We report on changes in the Sun's subsurface stratification inferred from helioseismology data. Using data from the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) for the last 9 years and, more precisely, the temporal variation of f-mode frequencies, we have computed the variation in the radius of subsurface layers of the Sun by applying helioseismic inversions. We have found a variability of the "helioseismic" radius in antiphase with the solar activity, with the strongest variations of the stratification being just below the surface, around 0.995 R , . In addition, the radius of the deeper layers of the Sun, between 0.975 and 0.99 R , , changes in phase with the 11-year cycle.

Context. Below 1 mHz, the power spectrum of helioseismic velocity measurements is dominated by the spectrum of convective motions (granulation and supergranulation) making it difficult to detect the low-order acoustic modes and the gravity modes. Aims. We want to better understand the behavior of solar granulation as a function of the observing height in the solar atmosphere and with magnetic activity during solar cycle 23. Methods. We analyze the Power Spectral Density (PSD) of eleven years of GOLF/SOHO velocity-time series using a Harvey-type model to characterize the properties of the convective motions in the solar oscillation power spectrum. We study then the evolution of the granulation with the altitude in the solar atmosphere and with the solar activity. Results. First, we show that the traditional use of a lorentzian profile to fit the envelope of the p modes is not well suitable for GOLF data. Indeed, to properly model the solar spectrum, we need a second lorentzian profile. Second, we show that the granulation clearly evolves with the height in the photosphere but does not present any significant variation with the activity cycle.

Recent models of variations of the Sun's structure with the 11-year activity cycle by Sofia et al. (2005) predict strong non-homologous changes of the radius of subsurface layers, due to subsurface magnetic fields and field-modulated turbulence. According to their best model the changes of the surface radius may be 1000 times larger than those at the depth of 5 Mm. We use f-mode oscillation frequency data from the MDI instrument of Solar and Heliospheric Observatory (SOHO) and measurements of the solar surface radius variations from SOHO and ground-based observatories during solar cycle 23 (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) to put constraints on the radius changes. The results show that the above model overestimates the change of the radius at the surface relative to the change at 5Mm.

International audienceWe hereby present a review on solar oblateness measurements. By emphasizing historical data, we illustrate how the discordance between experimental results can lead to substantial improvements in the building of new technical apparatus as well as to the emergence of new ideas to develop new theories. We stress out the need to get accurate data from space to enhance our knowledge of the solar core in order to develop more precise ephemerids and ultimately build possible new gravitational theories

Several works have reported changes of the Sun's subsurface stratification inferred from f -mode or p-mode observations. Recently a non-homologous variation of the subsurface layers with depth and time has been deduced from f -modes. Progress on this important transition zone between the solar interior and the external part supposes a good understanding of the interplay between the different processes which contribute to this variation. This paper is the first of a series where we aim to study these layers from the theoretical point of view. For this first paper, we use solar models obtained with the CESAM code, in its classical form, and analyze the properties of the computed theoretical f -modes. We examine how a pure variation in the calibrated radius influences the subsurface structure and we show also the impact of an additional change of composition on the same layers. Then we use an inversion procedure to quantify the corresponding f -mode variation and their capacity to infer the radius variation. We deduce an estimate of the amplitude of the 11-year cyclic photospheric radius variation.

The Astrophysical Journal, v. 649, p. 444-451, 2006. http://dx.doi.org/10.1086/506134International audienc

Irradiance variability has been monitored from space for more than two decades. Even if data are coming from different sources, it is well established that a temporal variability exists which can be set to as ≈ 0.1%, in phase with the solar cycle. Today, one of the best explanation for such an irradiance variability is provided by the evolution of the solar surface magnetic fields. But if some 90 to 95% can be reproduced, what would be the origin of the 10 to 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.

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