modelisation of atmospheric effects for solar
observations with high angular resolution (solar diameter measurements at astrolabe for example). It is
shown that the values obtained for L0 are smaller than those found for night observations. Two methods have
been developed to estimate these parameters. They have been investigated and applied to solar images
recorded at Calern Observatory Astrolabe (France). The obtained results are presented and discussed.
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| P.I.1 (poster) SATURATION MECHANISM OF THE TURBULENCE AMPLIFICATION USING HIGH SPECTRAL AND ANGULAR RESOLUTION OF VLT AND VLTI Merieme Chadid |
We know from laboratory experiments that the turbulence
level of a gas increases when it is crossed by a shock wave.
The theory correctly predicts the observational amplification rate up to
small Mach numbers (M<3). But, the existence of a supersonic turbulence is
still unknown. Moreover, due to technical limitations, laboratory experiments
cannot provide the solution. In this paper, we provide a
method to determe the amplification rate versus the Mach number by the
observation of strong shock waves occurring within the supersonic atmosphere
of pulsating stars using the spectral high resolution
spectrograph UVES of the VLT instrument. Moreover, some future plans are
mentionned to study semi-quantitatively this physical phenomenon with help of
the high angular resolution VLTi-mode. |
Updated 2000, August 23