On the role of asymmetries in the reversal of the solar magnetic field

Brun, Allan Sacha; DeRosa, Marc L.; Hoeksema, J. T.

doi:10.1017/s1743921313002275

Cited by 1 publication

(1 citation statement)

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“…In the past few decades, a wide variety of published nonlinear dynamo simulations in spherical geometries have been shown to exhibit cyclical behaviour (e.g., Gilman 1983;Ghizaru et al 2010, in the context of stellar astrophysics, or Goudard & Dormy 2008;Schrinner et al 2011;Simitev & Busse 2012; in the planetary context). However, to the extent that these simulations have exhibited systematic latitudinal propagation, this has generally been poleward (see, e.g., discussion in Brun et al 2013), in agreement with the Parker-Yoshimura rule (given the realized kinetic helicity and differential rotation) but in conflict with the observed solar butterfly diagram. More recently, a few groups have published examples of convective dynamos whose propagation is equatorward: e.g., Käpylä et al (2012Käpylä et al ( , 2013; Augustson et al (2013); Warnecke et al (2014).…”

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confidence: 64%

Helicity inversion in spherical convection as a means for equatorward dynamo wave propagation

Duarte¹,

Wicht

Browning³

et al. 2015

Mon. Not. R. Astron. Soc.

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We discuss here a purely hydrodynamical mechanism to invert the sign of the kinetic helicity, which plays a key role in determining the direction of propagation of cyclical magnetism in most models of dynamo action by rotating convection. Such propagation provides a prominent, and puzzling constraint on dynamo models. In the Sun, active regions emerge first at mid-latitudes, then appear nearer the equator over the course of a cycle, but most previous global-scale dynamo simulations have exhibited poleward propagation (if they were cyclical at all). Here, we highlight some simulations in which the direction of propagation of dynamo waves is altered primarily by an inversion of the kinetic helicity throughout much of the interior, rather than by changes in the differential rotation. This tends to occur in cases with a low Prandtl number and internal heating, in regions where the local density gradient is relatively small. We analyse how this inversion arises, and contrast it to the case of convection that is either highly columnar (i.e., rapidly rotating) or locally very stratified; in both of those situations, the typical profile of kinetic helicity (negative throughout most of the northern hemisphere) instead prevails.

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