Speaker
Illa Rivero Losada
Description
In a strongly stratified turbulent layer, a uniform
horizontal magnetic field can become unstable to
spontaneously form local flux concentrations due to a
negative contribution of turbulence to the large-scale
(mean-field) magnetic pressure. This mechanism, called the
negative effective magnetic pressure instability (NEMPI), is
of interest in connection with dynamo scenarios where most
of the magnetic field resides in the bulk of the convection
zone, and not at the bottom. Recent work using the
mean-field hydromagnetic equations has shown that NEMPI
becomes suppressed at rather low rotation rates with
Coriolis numbers as low as 0.1. Here we extend these earlier
investigations by studying the effects of rotation both on
the development of NEMPI and on the effective magnetic
pressure. We also quantify the kinetic helicity from direct
numerical simulations (DNS) and compare with earlier work.
To calculate the rotational effect on the effective magnetic
pressure we consider both DNS and analytical studies using
the $\tau$ approach. To study the effects of rotation on the
development of NEMPI we use both DNS and mean-field
calculations of the 3D hydromagnetic equations in a
Cartesian domain. We find that the growth rates of NEMPI
from earlier mean-field calculations are well reproduced
with DNS, provided the Coriolis number is below about 0.06.
In that case, kinetic and magnetic helicities are found to
be weak. For faster rotation, dynamo action becomes
possible. However, there is an intermediate range of
rotation rates where dynamo action on its own is not yet
possible, but the rotational suppression of NEMPI is being
alleviated. Production of magnetic flux concentrations
through the suppression of turbulent pressure appears to be
possible only in the upper-most layers of the Sun, where the
convective turnover time is less than 2 hours.
Primary author
Illa Rivero Losada
