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Licentiate thesis: Effects of rotation and stratification on magnetic flux concentrations
Illa R. Losada
(Nordita/ Stockholm University)
The formation of magnetic flux concentrations in the Sun is still a matter of debate.
One observable manifestations of such concentrations is sunspots.
A mechanism able to spontaneously form magnetic flux concentrations in strongly
stratified hydromagnetic turbulence and in the presence of a weak magnetic field
is the negative effective magnetic pressure instability (NEMPI).
This instability is caused by the local suppression of the turbulence
by the magnetic field.
Due to the complexity of the system, and in order to understand the fundamental
physics behind the instability,
the study started
by considering simplified conditions.
In this thesis we aim to move towards the
complexity of the Sun.
Here we want to know whether the instability can develop under rotation and in the
case of a polytropic stratification instead of the simpler isothermal stratification.
We perform different kinds of simulations, namely direct numerical simulations (DNS)
and mean field simulations (MFS) of strongly stratified turbulence
in the presence of weak magnetic fields.
We then study separately
the effects of rotation and the change in stratification.
It is found that slow rotation can suppress the instability.
For Coriolis numbers larger than $0.1$ the MFS
no longer result in growth, whereas the DNS start first with a
of the growth rate of the instability % with the speed-up of rotation is alleviated
and then, for $\Co > 0.06$, an increase owing to the fact that rotation leads to
the onset of the dynamo instability, which couples with NEMPI in a combined
In fact, the suppression implies a constraint on the depth where the instability
can operate in the Sun.
Since rotation is very weak in the uppermost layers of
the Sun, the formation of the flux concentration through this
instability might be a shallow phenomenon.
The same constraint is found when we study the effects of polytropic stratification
In this case, the instability also develops, but it is much more concentrated
in the upper parts of the simulation domain than in the isothermal case.
In contrast to the isothermal case, where the density scale height is constant in
the computational domain,
decrease their stratification deeper down,
so it becomes harder for NEMPI to operate.
With these studies we confirm that NEMPI can form magnetic flux concentrations
even in the presence of weak rotation and for polytropic stratification.
When applied to the Sun, the effects of rotation and
the change of stratification constrain the depth where NEMPI can develop to the
uppermost layers, where the rotational influence is weak and the stratification is strong enough.