Speaker
Sarah Jabbari
(KTH Royal Institute of Technology, Stockholm)
Description
In the presence of strong density stratification, turbulence can
lead to a
large-scale instability of a horizontal magnetic field if its strength
is in a
suitable range (within a few percent of the turbulent equipartition
value). This
instability is related to a suppression of the turbulent pressure so
that the
turbulence contribution to the mean magnetic pressure becomes
negative. This results
in the excitation of a negative effective magnetic pressure
instability (NEMPI). This
instability has so far only been studied for an imposed magnetic
field. We want to
know how NEMPI works when the mean magnetic field is
generated self-consistently by
an \alpha^2 dynamo, whether it is affected by global spherical
geometry, and whether
it can influence the properties of the dynamo itself. We adopt the
mean-field
approach which has previously been shown to provide a realistic
description of NEMPI
in direct numerical simulations. We assume axisymmetry and
solve the mean-field
equations with the Pencil-Code for an adiabatic stratification at a
total density
contrast in the radial direction of approximately 4 orders of
magnitude. NEMPI is
found to work when the dynamo-generated field is about 4% of
the equipartition value,
which is achieved through strong \alpha quenching. This
instability is excited in the
top 5% of the outer radius provided the density contrast across
this top layer is at
least 10. NEMPI is found to occur at lower latitudes when the
mean magnetic field is
stronger. For weaker fields, NEMPI can make the dynamo
oscillatory with poleward
migration. In conclusion, NEMPI is a viable mechanism for
producing magnetic flux
concentrations in a strongly stratified spherical shell in which a
magnetic field is
generated by a strongly quenched \alpha effect dynamo.
Primary author
Sarah Jabbari
(KTH Royal Institute of Technology, Stockholm)
