Speaker
Sarah Jabbari
(PhD student)
Description
We study a system of a highly stratified turbulent plasma.
In such a system, when the magnetic Reynolds number is large
enough and there is a background field of suitable strength,
a new effect will play role in concentrating magnetic fields
such that it leads to the formation of magnetic spots and
bipolar regions. This effect is due to the fact that the
turbulent pressure is suppressed by the large-scale magnetic
field, which adds a negative term to the total mean-field
(effective) pressure. This leads to an instability, which is
known as the negative effective magnetic pressure
instability (NEMPI). Direct numerical simulations (DNS) of
isothermally forced turbulence have shown that NEMPI leads
to the formation of spots in the presence of an imposed
field. Our main aim now is to use NEMPI to explain the
formation of active regions and sunspots. To achieve this
goal, we need to move progressively to more realistic
models. Here we extend our model by allowing the magnetic
field to be generated by a dynamo. A dynamo plays an
important role in solar activity. Therefore, it is of
interest to investigate NEMPI in the presence of
dynamo-generated magnetic fields. Mean-field simulations
(MFS) of such systems in spherical geometry have shown how
these two instabilities work in concert. In fact NEMPI will
be activated as long as the strength of the magnetic field
generated by the dynamo is in a proper range (for more
detail see Jabbari et al. 2013). In our new study, we use
DNS to investigate a similar system. The turbulence is
forced in the entire spherical shell, but the forcing is
made helical in the lower 30% of the shell, similar to the
model of Mitra et al. (2014). We perform simulations using
the Pencil Code for different density contrasts and other
input parameters. We applied vertical field boundary
conditions in the r direction. The results show that, when
the stratification is high enough, intense bipolar regions
form and as time passes, they expand, merge and create giant
structures. At the same time, the new structures appear at
different latitudes. By extending in φ direction, the size
of the bipolar regions decreases. When the helical zone is
thinner, the structures appear at a later time.
Primary author
Sarah Jabbari
(PhD student)
