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.
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.
%0 Generic
%1 citeulike:12086662
%A Jabbari, Sarah
%A Brandenburg, Axel
%A Kleeorin, Nathan
%A Mitra, Dhrubaditya
%A Rogachevskii, Igor
%D 2013
%K imported
%T Surface flux concentrations and spherical alpha-square dynamo
%U http://arxiv.org/abs/1302.5841
%X 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.
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.
@misc{citeulike:12086662,
abstract = {{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.
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.}},
added-at = {2019-03-25T08:20:55.000+0100},
archiveprefix = {arXiv},
author = {Jabbari, Sarah and Brandenburg, Axel and Kleeorin, Nathan and Mitra, Dhrubaditya and Rogachevskii, Igor},
biburl = {https://www.bibsonomy.org/bibtex/246c994ab9185faf9527919183b2a221f/ericblackman},
citeulike-article-id = {12086662},
citeulike-linkout-0 = {http://arxiv.org/abs/1302.5841},
citeulike-linkout-1 = {http://arxiv.org/pdf/1302.5841},
day = 23,
eprint = {1302.5841},
interhash = {007cb5edae1042c7091f30e482e8b68d},
intrahash = {46c994ab9185faf9527919183b2a221f},
keywords = {imported},
month = feb,
posted-at = {2013-02-27 17:55:20},
priority = {2},
timestamp = {2019-03-25T08:20:55.000+0100},
title = {{Surface flux concentrations and spherical alpha-square dynamo}},
url = {http://arxiv.org/abs/1302.5841},
year = 2013
}