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SUMMARY:Paradigm shifts of the Solar Dynamo - Axel Brandenburg (Nordita)
DTSTART:20081117T160000Z
DTEND:20081117T170000Z
UID:TALK12842@talks.cam.ac.uk
CONTACT:Dr Geoffroy Lesur
DESCRIPTION:Solar dynamo theory has experienced arguably three major parad
 igm\nshifts since its broad initial acceptance during the 1970s. Inevitabl
 y\,\nthese paradigm shifts have brought the modelling further away from th
 e\noriginal ideas that were based on dynamo theory. At the same time solar
 \ndynamo theory has lost much of its initial rigor that dynamo theory used
 \nto be based on. It its therefore important that the motivation for such\
 ndepartures from  the original theory are well justified. In the following
 \nwe comment briefly on each of the three paradigm shift.\n\nIn the Sun an
  oscillatory magnetic field is generated\, but new research\nnow shows tha
 t at large magnetic Reynolds numbers this can only happen\nif the Sun shed
 s small-scale magnetic twist through the surface while\nregenerating an in
 terlinked assembly of large-scale poloidal and toroidal\nmagnetic fields. 
 The Sun is believed to accomplish this through coronal\nmass ejections\, w
 hich are known to shed approximately the required amount\nof magnetic twis
 t or helicity.\n\nThe inclusion of the effects of coronal mass ejections i
 nto the model\nis believed to be one of the key factors of future solar dy
 namo models.\nOther factors include the recently discovered near-surface s
 hear layer\nof the Sun\, where the shear has the opposite radial gradient 
 than in the\nbulk of the convection zone\, and can lead to equatorward mig
 ration of\nsunspot activity\, which is a major problem in understanding th
 e solar\ndynamo. Finally\, the discovery that convection pumps magnetic fi
 elds\ndownward and thus opposes magnetic buoyancy losses is another factor
 \nthat makes so-called distributed dynamo model viable. Here\, the magneti
 c\nfield resides in the entire convection zone\, and is thus not confined 
 to\nthe thin layer just beneath the convection zone\, which is still assum
 ed\nin many models.\n\nRegardless of the nature and the location of the dy
 namo\, the effect of\nmagnetic helicity fluxes is crucial for allowing the
  dynamo to reach\nsignificant saturation levels. This is most dramatically
  demonstrated\nwith two examples of convective dynamo action in the presen
 ce of either\nvertical or horizontal shear. In both cases high saturation 
 levels\nare reached provided the contours of constant shear cross a bounda
 ry\nthat transmits magnetic helicity fluxes. Shear also allows for\ncyclic
  dynamo action with a frequency that scales with the quenched\nvalue of th
 e magnetic diffusivity. This quantity is now determined\nwith the test fie
 ld method which will also be discussed in my talk.
LOCATION:MR14\, DAMTP\, Pav. F
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