TY - JOUR
T1 - Magnetic flux concentrations from dynamo-generated fields
AU - Jabbari, S.
AU - Brandenburg, A.
AU - Losada, I. R.
AU - Kleeorin, N.
AU - Rogachevskii, I.
PY - 2014
Y1 - 2014
N2 - Context. The mean-field theory of magnetized stellar convection gives rise to two distinct instabilities: the large-scale dynamo instability, operating in the bulk of the convection zone and a negative effective magnetic pressure instability (NEMPI) operating in the strongly stratified surface layers. The latter might be important in connection with magnetic spot formation. However, as follows from theoretical analysis, the growth rate of NEMPI is suppressed with increasing rotation rates. On the other hand, recent direct numerical simulations (DNS) have shown a subsequent increase in the growth rate. Aims. We examine quantitatively whether this increase in the growth rate of NEMPI can be explained by an α2 mean-field dynamo, and whether both NEMPI and the dynamo instability can operate at the same time. Methods. We use both DNS and mean-field simulations (MFS) to solve the underlying equations numerically either with or without an imposed horizontal field. We use the test-field method to compute relevant dynamo coefficients. Results. DNS show that magnetic flux concentrations are still possible up to rotation rates above which the large-scale dynamo effect produces mean magnetic fields. The resulting DNS growth rates are quantitatively reproduced with MFS. As expected for weak or vanishing rotation, the growth rate of NEMPI increases with increasing gravity, but there is a correction term for strong gravity and large turbulent magnetic diffusivity. Conclusions. Magnetic flux concentrations are still possible for rotation rates above which dynamo action takes over. For the solar rotation rate, the corresponding turbulent turnover time is about 5 h, with dynamo action commencing in the layers beneath.
AB - Context. The mean-field theory of magnetized stellar convection gives rise to two distinct instabilities: the large-scale dynamo instability, operating in the bulk of the convection zone and a negative effective magnetic pressure instability (NEMPI) operating in the strongly stratified surface layers. The latter might be important in connection with magnetic spot formation. However, as follows from theoretical analysis, the growth rate of NEMPI is suppressed with increasing rotation rates. On the other hand, recent direct numerical simulations (DNS) have shown a subsequent increase in the growth rate. Aims. We examine quantitatively whether this increase in the growth rate of NEMPI can be explained by an α2 mean-field dynamo, and whether both NEMPI and the dynamo instability can operate at the same time. Methods. We use both DNS and mean-field simulations (MFS) to solve the underlying equations numerically either with or without an imposed horizontal field. We use the test-field method to compute relevant dynamo coefficients. Results. DNS show that magnetic flux concentrations are still possible up to rotation rates above which the large-scale dynamo effect produces mean magnetic fields. The resulting DNS growth rates are quantitatively reproduced with MFS. As expected for weak or vanishing rotation, the growth rate of NEMPI increases with increasing gravity, but there is a correction term for strong gravity and large turbulent magnetic diffusivity. Conclusions. Magnetic flux concentrations are still possible for rotation rates above which dynamo action takes over. For the solar rotation rate, the corresponding turbulent turnover time is about 5 h, with dynamo action commencing in the layers beneath.
KW - Dynamo
KW - Hydrodynamics
KW - Magnetohydrodynamics (MHD)
KW - Sunspots
KW - Turbulence
UR - http://www.scopus.com/inward/record.url?scp=84906854826&partnerID=8YFLogxK
U2 - 10.1051/0004-6361/201423499
DO - 10.1051/0004-6361/201423499
M3 - Article
AN - SCOPUS:84906854826
VL - 568
JO - Astronomy & Astrophysics
JF - Astronomy & Astrophysics
SN - 0004-6361
M1 - A112
ER -