Speaker
Jörn Warnecke
(Max-Planck-Institut für Sonnesystemforschung)
Description
Abstract: This work presents an extensive study of the
previously discovered formation of bipolar flux
concentrations in a two-layer model. We relate the formation
process to the negative effective magnetic pressure
instability (NEMPI), which is a possible mechanism to
explain the origin of sunspots. In our simulations we use a
Cartesian domain of isothermal stratified gas which is
divided into two layers. In the lower layer, turbulence is
forced with transverse non-helical random waves, whereas in
the upper layer no flow is induced. An initially weak
uniform horizontal magnetic field is imposed in the entire
domain. In this study we vary the stratification by changing
the gravitational acceleration, magnetic Reynolds number,
the strength of the imposed magnetic field and the size of
the domain to investigate their influence on the formation
process. Bipolar magnetic structure formation takes place
over a large range of parameters. The magnetic structures
become more intensive for higher stratification. The large
fluid Reynolds numbers allow for the generation of flux
concentrations when the magnetic Prandtln umber is between
0.1 and 1. The magnetic field in bipolar regions increases
with higher imposed field strength until the field becomes
comparable to the equipartition field strength of the
turbulence. A larger horizontal extent enables the flux
concentrations to become stronger and more coherent. The
size of the bipolar structures turns out to be independent
of the domain size. Bipolar flux concentrations are
correlated with strong large-scale downward and converging
flows and can therefore be explained by NEMPI.
