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VERSION:2.0
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BEGIN:VEVENT
DTSTART;TZID=Atlantic/Canary:20121211T123000
DTEND;TZID=Atlantic/Canary:20121211T133000
UID:iactalks-457
X-WR-CALNAME: IAC Talks: Open Astronomy Seminars
X-ORIGINAL-URL: /iactalks/Talks/view/457
CREATED:2012-12-11T12:30:00+00:00
X-WR-CALDESC: IAC Talks upcomming talks
SUMMARY:Testing new ideas of sunspot formation: the negative effective magn
 etic pressure instability 
DESCRIPTION:Testing new ideas of sunspot formation: the negative effective 
 magnetic pressure instability \nMrs. Illa Rivero\n\nThe formation of activ
 e regions and its most visible outcome-sunspots-are still a matter of rese
 arch. Magnetic flux tubes theory tends to explain the formation of sunspot
 s, but it still faces some unresolved questions: How are they generated? W
 hy can they survive all along the convective zone? How do they rise? I wil
 l review this theory and introduce a new way to understand sunspot formati
 on: the negative effective magnetic pressure instability (NEMPI). NEMPI wa
 s predicted long ago (Kleeorin et al., 1989, 1990; Kleeorin \& Rogacke
 vskii, 1994; Kleeorin et al., 1996) but has only been seen recently (Brand
 erburg et. al., 2011). It arised as a effect of strong stratication and th
 e presence of turbulence with a weak mean magnetic field. Under suitable c
 onditions, a large-scale instability resulting in the formation of non-uni
 form magnetic structures, can be excited over the scale of many turbulent 
 eddies or convection cells. This instability is caused by a negative contr
 ibution of turbulence to the effective (mean-field) magnetic pressure and 
 has previously been discussed in connection with the formation of active r
 egions and perhaps sunspots. Now, we want to understand the effects of rot
 ation on this instability in both two and three dimensions. We use mean-fi
 eld magnetohydrodynamics in a parameter regime in which the properties of 
 the negative effective magnetic pressure instability have previously been 
 found to be in agreement with those of direct numerical simulations. We fi
 nd that the instability is suppressed already for relatively slow rotation
  with Coriolis numbers (i.e. inverse Rossby numbers) around 0.2. The suppr
 ession is strongest at the equator. In the nonlinear regime, we find trave
 ling wave solutions with propagation in the prograde direction at the equa
 tor with additional poleward migration away from the equator. The prograde
  rotation of the magnetic pattern near the equator is argued to be a possi
 ble explanation for the faster rotation speed of magnetic tracers found on
  the Sun. In the bulk of the domain, kinetic and current helicities are ne
 gative in the northern hemisphere and positive in the southern.
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