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SUMMARY:Magnetic flux concentrations from dynamo-generated fields
DTSTART;VALUE=DATE-TIME:20140129T123000Z
DTEND;VALUE=DATE-TIME:20140129T131500Z
DTSTAMP;VALUE=DATE-TIME:20200921T184800Z
UID:indico-event-4328@indico.fysik.su.se
DESCRIPTION:The mean-field theory of magnetized stellar convection gives r
ise to the two possibility of distinct instabilities: the large-scale dyna
mo instability\, operating in the bulk of the convection zone\, and a nega
tive effective magnetic pressure instability (NEMPI) operating in the stro
ngly stratified surface layers. The latter might be important in connectio
n with magnetic spot formation\, but the growth rate of NEMPI is suppresse
d with increasing rotation rates\, although recent direct numerical simula
tions (DNS) have shown a subsequent increase in the growth rate. We examin
e quantitatively whether this increase in the growth rate of NEMPI can be
explained by an alpha squared mean-field dynamo\, and whether both NEMPI a
nd the dynamo instability can operate at the same time. We use both DNS an
d mean-field simulations (MFS) to solve the underlying equations numerical
ly either with or without an imposed horizontal field. We use the test-fie
ld method to compute relevant dynamo coefficients. DNS show that magnetic
flux concentrations are still possible up to rotation rates above which th
e large-scale dynamo effect produces mean magnetic fields. The resulting D
NS growth rates are quantitatively well 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. Magnetic flux concentrations are st
ill possible for rotation rates above which dynamo action takes over. For
the solar rotation rate\, the corresponding turbulent turnover time is abo
ut 5 hours\, with dynamo action commencing in the layers beneath.\n\narXiv
:1401.6107\n\nhttps://indico.fysik.su.se/event/4328/
LOCATION: 122:026
URL:https://indico.fysik.su.se/event/4328/
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