Gilbert Ricort

CONTRIBUTION TO THE OBSERVATION OF THE PHOTOSPHERIC OSCILLATIONS

Observations of the 300 s photospheric oscillation on large solar surfaces (up to 5′20″ in diameter) using a sodium optical resonance cell seem to show that the power at long horizontal wavelengths is larger than previous results would indicate. In order to get more information about the spatial distribution of the energy, a new observational method has been perfected, which will allow us to obtain the spatiotemporal power spectrum.In some of our observations, a long-period oscillation (about 40 min) appears, with an amplitude comparable to that of the 300-s oscillation, and which seems to be correlated with the occurence of chromospheric flares.

Contribution to the space-time study of stellar speckle patterns

The temporal behavior of stellar speckle patterns is statistically analyzed. The time-only power spectrum is shown to be the sum of two exponentially decreasing functions defining two characteristic time constants. The corresponding correlation is the sum of two Lorentzian functions. This is consistent with the first-order expansion of the power spectrum deduced from the multiple-layer model for atmospheric turbulence. However, this model fails to account for the experimental data that show a strong correlation between the spatial structure of a speckle pattern and its temporal behavior. This leads to the introduction of a new empirical model, called the randomly jittered speckle pattern model, which gives a preponderant place to image motion. The speckle lifetime then appears to be substantially longer than the corresponding measured time constant. As a consequence, a preliminary compensation of the image motion appears to be particularly interesting in speckle interferometry or active optics experiments.

Determination of Fried's parameter r 0 prediction for the observed r.m.s. contrast in solar granulation

Frieds seeing parameter r0 can be determined from the observed r.m.s. contrast of solar granulation. The computation is based on results obtained from solar speckle interferometry. The influence of the telescope diameter on solar observations is shown graphically, and predictions for the observed contrast in solar granulation are made as a function of wavelength. The importance of seeing conditions is discussed, and a conclusion is made on the optimum telescope size to be used for given turbulence conditions

Measurements of Stellar Speckle Interferometry Lens-atmosphere Modulation Transfer Function

Lens-atmosphere modulation transfer function measurements are presented for speckle interferometry using the 193 cm telescope of the Observatoire de Haute Provence, France. A single photoelectric scanning detector is used to record the speckle pattern, and real-time data processing is achieved using a power spectrum analyser. Normal and log-normal models are used to compare theory with experiment. The results are in substantial agreement with Korffs log-normal assumptions.

Defocusing effects in astronomical speckle interferometry

Abstract Theoretical transfer functions for astronomical speckle interferometry, through a defocused telescope, have been computed assuming a log-normal complex amplitude for the incoming wavefront. They are found in reasonable agreement with recently published observations. The theory also applies to the effect of astigmatism.

Direct measurement of the formation height difference of the 630 nm Fe I solar lines

Context. Spectral lines formed over a limited height range in either a stellar or planetary atmosphere provide us with information about the physical conditions within this height range. In this context, an important quantity is the so-called line formation depth. It is usually determined from numerical calculation of the atmospheric opacity in the line of interest and then converted into geometrical depth by using atmospheric models. Aims. We develop a radically different approach, which allows us to measure directly line formation depths from spectroscopic observations without relying on assumptions about an atmospheric model. This method requires spatially resolved observations, which up to now have been available only for solar or planetary studies. We apply this method to images of the solar granulation. Methods. The method was presented and tested numerically in previous papers. It is based on the measurement of the perspective shift between images at different wavelengths, formed at different heights, when they are observed away from disk center. Because of the Fourier transform properties, this shift gives rise to a deterministic linear phase term in the cross spectrum of the images. Results. The method is applied to observations of solar quiet regions performed with the SOT spectropolarimeter on HINODE in the Fe I line pair at 630.15 and 630.25 nm. We derive the difference in formation heights between the two lines and its center-to-limb variations. We show that the high sensitivity of the measurements allows us to detect variations in the line formation heights between magnetized and non-magnetized regions of the solar atmosphere. Conclusions. Our results are the first direct measurements of line formation depths in the solar photosphere. Cross spectral analysis provides us with a new observable quantity, which may be measured with an accuracy well bellow the spatial resolution of the observations. We recall that the Fe I line pair at 630.15 and 630.25 nm is often used to determine solar magnetic fields by spectropolarimetric observations and inversion methods. The difference in the line formation heights that we measure should be taken into account in the inversion procedures.

Differential speckle interferometry: in-depth analysis of the solar photosphere

Aims.We present the results of an experiment performed at the solar telescope THEMIS in 2002 to measure the depth over which the solar granulation extends in the photosphere. Methods: Observations made in the 523.3 nm and 557.6 nm photospheric non-magnetic iron lines were correlated with images in the continuum using spectrograms. The difference in depth between the different levels in the photosphere is projected into a difference of position along the slit of the spectrograph, using a perspective effect similar to the well-known Wilson effect for sunspots. This requires measuring displacements, ones much smaller than the telescope resolution. This is made possible by using a differential speckle interferometric technique, cross-correlating images taken in the continuum and the line. The method is not adapted to following displacements of structures in the core of strong lines, due to their difference in shapes with the structures observed in the continuum. In this case, a sequential cross-spectrum method is developed to cross-correlate images taken at close wavelengths. Results: The raw results are surprising: displacements measured in the blue and the red wings of a line have opposite signs! North and South observations, however, clearly show the expected behavior attributed to a perspective effect. After a description of the observations, we give a first interpretation of the results. The main part of the observed displacement comes from the effect of unresolved Doppler shifts produced by horizontal velocities in the solar photosphere. The perspective effect we seek appears as a second-order term; we find that its amplitude is 2 or 3 times larger than predicted by theoretical 1D models. In the core of strong lines we detect a contrast inversion that also shows up in the cross-correlation function as an anti-correlation peak at line center. Conclusions: .This first use of the differential speckle interferometry technique on the Sun is quite promising for 3D studies at high spatial resolution. Further observations with very good image quality are needed to take advantage of this new technique. THEMIS is operated on the Island of Tenerife by CNRS-CNR in the Spanish Observatorio del Teide of the Instituto de Astrofisica de Canarias.

Changes in the atmospheric-lens modulation transfer function used for calibration in solar speckle interferometry

A method is described for the calibration of power spectra of nonstellar astronomical objects, by the use of changes in seeing conditions during speckle-interferometric measurements. Results are given for an application of this technique to solar granulation. The correction of 40 analyses, determined with Fried’s parameter r0 ranging between 2.5 and 11.5 cm, provides satisfactory convergence, and thus permits estimation of the solar granulation power spectrum for frequencies up to 2.5 arc sec−1.

A comparison between estimations of Fried's parameter r o simultaneously obtained by measurements of solar granulation contrast and of the variance of angle-of-arrival fluctuations

Estimations of Frieds parameter ro are performed simultaneously using the same telescope, from the observed solar granulation contrast and from the variance of angle-of-arrival fluctuations. The results are well correlated and the mean ratio between the values for ro obtained from the 2 methods is found to be equal to 1.4.The sensitivity and accuracy of the methods are discussed in terms of their range of application (site testing, seeing measurements during solar observations ...). Temporal power spectra of turbulent energy are computed and evidence of long period oscillations of about 10 min is found.

Temporal autocorrelation functions of solar speckle pattern

Abstract A study of day-time temporal autocorrelation functions of speckles was performed on the solar granulation with the McMath telescope of the Kitt Peak National Observatory. In the space of only a few minutes, rapid variations of time constants are observed. For some observations, a single and high value T of time-constant is obtained: average = 170 ms, standard deviation = 80 ms. The temporal autocorrelation functions are consistent with theoretical curves obtained from a single layer atmospheric turbulence model. For other observations, a super-imposed short time-constant τ, showing little variations around 5.5 ms (standard deviation 1 ms), is also present. It is shown that T depends on the diameter of the telescope D and that the value τ of the short time scale, when it occurs, is correlated with the Fried parameter r 0 .