Effect of Density Stratification on the Thermal Convection in a Rotating Spherical Shell

Nishikawa, Nobuhide; Kusano, K.

doi:10.1086/344138

Cited by 5 publications

(2 citation statements)

References 21 publications

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“…But in simple parametrizations of the solar dynamo, the layer where this reversal is achieved is typically taken to be confined to a region near the bottom of the convective envelope or below its base (e.g., Charbonneau 2010). The convergence of flows to feed plumes that are beginning to rise buoyantly from a bottom thermal boundary layer is also well known (Julien et al 1996;Nishikawa & Kusano 2002;Dubé & Charbonneau 2013;Guervilly et al 2014) and is likewise somewhat distinct from the distributed helicity profiles described here, though the same basic physical mechanisms underlie all these. This will be demonstrated in the following Sections.…”

Section: "Inverted" Helicity Configurationmentioning

confidence: 99%

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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Section: "Inverted" Helicity Configurationmentioning

confidence: 99%

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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“…In Babcock-Leighton models, the poloidal field is regenerated by the decay of active regions and by their subsequent poleward migration due to interactions with meridional flows. Recent work invokes either surface-bound shallow convection (DeRosa et al 2002;Robinson et al 2003;Nishikawa & Kusano 2002;Ru ¨diger et al 2003) or rotationally-influenced magneto-shear instabilities and turbulence in the tachocline (Miesch 2003;Petrovay 2003;Cally 2003;Kim & MacGregor 2003;Choudhuri 2003;Cline et al 2003;Marik & Petrovay 2002;Zhang & Liao 2003;Hathaway et al 2003) as drivers of the a-effect, so there is competition between surface-driven and deep-seated a-engines (Mason et al 2002). Of course, the communication between the tachocline and surface field requires buoyantly-rising flux tubes that have a sufficiently long memory to remember their magnetic helicity at emergence, which has been scrutinized in a number of studies and MHD simulations (Choudhuri 2003;Cline et al 2003;Fan & Gibson 2003;Fan et al 2003b;Linton & Antiochos 2002;Abbett & Fisher 2003;Rempel 2003;Rempel & Dikpati 2003;Sa ´nchez Almeida et al 2003;Kuzanyan et al 2003).…”

Section: The Workings Of the Solar Dynamomentioning

confidence: 99%

Astrophysics in 2003

Trimble

Aschwanden

2004

PUBL ASTRON SOC PAC

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Five coherent sections appear this year, addressing solar physics, cosmology (with WMAP highlights), gamma-ray bursters (and their association with Type Ia supernovae), extra-solar-system planets, and the formation and evolution of galaxies (from reionization to assemblage of Local Group galaxies). There are also eight incoherent sections that deal with other topics in stellar, galactic, and planetary astronomy and the people who study them. INTRODUCTIONSTriskaidekaphobics may prefer not to read this installment of ApXX, and their numbers may well be augmented even before the introduction is over. This is, in fact, the 13th overview of 365.24 days of the astronomy and astrophysics literature. The earlier ones are cited here as Ap91, Ap92, …, Ap02 and appear in volumes 104-115 of PASP. The authors have tried to read systematically the contents of about 30 journals and other periodicals. Only about 10% of what has been read gets cited, and being cited is not always an unqualified compliment.Section 2 was assembled using papers found on the Astrophysics Data Service, maintained with support from NASA and including Solar

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Simulation study of the symmetry-breaking instability and the dipole field reversal in a rotating spherical shell dynamo

Nishikawa

Kusano

2008

Physics of Plasmas

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The reversal mechanism of a dipole magnetic field generated by dynamo action in a rotating spherical shell is investigated by a three-dimensional nonlinear magnetohydrodynamic simulation as well as a linear stability analysis. The emphasis of the study is on understanding the relationship between dipole reversal and the symmetry properties of the dynamo solution. As a result, first, it is found that there is a threshold of the magnetic Prandtl number, below which the dipole field is never reversed, and above which the reversal occurs at irregular intervals like the paleomagnetic evolution of the geodynamo. Second, it is shown that the dynamo process responsible for the generation of a dipole field (called “a-dynamo” in this paper) consists only of the antimirror symmetric magnetic field and the mirror symmetric velocity field with respect to the equatorial plane. Third, it is found that the components of the opposite symmetry to the a-dynamo grow only during the polarity reversal events and quickly decay afterwards. This indicates that the dipole field reversal and the loss of equatorial symmetry are tightly connected. In fact, it is clearly demonstrated by numerical analyses that the a-dynamo process is linearly unstable for the perturbation of opposite symmetry when the magnetic Prandtl number exceeds the threshold for dipole reversal. Mode coupling between the longitudinal Fourier components plays a crucial role in creating the instability. Based on the above results, it is proposed that symmetry-breaking instability could be the mechanism for dipole field reversal in the geodynamo process. The energy conversion between components of different symmetry is also analyzed in the quasistable polarity phase and in the polarity reversal phase, respectively.

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Effect of Density Stratification on the Thermal Convection in a Rotating Spherical Shell

Cited by 5 publications

References 21 publications

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

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

Astrophysics in 2003

Simulation study of the symmetry-breaking instability and the dipole field reversal in a rotating spherical shell dynamo

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