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Astron. Astrophys. 317, 919-924 (1997)

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1. Introduction

The solar core, especially the inner core, is where we expect to see the imprint of solar evolution in the hydrogen abundance profile and the angular momentum distribution. Techniques of helioseismology enable one to sound this region. In helioseismology, one does not directly probe the chemical composition, instead one investigates the sound speed, the behavior of which in the inner core is affected by the mean molecular weight. The angular velocity of rotation is directly probed from the fine structure by measuring rotational splitting in the spectrum of solar oscillations. However, the contribution to rotational splitting arising from the core is very small- comparable to the errors in the splittings.

The Sun's p-modes are the only tool we have for seismic investigations. This tool is not ideal because only a small fraction of the observed modes exhibit any sensitivity to core properties. The ones that do are of low degree(l). However, even these modes are primarily sensitive to the Sun's envelope structure which includes its near surface, where the Sun is more complicated and still quite poorly understood. The primary problem here is the effect of vigorous convection both on the mean structure and the adiabaticity of the individual modes of vibration. Inversions of p-mode data to determine the Sun's internal structure have treated the near surface complications as though they possess spherical symmetry. This method used is not applicable to removing the effect of the near surface magnetic field because the perturbation it causes depends on latitude.

In this work, we demonstrate that the magnetic perturbation is significant. We then show how to account for it through the use of the fine structure in the spectrum of solar oscillations. More specifically through the even-a coefficients of Duvall, Harvey, Pomerantz(1986),

[EQUATION]

where [FORMULA] the cyclic frequency of an individual [FORMULA] -mode, [FORMULA], P is a Legendre polynomial, and N is the order of the Legendre expansion provided by the observers. The [FORMULA] are the radial order, angular degree and azimuthal order of the oscillation, respectively. The fine structure in each [FORMULA] multiplet is labelled by m. The odd a -coefficients arise from the linear effect of rotation. While the even-a 's arise from asphericities, like those from magnetic and centrifugal forces.

Before discussing the effect of the surface magnetic field on the oscillations which sound the core, one needs to have a target precision for the frequencies which probe the core.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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