Issue |
A&A
Volume 397, Number 3, January III 2003
|
|
---|---|---|
Page(s) | 1097 - 1107 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20021594 | |
Published online | 21 January 2003 |
Thermal properties of magnetic flux tubes
II. Storage of flux in the solar overshoot region
Max-Planck-Institut für Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany e-mail: rempel@linmpi.mpg.de
Corresponding author: rempel@ucar.edu
Received:
4
September
2002
Accepted:
30
October
2002
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
, which is not predicted
by most current overshoot models (e.g., Skaley & Stix 1991;
van Ballegooijen 1982; Schmitt et al. 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
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
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.
Key words: MHD / Sun: magnetic fields / Sun: interior
© ESO, 2003
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