Francis Laclare

PICARD: Simulataneous measurements of the solar diameter, differential rotation, solar constant and their variations

Abstract PICARD is a CNES micro-satellite mission due for flight by the end of 2002, named after the name of a French astronomer who first observed with consistency the solar diameter changes during the Maunder minimum in the 16th century. It consists of two instruments measuring (i) the solar diameter and differential rotation, and (ii) the total solar irradiance. These quantities are fundamental for the understanding of the solar-Terrestrial relations, e.g. the influence of the Sun on the Earths climate, and of the internal structure of the Sun. The continuous — or nearly continuous — viewing of the Sun from an appropriate orbit, the 5 minutes sampling rate and the very low noise measurements, will allow g-modes detection and precise diameter measurements besides accurately establishing the relationship between irradiance and diameter changes. Providing an absolute measure of the solar diameter to 1 milliarcsecond, PICARD is the first step towards instruments capable of accurate and perennial measurements, for the centuries to come, of the solar-terrestrial influence. The objectives of the mission, instrument capabilities, observing modes and performances are described.

Solar diameter measurements with Calern Observatory astrolabe and atmospheric turbulence effects

In order to deduce significant astrophysical results from solar diameter measurements it is necessary to take an accurate account of instrumental and atmospheric effects. This paper presents a comparison between visual and CCD camera measurements performed by means of the Calern Observatory solar astrolabe during the last 4 years; this allows us to evaluate visual measurements done previously (from 1975 to 1989). Then, a study of atmospheric effects is developed. From CCD measurements, the image quality, expressed by Frieds seeing parameter,r0, is estimated and related to the errors occurring in solar diameter measurements. A statistical analysis gives about 0.26 arc sec (or 0.13 arc sec for the semi-diameter) as the lowest value that this error may reach at Calern Observatory. One conclusion of this work is that it is important in the future to have image quality observations, obtained using a dedicated monitor, in order to evaluate and classify the measurements. A survey of the seeing might so lead to improve the precision of the results by weighting each diameter estimation and eventually to schedule the observations.

SHORT vs LONG TIME SERIES : EXAMPLE OF THE SOLAR DIAMETER

The significance of a time series obviously changes with its length and accuracy; data about the Sun are no exception. In this short paper, we compare the 23-year long series of visual observations of the solar semi-diameter at Calern Observatory with the activity cycle to find an apparent opposing phase. The comparison of the sub-series of the same, carried out during 26-month long measurements of SOHO/MDI (April 1996, June 1998), reveals an apparent phase with the same activity cycle, in line with most of the recent results. Consequently, the importance of long and coherent ground measurements of the solar diameter has to be stressed.

FEATURE EXTRACTION FROM SOLAR IMAGES USING WAVELET TRANSFORM: IMAGE CLEANING FOR APPLICATIONS TO SOLAR ASTROLABE EXPERIMENT

Good edge extraction from temporal series of solar images is fundamental in the solar astrolabe experiment. Noise and spots in images cause however, difficulties to extract an accurate solar edge. We present in this paper a new image-processing method which solves this problem and thus improves the characteristics of the solar astrolabe experiment. The method is based upon the use of wavelet transform in the solar image analysis. It is developed to remove image defects (parasite spots) and noise without reducing image resolution. Solar images obtained at Calern Observatory astrolabe (France) are then processed using this method. Solar edges of these images are extracted and trajectories reconstructed.

Spectral analysis of solar diameter measurements recorded at Calern Observatory astrolabe during two solar cycles

Solar diameter measurements performed with the Calern Observatory astrolabe (O.C.A - France) during more than two solar cycles show temporal variations. Due to the weather, seasonal effects and instrumental characteristics, recorded solar data are non uniformly sampled and present temporal gaps. Thus, to analyze these data, diameter measurements averaged over one or more months have generally been considered. This limits the accessible harmonic terms to a low frequency range. To determine short-term periodicities from the observed variations, all daily solar data need to be considered and also corrected from the zenithal distance. In the present work, we use two methods to analyze solar diameter measurements recorded at Calern Observatory astrolabe during the observation period 1975-1996. They are based on least square fits and deconvolution of the observation window function. Results deduced from the analysis confirm harmonic terms already found by other authors but also reveal new higher frequencies.

Atmospheric Turbulence and Solar Diameter Measurement

Object images obtained by mean of ground based instrument are degraded by the earth atmosphere. Indeed, the wave-front at the entrance of the instrument pupil, present phase and amplitude random fluctuations depending of the time, the position and the line of sight. The recorded images are consequently filtered leading to bad measurements of the studied object parameters. To qualify the atmospheric degradations, several parameters are commonly defined. For solar diameter measurements performed with an astrolabe, errors due to the atmospheric turbulence are directly related to these parameters. After, a brief recall of the basic properties of the atmospheric turbulence, a method allowing to generate realistic random wave-fronts will be presented. They will be used to study errors on diameter measurements performed with a solar astrolabe. The obtained results will clearly show the necessity to have a seeing monitor observing together with the solar experiment. A method allowing to obtain the observation conditions for solar observation will then be given.

Analysis of Diameter Measurements Performed at Calern Observatory Astrolabe

Solar diameter measurements performed at Calern Observatory astrolabe during more than two solar cycles show variations at various time scales in a broad range. Due to seasonal effects and instrumental characteristics, the recorded data are not sampled uniformly and present some gaps during these years. This time series has been analyzed by means of a deconvolution of the window function, as an alternative method to the standard least square fits of harmonic functions. The results show an extremely important improvement of the power spectrum. Harmonic terms already found by other authors are confirmed but new frequencies are also revealed by this analysis.