A Search for an Obliquely Rotating Core

Lead Investigator: A.G. Kosovichev
Other Team Members: T. Appourchaux, T. Bai, D.O. Gough, J.T. Hoeksema, P.H. Scherrer, P.A. Sturrock, T. Toutain

Abstract/Technical Summary

Use combined MDI-VIRGO datasets to determine whether the core of the Sun rotates about an axis that is inclined to the axis of rotation of the envelope by analyzing normal mode eigenfunctions and rotational splitting of oscillation eigenfrequencies of low- and intermediate-degree modes, and by comparing the results with the surface distribution of flares and magnetic field.

Investigation Plan

Bai & Sturrock (1993) have recently suggested from their analysis of the longitudinal distribution of major solar flares that the core of the Sun rotates about an axis which is inclined by to the axis of rotation of the envelope. We propose a seismological study of this problem.

The helioseismic inversion techniques presume that the Sun's interior rotates about a unique axis. A variation with radius of the direction of the rotation axis would modify the form of rotational splitting of oscillation eigenfrequencies. But so too does a variation with depth and latitude in the magnitude of the angular velocity. One type of variation can mimic the other, and so frequency information alone cannot differentiate between them. Therefore, in the case of the obliquely rotating core, the inversion techniques will simply detect a decrease of the angular velocity in the core (Gough & Kosovichev, 1993; Goode & Thompson, 1992). Gough & Kosovichev (1993) have developed a non-linear analysis technique to determine the radial dependence of the angular velocity in the obliquely rotating core. From the BBSO data, they concluded that it cannot yet be determined whether the solar core is oblique.

What is different, however, is the structure of the eigenfunctions. Therefore, in principle, one might hope to untangle the two phenomena using information about both the frequencies and the amplitudes of the oscillations. Gough, Kosovichev & Toutain (1995) have demonstrated that the effect of the obliquely rotating core could be seen in the relative amplitudes of the rotational-multiplet components. From the analysis of the IPHIR low-degree data, they concluded that the obliquely rotating core proposed by Bai & Sturrock is unlikely to be larger than 0.2R_SUN. However, relatively small variations of the rotation axis by few degrees are still possible.

New accurate frequency measurements from VIRGO and MDI data will lead to a more definite conclusion about possible variations of the Sun's rotation axis.

The investigation plan is

1. obtain accurate measurements of the eigenfunctions and eigenfrequencies of the normal modes from MDI Medium-l and VIRGO data;

2. compare the measurements with the theoretical models of oblique rotation and determine the most plausible parameters (radius and angle) of the obliquely rotating core;

3. compare the results with the surface distribution of flares and large-scale magnetic field;

4. analyze the hydrodynamic stability of the obliquely rotating core.

Bibliography

Bai, T. & Sturrock, P. 1993, Astrophys. J., 409, 476

Goode, P.R. & Thompson, M.J., 1992, Astrophys. J., 395, 307-315

Gough, D.O. & Kosovichev, A.G. 1993, in Proc. IAU Colloq. 137, Inside the Stars, ed W.W. Weiss, A. Baglin, p. 556

Gough, D.O., Kosovichev, A.G. & Toutain, T. 1995, in GONG '94 : Helio- and Asteroseismology from the Earth and Space, ed. R. K. Ulrich, E.J. Rhodes, W. Däppen (ASP Conference series)