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| Paper: |
Solar Oblateness Variations and Astrophysical Consequences |
| Volume: |
154, Cool Stars, Stellar Systems and the Sun: Tenth Cambridge Workshop |
| Page: |
685 |
| Authors: |
Rozelot, J. P. |
| Abstract: |
Since about 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 diameters. Reliable data are only available since about the last 15 years. The amplitude of the observed variations of the solar radius made at the Calern Observatory (France) does not exceed 0.7" and has been probably less (0.3") during the last five years. Recently, new measurements of the solar oblateness made at the Pic du Midi Observatory (France) seem to confirm a possible oscillation with the solar cycle. Furthermore, an upper bound to the solar oblateness can be derived from the observations and compared with theoretical computations of the angular momentum of the Sun. Between July 1993 and July 1994 the reported mean value difference between the equatorial radius and the polar one is 11.5 +/- 3.4 arc ms. New measurements at the end of 1996, during the last minimum, yields lower values, consistent with previous observations. From the observed values of the quadrupole moment of the Sun, it can be concluded that the shape of the helioid is really neither spherical nor oblate. A bump seems to occur in the royal zones and a depletion is observed at a latitude of about (70^{\circ}). Helioseismic measurements should eventually place useful limits on the strength and uniformity of the internal magnetic fields which may be responsible for the solar surface changes of different oscillation modes. A full understanding of the physical character of such an oscillation of the geometry of the Sun is still missing (rotating core?), as well as the question of this out-of-roundness of our nearby star. SOHO's measurements seem to confirm this assertion, as hyperfine structure of high frequencies mode recently observed by VIRGO, can be only interpreted by the presence of a core below the convective zone (rotation rate between 1.5 and 2 times the surface velocity). However, correlation of the data with the solar irradiance variations may produce a forecasting model of the Sun, helpful to study, among other problems, the response on the Earth climatic system. |
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