Search

A large number of acoustic frequencies have already been detected, leading to a “seismic” model of the Sun rather close to the actual standard solar models. The core however is not yet well constrained by these observations and frequencies of low degree, low frequency modes which penetrate deeply into the solar core are needed. We present here a study on the sensitivity of low degree low frequency (50 – 900 μHz) modes to the structure of the solar interior, in order to help their detection and identification in the low frequency spectrum observed by SoHO experiments like VIRGO and GOLF. The frequencies of p and g modes have been computed for a set of solar models with updated physics (Morel et al., 1997). We analyze their sensitivity to solar parameters like age and metallicity, and to various physical processes, like convective core overshoot and mass loss during the beginning of solar evolution

After 8 months of nearly continuous measurements the GOLF instrument, aboard SOHO, has detected acoustic mode frequencies of more than 100 modes, extending from 1.4 mHz to 4.9 mHz. In this paper, we compare these results with the test available predictions coming from solar models. To verify the quality of the data, we examine the asymptotic seismic parameters; this confirms the improvements achieved in solar models during the last decade. Using the GOLF set of frequencies for l = 0, 1, 2, 3 combined with the LOWL second year data set for l > 3, we then cany out inversions to infer properties of the solar core. This largely confirms the previous results down to around 0.1 R, while there remain differences, even closer to the centre, where the present study shows an externe sensitivity of the inversion results to the values of the frequencies. We finally consider physical processes which may influence directly or indirectly the solar core structure.

First results from the VIRGO experiment (Variability of solar IRradiance and Gravity Oscillations) on the ESA/NASA Mission SOHO (Solar and Heliospheric Observatory) are reported. The observations started mid-January 1996 for the radiometers and sunphotometers and near the end of March for the luminosity oscillation imager. The performance of all the instruments is very good, and the time series of the first 4–6 months are evaluated in terms of solar irradiance variability, solar background noise characteristics and p-mode oscillations. The solar irradiance is modulated by the passage of active regions across the disk, but not all of the modulation is straightforwardly explained in terms of sunspot flux blocking and facular enhancement. Helioseismic inversions of the observed p-mode frequencies are more-or-less in agreement with the latest standard solar models. The comparison of VIRGO results with earlier ones shows evidence that magnetic activity plays a significant role in the dynamics of the oscillations beyond its modulation of the resonant frequencies. Moreover, by comparing the amplitudes of different components of p-mode multiplets, each of which are influenced differently by spatial inhomogeneity, we have found that activity enhances excitation.

Asymptotic properties of low degree gravity modes and their relation to the stratification of the model through the Brunt-Vaissala frequency are discussed for a solar type model and for a 10 M ⊙ model. For the solar model, taking into account the quasi-adiabaticity of the solar convection zone, it is shown that two global constraints on the deep interior of the solar model can be derived from a set of g-modes periods in the observed range. However modes with large periods, i.e. larger than those up to now observed, are required to obtain informations on the stratification just below the convection zone. For the 10 M ⊙ model, the preliminary results show that the existence of a layer with a large mean molecular weight gradient destroys the equidistance of the periods of gravity modes of a given degree and that some informations on the properties of this layer can be obtained from the analysis of the g-modes periods.