(LBKR13)
Losada, I. R., Brandenburg, A., Kleeorin, N., & Rogachevskii, I.: 2013, ``Competition of rotation and stratification in flux concentrations,'' *Astron. Astrophys.*, in press
(arXiv:1212.4077, HTML)

### Additional material

As a result of negative effective magnetic pressure,
regions with enhanced magnetic field strength must have a higher
gas pressure to compensate for the negative magnetic contribution.
This higher gas pressure makes these structures denser and thus
heavier, so they sink!
This is now seen in DNS, and is strongly reminiscent to earlier
mean-field results.

Potato sack effect in DNS for Re_{M}=6
[animation].

Potato sack in
BKR10;
for an animation, see also
here.

NEMPI has now been verified for magnetic Reynolds numbers between 1 and 70.
(These Reynolds numbers are based on the wavenumber of the energy-carrying
eddies, which are small compared to the domain.
Reynolds numbers based on the scale of the full domain are about 100 times
bigger and range from 100 to 7000.)
For magnetic Reynolds numbers below 1, NEMPI no longer exists.
This is significant, because it explains why it has not been seen
in earlier work based on quasilinear theory.
Below is an example of the effective magnetic pressure as a function
of the mean magnetic field, showing a minimum for field strengths
below about half the equipartition value.

Demonstration that ½(1-*q*_{p})*B*^{2} is
negative even for a range of Re_{M} values above ∼1.7
(for which there is still no minimum) and below ∼3.5 (for which there is).

To be able to say something about larger values of Re_{M},
we have to reduce the scale separation ratio.
In the figure below, we show the Re_{M} of three fit
parameters β_{p}, β_{*}, and *q*_{p0}.
These parameters characterize the shape of the P_{eff} curve
(solid lines above) and fit the numerics well for weak fields (dotted lines).

Convergence of fit parameters β_{p}, β_{*}, and
*q*_{p0}.

Re_{M}=74 with 256^{3} meshpoints
[animation].

Repetitive structures in a box that is 8 times wider,
with just *y* averaging (upper panel)
and with *yt* averaging (lower panel).
Re_{M}=36.

### Powerpoint presentation: