AN UPPER BOUND TO THE SOLAR OBLATENESS

Abstract

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 diameters. Thus the complete geometry of the Sun is may be not as ideal as a perfect sphere. The aim of this paper is to briefly review up to now the figure of the Sun, to deduce an upper bound to the solar oblateness (if any).

To sum up over 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) seems to confirm a possible oscillation with the solar cycle, as previously conjectured by a Princeton group. 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.

Let us mention that 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, consistent with previous observations. Effects of the atmosphere are briefy discussed. 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 knowledge of the physical character of such an oscillation of the geometry of the Sun is still pending (rapidly rotating core?), as well as the question of this out-of-roundness of our nearby star.