Calern Observatory Research Articles

Monitoring fast solar chromospheric activity: the MeteoSpace project

We present in this reference paper an instrumental project dedicated to the monitoring of solar activity during solar cycle 25. It concerns the survey of fast evolving chromospheric events implied in Space Weather, such as flares, coronal mass ejections, filament instabilities and Moreton waves. Coronal waves are produced by large flares around the solar maximum and propagate with chromospheric counterparts; they are rare, faint, difficult to observe, and for that reason, challenging. They require systematic observations with automatic, fast and multi-channel optical instruments. MeteoSpace is a high cadence telescope assembly specially designed for that purpose. The large amount of data will be freely available to the solar community. We describe in details the optical design, the qualification tests and capabilities of the telescopes, and show how waves can be detected. MeteoSpace will be installed at Calern observatory (Côte d’Azur, 1270 m) and will be in full operation in 2023.

Contribution of statistical site learning to improve optical turbulence forecasting

ABSTRACT The forecast for atmospheric and turbulence conditions above astronomical observatories is of interest to the astronomical community because it allows observations to be planned with maximum efficiency, a process called flexible scheduling. It can also be used to simulate long-term site testing to provide local information useful for the conception of focal and post-focal instrumentation. We have presented our forecasting tool in previous publications, but in this paper we focus on the importance of using local measurements to improve the predictive turbulence model and to better consider the local specificities of a given site, a process we call site learning. For this study, we use a local data base provided by the Calern Atmospheric Turbulence Station, which has been operational since 2015 at Calern Observatory. In addition, we use a set of several months of predictions to feed the turbulence model, taking into account daytime and nighttime conditions. This upgrade improves the quality of our forecasting by reducing the absolute bias between measurements and predictions from 25 to 50 per cent for each layer of the $C_n^2$, by 25 per cent for the seeing, and by 70 per cent for the isoplanatic angle.

PML: a generalized monitor of atmospheric turbulence profile with high vertical resolution.

The future generation of extremely large telescopes will be certainly equipped with wide-field adaptive optics systems. All the components of such adaptive optics systems have to be precisely specified, and most of the technical specifications are related to atmospheric turbulence parameters, particularly the profile of the refractive index structure constant Cn2(h). The monitor Profiler of Moon Limb (PML) for the extraction of the Cn2(h) profile with high vertical resolution for nighttime and daytime conditions by the observation of the moon limb or sun edge has been developed and is now routinely exploited at the Calern Observatory on the French Riviera. The PML instrument uses a differential method with two small subapertures through which the moon limb or sun edge are observed, leading to a continuum of double stars that allows a scan of the whole atmosphere with high resolution in altitude. The PML is an autonomous instrument aided by a set of equipment such as an all-sky camera, a small meteorological station, and an automatic system to cover the two subapertures with solar filters to switch from night/moon to day/solar observations. In addition, the PML instrument provides in real time a large characterization of the atmospheric turbulence since it can measure other turbulence parameters, such as the total seeing and the isoplanatic angle.

Optical instrumentation for chromospheric monitoring during solar cycle 25 at Paris and Côte d’Azur observatories

We present the observing program proposed by Paris and Côte d’Azur Observatories for monitoring solar activity during the upcoming cycle 25 and providing near real time images and movies of the chromosphere for space-weather research and applications. Two optical instruments are fully dedicated to this task and we summarize their capabilities. Short-term and fast-cadence observations of the chromosphere will be performed automatically at Calern observatory (Côte d’Azur), where dynamic events, as flare development, Moreton waves, filament instabilities and Coronal Mass Ejections onset, will be tracked. This new set of telescopes will operate in 2021 with narrow bandpass filters selecting Hα and CaII K lines. We present the instrumental design and a simulation of future images. At Meudon, the Spectroheliograph is well adapted to the long-term and low-cadence survey of chromospheric activity by recently improved and optimized spectroscopic means. Surface scans deliver daily (x, y, λ) datacubes of Hα, CaII K and CaII H line profiles. We describe the nature of available data and emphasize the new calibration method of spectra.

A generalized differential image motion monitor

We present the Generalised Differential Image Motion Monitor. It is a compact instrument dedicated to measure 4 parameters of the optical turbulence: seeing, isoplanatic angle, coherence time and wavefront coherence outer scale. GDIMM is based on a small telescope (28cm diameter) equipped with a 3-holes mask at its entrance pupil. The instrument is fully automatic, and performs continuous monitoring of turbulence parameters at the Calern Observatory (France). This paper gives a description of the instrument, data processing and error budget. We present also statistics of 3.5 years of monitoring of turbulence parameters above the Calern Observatory.

Solar seeing monitor MISOLFA: A new method for estimating atmospheric turbulence parameters

Daily observation conditions are needed when observing the Sun at high angular resolution. MISOLFA is a daytime seeing monitor developed for this purpose that allows the estimation of the spatial and temporal parameters of atmospheric turbulence. This information is necessary, for instance, for astrometric measurements of the solar radius performed at Calern Observatory (France) with SODISM II, the ground-based version of the SODISM instrument of the PICARD mission. We present a new way to estimate the spatial parameters of atmospheric turbulence for daily observations. This method is less sensitive to vibrations and guiding defaults of the telescope since it uses short-exposure images. It is based on the comparison of the optical transfer function obtained from solar data and the theoretical values deduced from the Kolmogorov and Von Ka`rma`n models. This method, previously tested on simulated solar images, is applied to real data recorded at Calern Observatory in July 2013 with the MISOLFA monitor. First, we use data recorded in the pupil plane mode of MISOLFA and evaluate the turbulence characteristic times of angle- of-arrival fluctuations: between 5 and 16 milliseconds. Second, we use the focal plane mode of MISOLFA to simultaneously record solar images to obtain isoplanatic angles: ranging from 1 to 5 arcseconds (in agreement with previously published values). These images and our new method allow Fried’s parameter to be measured; it ranges from 0.5 cm to 4.7 cm with a mean value of 1.5 cm when Kolmogorov’s model is considered, and from less than 0.5 to 2.6 cm with a mean value of 1.3 cm for the Von Ka`rma`n model. Measurements of the spatial coherence outer scale parameter are also obtained when using the Von Karman model; it ranges from 0.25 to 13 meters with a mean value of 3.4 meters for the four days of observation that we analyzed. We found that its value can undergo large variations in only a few hours and that more data analysis is needed to better define its statistics.

MISOLFA: a generalized monitor for daytime spatio-temporal turbulence characterization

Ground based solar observations are strongly affected by optical turbulence. The concept of a new instrument which allows to measure both spatial and temporal parameters of atmospheric turbulence has been proposed in the late 90s. The instrument MISOLFA (Moniteur d'Images Solaire Franco-Algerien) is based on this concept and has been developped over the past ten years in the framework of a ground-based solar astrometry program and in parallel to the developpement of several night time turbulence monitors at Calern Observatory, south of France. In this paper we first describe its instrumental concept, the technical choices that were made to meet the specifications and discuss the difficulties encountered. Using numerical simulations, we present and test the methods that can be used in order to estimate the turbulence parameters from both MISOLFA image and pupil planes. The effect of finite outer scale on Fried parameter estimation from a simple estimate of the Angle-of-Arrival variance is clearly shown. Finally we present the first results obtained with the instrument fully operating in its two observing planes. We obtained a mean value of Angle-of-Arrival coherence time of 5.3 ms and good agreement is found between spatial parameters obtained with image and pupil planes.

Periodicity of the solar radius revisited by using empirical mode decomposition and the Lomb–Scargle method**Supported by the National Natural Science Foundation of China.

Using the Hilbert–Huang transform and the Lomb–Scargle method, we investigate periodicities in the daily solar radius data during the time interval from February 1978 to October 1999 derived from Calern Observatory. The following prominent periods are found: (1) the rotation cycle signal; (2) several mid-term periods including 122, 162.9 and 225 days, annual-variation periodicities (319 and 359 days), quasi-triennial oscillations (3.46 and 3.94 years); (3) the 11-year Schwabe cycle, which is in anti-phase with solar activity. This result indicates that the strong magnetic field associated with the Sun has a greater inhibitive effect on the radius variation.

ON THE VARIATION OF SOLAR RADIUS IN ROTATION CYCLES

The Date Compensated Discrete Fourier Transform and CLEANest algorithm are used to study the temporal variations of the solar radius observed at Rio de Janeiro Observatory from 1998 March 2 to 2009 November 6. The CLEANest spectra show several significant periodicities around 400, 312, 93.5, 86.2, 79.4, 70.9, 53.2, and 26.3 days. Then, combining the data on the daily solar radius measured at Calern Observatory and Rio de Janeiro Observatory and the corresponding daily sunspot areas, we study the short-term periodicity of the solar radius and the role of magnetic field in the variation of the solar radius. The rotation period of the daily solar radius is determined to be statistically significant. Moreover, its temporal evolution is anti-phase with that of sunspot activity, and it is found anti-phase with solar activity. Generally, the stronger solar activity is, the more obvious is the anti-phase relation of radius with solar activity. This indicates that strong magnetic fields have a greater inhibitive effect than weak magnetic fields on the variation of the radius.

PICARD SOL, a new ground-based facility for long-term solar radius measurements: first results

PICARD SOL is the ground component of the PICARD mission and is operational since March 2011. A set of instruments including the replica of the space instrument and several atmospheric monitors was set up at Calern observatory in order to compare solar radius measured in space and on ground and to better understand and calibrate atmospheric effects on ground based measurements. SODISMII provides full disk images of the chromosphere and photosphere of the Sun in five narrow pass bands ranging from the near ultraviolet to the near infrared. Our preliminary results show a very good instrumental stability. After plate scale calibration using star doublet observations and corrections for atmospheric refraction, first estimates of the mean solar radius at the five wavelengths (393.37, 535.7, 607.1, 782.2, and 1025.0nm) are deduced from measurements recorded between May 2011 and December 2012.

LONG-TERM PERIODICITY VARIATIONS OF THE SOLAR RADIUS

In order to study the long-term periodicity variations of the solar radius, daily solar radius data from 1978 February to 2000 September at the Calern Observatory are used. Continuous observations of the solar radius are difficult due to the weather, seasonal effects, and instrument characteristics. Thus, to analyze these data, we first use the Dixon criterion to reject suspect values, then we measure the cross-correlation between the solar radius and sunspot numbers. The result indicates that the solar radius is in complete antiphase with the sunspot numbers and shows lead times of 74 months relative to the sunspot numbers. The Lomb-Scargle and date compensated discrete Fourier transform methods are also used to investigate the periodicity of the solar radius. Both methods yield similar significance periodicities around similar to 1 yr, similar to 2.6 yr, similar to 3.6 yr, and similar to 11 yr. Possible mechanisms for these periods are discussed. The possible physical cause of the similar to 11 yr period is the cyclic variation of the magnetic pressure of the concentrated flux tubes at the bottom of the solar convection zone.

A Comparison among Solar Diameter Measurements Carried Out from the Ground and outside Earth's Atmosphere

The solar diameter has been measured since the 17th century, using different methods and instruments, and without a clear strategy of measurement, which could explain the lack of coherence between the results obtained. The present study confirms that the discrepancies between solar diameter measurements are mostly due to differences in instrumental characteristics, the spectral domain of observation of each investigation, and atmospheric turbulence for ground measurements. We show that correcting the measurements for the effects introduced by the properties of each instrument reduces the differences between the radius measurements to the level of uncertainty of each instrument. This study makes use of simulated and real data. For the simulated data, we use an empirical model of the solar limb shape, and for observations, we use measurements carried out with the Solar Disk Sextant experiment, the CCD solar astrolabe of Calern Observatory, and the Michelson Doppler Imager on board SOHO.

Ground solar radius survey in view of microsatellite missions

For the last 25 years, ground time series of the solar radius have shown (different) apparent variations according to different instruments. The origin of these variations may search in the observer, the instrument, the atmosphere and up to the sun. Ground instruments are automated to reduce the “personnal equation” and place that origin in the atmosphere and/or in the sun. Astrometric satellites scheduled at the end of this decade will perform non ambiguous diameter measurements. A survey of the Solar radius has been initiated in 1975 by Francis Laclare, at the Calern site of the Observatoire de la Côte d’Azur, which have been chosen for hosting the ground segment of the Centre National d’Etudes Spatiales (CNES) Microsatellite PICARD mission, to be launched in 2008. This reference series was obtained by Visual observations of the Sun, with a Solar Astrolabe whose metrological character has to be stressed. Considering the Visual series results, we have compared the solar diameter variations with the solar activity cycle, and we found an opposite phase, for the whole series and at the different times of the cycles. Parallel to that series, CCD measurements were made with the same instrument and gave results which are perfectly blended together, within our quoted uncertainties. Located next to the Solar Astrolabe, DORAYSOL (Définition et Observation du Rayon Solaire) is a second generation instrument, which keeps the major features of the design of its predecessor and, which is designed to increase the number of CCD measurements and to be eventually automated. Since 1999, both series overlap correctly within our quoted uncertainties. Some information is added to explain the pattern of the PICARD mission ground segment, next to those instruments at Calern Observatory, as well as the international network intended to carry out the Sun’s Radius ground survey ( R2S3: Réseau de Suivi au Sol du Rayon Solaire).

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.

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.

Wavelet Analysis of Solar Diameter Measurements Performed at Calern Observatory Astrolabe

Since 1975, a visual program of solar diameter measurement is pursuedat Calern Observatory (France). These observations allowed to put inevidence variations in solar data. Several studies were developed inorder to analyze the data using appropriate me thods (deconvolution ofthe observation window function and least square fits) (Moussaouiet al., 1999). We present in this work a time-frequency studyof the found harmonic terms. It is developed using the wavelettransform. The obtained results are presented and discussed.

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.

Analysis of solar radius determination obtained by the modern CCD astrolabe of the Calern Observatory

A semi-empirical model of solar images is presented here and used to derive the solar radius from astrolabe observations made at the Côte d'Azur Observatory (OCA), at the Calern station. This model was elaborated in order to remove a systematic effect existing in the measurements. This effect is caused by the center to limb darkening existing in the apparent solar light intensity and is magnified by atmospheric and instrumental effects. The result is that the apparent or observed solar radius is always smaller than the true one. After a description of the observational methods used, a definition of the true solar radius is given and the model is described (Eqs. (1) or (3)). A new set of results is obtained using the model, which corresponds to observations made with a rotating shutter (used to separate the two solar images present in the focal plane). All the results are given for the unit distance. This paper presents two series of solar apparent radius (see Table 1). The first one (column derivative on Table 1), obtained in a previous analysis uses only numerical methods, is affected by the perturbing effects of the center to limb darkening, atmospheric turbulence and attenuation and conducts to the mean value . Taking account of all the informations given by the CCD observations, correlations between the Fried parameter r0 and the derivative width, and between r0 and the solar radius R, are found, in the results obtained by purely numerical methods. These results give us the possibility to evaluate, for , the corresponding values of the derivative width and the solar radius outside the terrestrial atmosphere. In these conditions, the mean solar radius is found to be equal to . The second series (column model on Table 1) obtained by the use of the presented model, conducts directly to a corrected mean result . One can see that the corrected result of the first method agrees very well with the one obtained using the model, which has a better precision. By the same way, the parameters b, which represent the slope of the solar limb near the true inflection point, and p, which define the slope of the darkening effect near the limb are given through and outside the atmosphere. As we hoped, the slope b of the derivative is much greater outside than inside the atmosphere (about two times). The contrary occurs for p, as the darkening is increased by the presence of the atmosphere and dust … The comparison with other results and method shows that the model has the strong advantage to give directly the correct result without any supplementary correction. Some observations done without the rotating shutter at Calern, Rio de Janeiro and San Fernando Observatories will be analyzed in a near future using our model, applied to these existing data. We hope so to analyze the future observations from Antalya where a CCD astrolabe will start a new campaign.

Variations apparentes du rayon solaire observées à l'observatoire de la Côte d'Azur (Astrolabe solaire du site de Calern : 1975–1998)

The results here discussed were produced by a program of visual observations carried out at the Calern Observatory from 1975 to 1998. The sun radius data collected display some variability, in particular an oscillation of 0.2′' amplitude and 11.8 years period which seems anticorrelated with the sunspot cycle. These apparent variations might also be put together with other structural solar parameters: frequency shifts of low-degree p-modes or neutrino flux. Moreover, the series shows a deviation from the mean diameter as a function of heliographic latitude. The origin of these apparent variations is currently unclear and will be better understood by ground measurements over a long period of time (e.g. the DORAYSOL Instrument and Program at Calern) and by the results expected from the future PICARD mission of the French Space Agency CNES.

Absolute declinations with the photoelectric astrolabe at Calern Observatory (OCA)

A regular observational programme with a photoelectric astrolabe have been performed at Observatoire du Calern (Observatoire de la Cote d'Azur, OCA, ϕ = ; λ = , Calern, Caussols, France) for the last twenty years. It has been almost fully automatized between 1984 and 1987. Since 1988 the photoelectric astrolabe was used without any modification. In addition to determining the daily orientation of the local vertical, the yearly analysis of the residuals permits to derive corrections to the used star catalogue (Vigouroux et al. 1992). A global reduction method was applied for the ASPHO observations. The new form of the equations (Martin & Leister 1997) give us the possibility of using the entire set of the observing program using data taken at two zenith distances ( 30 o and 45 o ). The program contains about 41648 stars' transits of 269 different stars taken at Observatoire du Calern (OCA). The reduction was based on the HIPPARCOS system. We discuss the possibility of computing absolute declinations through stars belonging simultaneously to the 30 o and 45 o zenith distances programmes. The absolute declination corrections were determined for 185 stars with precision of 0.027″ and the value of the determined equator correction is -0.018 ″ ± 0.005″. The instrumental effects were also determined. The mean epoch is 1995.29.