On the stability of the 11-year solar cycle period (and a few others)

The existence of an 11.1-yr periodic variation in the sunspot number record has been recognized for many years; however, periodicities other than this remain questionable. Power spectral analysis of the International sunspot number is performed and the results are compared with those for the same period using values that were taken randomly inside the error bars. The findings are that only a few periodicities show noticeable peaks. These include periodicities of 8.49, 10.01, 10.58, 11.10, 12.50, 58.50, and 97.20 yr. On the basis of these seven periodicities, one can loosely simulate the observable sunspot record (r = 0.75). We find that discrepancies in number and value of periodicities with other authors appear to be related to the length of the sunspot record used in the analysis and to the occurrence of 0.3-yr windows around the inferred periodicities.     

AN UPPER BOUND TO THE SOLAR OBLATENESS

Solar Physics - For more than a decade, it has been suggested that the diameter of the Sun may vary in time. This not only concerns variations of the equatorial diameter, but also any other...     

A new estimate of the quadrupole moment of the sun

Recent solar observations at Pic du Midi are reported that yield a value of J ₂=(2.57 ± 2.36) x 10^–6 for the quadrupole moment of the Sun. These observations were conducted from July 1993 to July 1994 after several improvements of the scanning heliometer. This instrument operates by fast photoelectric scans of opposite limbs of the Sun quasi-simultaneously, which provides the distance between both inflection points of the limb profiles. Any number of solar diameters in any position angle can be measured within a time interval short enough to minimize the scattering of the observational parameters. Errors due to atmospheric deterioration are discussed. From our results, compared to previous values obtained by other authors, it can be concluded than an upper limit for J ₂ is probably 1.0 × 10^-5.     

Are Non-Magnetic Mechanisms Such As Temporal Solar Diameter Variations Conceivable for an Irradiance Variability?

Irradiance variability has been monitored from space for more than two decades. Even though data come from different sources, it is well established that a temporal variability exists ≈0.1%, in phase with the solar cycle. Today, one of the best explanations for such an irradiance variability is provided by the evolution of the solar surface magnetic fields. But if some 90–95% can be reproduced, what would be the origin of the 10–5% left? Non-magnetic effects are conceivable. In this paper we will consider temporal variations of the diameter of the Sun as a possible contributor for the remaining part. Such an approach imposes strong constraints on the solar radius variability. We will show that over a solar cycle, variations of no more than 20 mas of amplitude can be considered. Such a variability – far from what is reported by observers conducting measurements by means of ground-based solar astrolabes – may explain a little part of the irradiance changes not explained by magnetic features. Further, requirements are needed that may help to reach a conclusion. Dedicated space missions are necessary (for example PICARD, GOLF-NG or SDO, scheduled for a launch around 2008); it is also proposed to reactivate SDS flights for such a purpose.     

Analyse des renforcements coronaux a travers quelques acquisitions spectroscopiques récentes des émissions monochromatiques du fer ionisé (X à XV)

Résumé L'obtention de spectres coronaux fournit, après analyse, un certain nombre d'informations dont on va tenter ici de tirer quelques conclusions par confrontation avec des calculs théoriques.On discute d'abord les conditions de validité du problème. D'une part, une approche théorique délicate, où les calculs ne peuvent être conduits jusqu'à leur formulation numérique qu'au prix d'hypothéses critiquables (insistant davantage sur le problème de 1'équilibre cinétique et du vent solaire), et d'autre part, des observations sûres reflétant la complexité des structures coronales (négligeant cependant les difficultés inhérentes aux microstructures).Les calculs théoriques ont été effectués en tenant compte dans 1'évaluation des paramètres physiques fondamentaux, tels sections de chocs, des progrès récents en physique atomique. Les observations ont été effectuées à 1'Observatoire du Pic du Midi, dont les résultats d'ensemble et les problèmes divers de photométrie (calibrations précises, réductions des mesures, etc....) ont été exposés et discutés dans un article séparé.La résolution des équations classiques de l'équilibre statistique, qui se réduisent pour un ion déterminé a un système d'équations linéaires si l'on prend div = 0, fournit les valeurs des populations relatives des différents niveaux énergétiques. On a pu ainsi étudier le comportement de nombreuses raies spectrales, comprenant donc les six raies interdites observées, en fonction de la température et de la densité électronique.On cherche alors à interpréter les résultats de certaines observations. On a pu ainsi donner une explication possible de quelques anomalies constatées dans le comportement de Fe x, indicateur des centres actifs jeunes. A été mise également en évidence une corrélation assez étroite entre les intensités de Fe xi et Fe xiv, indicateurs des régions émissives à haute température. Fe xiii se révèle être par contre un indicateur sensible des régions de forte densité électronique. Le cas de Fexii est discuté à part.On tente alors de tirer des indications sur les conditions physiques existant dans les renforcements coronaux. On montre, à partir du tracé des courbes d'isorapports d'intensité, pour deux valeurs du facteur de dilution, qu'il peut y avoir un argument en faveur d'un transport de matière dans le plasma coronal. Vers 1,1 rayon solaire environ, un domaine possible de température et de densité peut être considéré: les fluctuations permises à 1'intérieur de cette région traduisent des hétérogénéités dans le renforcement coronal, principalement dans la phase jeune de développement du centre actif sousjacent. La variation de l'atmosphère étudiée avec l'altitude montre que les effets de diffusion des éléments lourds donnent un bon accord théorie-expérience entre 60000 et 90000 km du limbe.Diverses conséquences possibles sont alors envisagées au niveau des structures. On montre ainsi que les fluctuations d'intensité observées s'expliquent mieux en termes de variations de densité électronique qu'en termes de variations de température.Enfin, on étudie la non uniformité en température et en densité le long de la ligne de visée. Des conclusions non abusives peuvent être difficilement tirées; si à température (resp. densité) constante, on fait varier la densité (resp. température), les hétérogénéités en densité ne peuvent pas atteindre plus de 10% dans la zone de température envisagée. De nouveaux raffinements doivent être apportés, et on montre qu'une distribution gaussienne de la densité, jointe à une variation bicarrée de la température, le long de la ligne de visée, rendent mieux compte des observations.

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Theoretical computation of the coronal spectrum have been performed and compared with observations carried out at the Pic du Midi Observatory, presented in a separate paper. The solution of the classical equations of statistical equilibrium for each of the ions led to a system of linear equations, if we take div = 0, and gives the values of the relative populations of the various energy levels. This enables one to study the behaviour of numerous spectral lines, including the six observed forbidden lines, as functions of temperature and electron density.A possible explanation can be given of some anomalies found in the behaviour Fe x, indicator of young active centers. A strong correlation between the intensity of Fe xi and Fe xiv indicates high temperature regions. Fe xiii is on the contrary a sensitive indicator of strong electronic density regions. The case of Fe xii is discussed apart. Arguments are given for the inflow of matter into coronal enhancements, derived from the study of isoratio curves of the intensity, for two values of the dilution factor. Indications for fluctuations in temperature and density are found at about 1.1 R . The fluctuations occur mainly in the young phase of development of the corresponding active centers.The incorporation of the effect of diffusion of heavy elements gives a good fit between theory and observation at altitudes between 60000 and 90000 km. The observed fluctuations of the intensities are better explained in terms of electronic density variations than of those of the temperature. The non-uniformity of temperature and density is studied along the line of sight: if at a constant temperature the density varies, the inhomogeneities in the density are always smaller than 10%. It is shown that along the line of sight a gaussian distribution of the density, together with a bi-squared variation of the temperature fits best with our observations.

     

Physical Characteristics of Umbral Dots Derived from a High-Resolution Observations

Solar Physics - We study the features of the magnetic field variations within the 2011 June 7 eruptive event that includes a large filament eruption, a flare, and a CME formation. The magnetic...     

Etude de l'eclipse solaire partielle du 25 Fevrier, 1971 au Pic du Midi

The scattering in the neighbourhood of the Sun is investigated on photographs taken in Pic-du-Midi Observatory during the partial solar eclipse of february 25th 1971. Briefly mentioned, the diffraction theory is used to determine the image of a crescent with incoherent illumination, and to compare the ‘diffracted’ isophote curves with those drawn from the plates. An accurate photometry of the plates enables us to give charts of the electronic density in a coronal sector, and also to determine some physical quantities, values of which are rather uneasy to get from usual observations, such as the polarisation ratio and the electron density of the first Fexiv excited level. In this last case, an original calculation was used to find the source function. A superimposition of a coronal enhancement in white light with a monochromatic arch, gives coherent results without any major assumptions. These assumptions are explained step by step and some interpretations are compared with those given by other authors.     

On the theory of the oblateness of the Sun

In this paper we first emphasize why it is important to know the successive zonal harmonics of the Sun's figure with high accuracy: mainly fundamental astrometry, helioseismology, planetary motions and relativistic effects. Then we briefly comment why the Sun appears oblate, going back to primitive definitions in order to underline some discrepancies in theories and to emphasize again the relevant hypotheses. We propose a new theoretical approach entirely based on an expansion in terms of Legendre's functions, including the differential rotation of the Sun at the surface. This permits linking the two first spherical harmonic coefficients (J ₂ and J ₄) with the geometric parameters that can be measured on the Sun (equatorial and polar radii). We emphasize the difficulties in inferring gravitational oblateness from visual measurements of the geometric oblateness, and more generally a dynamical flattening. Results are given for different observed rotational laws. It is shown that the surface oblateness is surely upper bounded by 11 milliarcsecond. As a consequence of the observed surface and sub-surface differential rotation laws, we deduce a measure of the two first gravitational harmonics, the quadrupole and the octopole moment of the Sun: J ₂=−(6.13±2.52)×10⁻⁷ if all observed data are taken into account, and respectively, J ₂=−(6.84±3.75)×10⁻⁷ if only sunspot data are considered, and J ₂=−(3.49±1.86)×10⁻⁷ in the case of helioseismic data alone. The value deduced from all available data for the octopole is: J ₄=(2.8±2.1)×10⁻¹². These values are compared to some others found in the literature. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005238718479