Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003a%26a...397.1097r&link_type=abstract
Astronomy and Astrophysics, v.397, p.1097-1107 (2003)
Astronomy and Astrophysics
Astrophysics
13
Mhd, Sun: Magnetic Fields, Sun: Interior
Scientific paper
We consider the consequences of radiative heating for the storage of magnetic flux in the overshoot region at the bottom of the solar convection zone. In the first part of the paper, we study the evolution of axisymmetric flux tubes (flux rings), which are initially in neutrally buoyant mechanical equilibrium. Radiative heating leads to a slow upward drift of the flux ring with a velocity depending on the degree of subadiabaticity of the stratification. Maintaining the flux tubes within the overshoot region for time intervals comparable with the solar cycle period requires a strongly subadiabatic stratification with delta =nabla -nabla ad < -10-4, which is not predicted by most current overshoot models (e.g., Skaley & Stix \cite{skaley91}; van Ballegooijen \cite{Ballegooijen:1982b}; Schmitt et al. \cite{Schmitt:etal:1984}). The drag force exerted by equatorward flow due to meridional circulation permits states of mechanical and thermal equilibrium in the overshoot region, but these apply only to very thin magnetic flux tubes containing less than 1% of the flux of a large sunspot. In the second part, we consider the influence of radiative heating (and cooling) on magnetic flux stored in the form of a magnetic layer. In contrast to the case of isolated flux tubes, the suppression of the convective energy transport within the magnetic layer affects the overall stratification of the overshoot region. In the case of a quenching of the convective heat conductivity by a factor of the order 100, the overshoot layer receives a net cooling leading to a stronger subadiabaticity, so that values of delta < -10-4 are reached. The stabilization of the stratification relaxes the conditions for flux storage. Stronger quenching of the heat conductivity leads to larger temperature perturbations (of both signs) and to the destabilization of the upper part of the overshoot layer, with the likely consequence of rapid magnetic flux loss.
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