Dynamo action in a rotating spherical shell at high Rayleigh numbers

Futoshi Takahashi, Masaki Matsushima

Research output: Contribution to journalArticle

14 Citations (Scopus)

Abstract

We investigate convection-driven dynamos in a rotating spherical shell with the Rayleigh number Ra up to about 53 times the critical value Rac, emphasizing Rayleigh number dependence of the thermal convection and the magnetic field generated by dynamo action. The Rayleigh numbers used in calculations are chosen so as to be in a range which allows us to study the sequence of bifurcation. In the low-Ra-dynamo regime, the flow structure is characterized by columnar convection cells, which mainly generate the magnetic field that is predominantly dipolar. Force balance is essentially in a geostrophic state. Both the magnetic energy and the kinetic energy increase with increase in Ra. In the moderate-Ra-dynamo regime, convective motions appear inside the tangent cylinder (TC), where the azimuthal magnetic field is generated through the so-called ω effect. However, the magnetic energy shows saturation due to relatively inefficient magnetic field generation. In the high-Ra-dynamo regime, dominance of convection inside the TC is remarkable. The advection processes play important roles both in force balance and in magnetic field generation. The magnetic field is generated very inefficiently, leading to the reduced magnetic energy in spite of higher kinetic energy. These three dynamo regimes exhibit distinctive differences in the process of generating magnetic field and characteristic dissipation scales.

Original languageEnglish
Article number076601
Pages (from-to)1-10
Number of pages10
JournalPhysics of Fluids
Volume17
Issue number7
DOIs
Publication statusPublished - Jan 1 2005
Externally publishedYes

Fingerprint

spherical shells
Rayleigh number
magnetic fields
tangents
convection
kinetic energy
convection cells
rotating generators
advection
free convection
energy
dissipation
saturation

All Science Journal Classification (ASJC) codes

  • Condensed Matter Physics

Cite this

Dynamo action in a rotating spherical shell at high Rayleigh numbers. / Takahashi, Futoshi; Matsushima, Masaki.

In: Physics of Fluids, Vol. 17, No. 7, 076601, 01.01.2005, p. 1-10.

Research output: Contribution to journalArticle

@article{b775cd051b1341d08ddc3dddff5b9c1c,
title = "Dynamo action in a rotating spherical shell at high Rayleigh numbers",
abstract = "We investigate convection-driven dynamos in a rotating spherical shell with the Rayleigh number Ra up to about 53 times the critical value Rac, emphasizing Rayleigh number dependence of the thermal convection and the magnetic field generated by dynamo action. The Rayleigh numbers used in calculations are chosen so as to be in a range which allows us to study the sequence of bifurcation. In the low-Ra-dynamo regime, the flow structure is characterized by columnar convection cells, which mainly generate the magnetic field that is predominantly dipolar. Force balance is essentially in a geostrophic state. Both the magnetic energy and the kinetic energy increase with increase in Ra. In the moderate-Ra-dynamo regime, convective motions appear inside the tangent cylinder (TC), where the azimuthal magnetic field is generated through the so-called ω effect. However, the magnetic energy shows saturation due to relatively inefficient magnetic field generation. In the high-Ra-dynamo regime, dominance of convection inside the TC is remarkable. The advection processes play important roles both in force balance and in magnetic field generation. The magnetic field is generated very inefficiently, leading to the reduced magnetic energy in spite of higher kinetic energy. These three dynamo regimes exhibit distinctive differences in the process of generating magnetic field and characteristic dissipation scales.",
author = "Futoshi Takahashi and Masaki Matsushima",
year = "2005",
month = "1",
day = "1",
doi = "10.1063/1.1972016",
language = "English",
volume = "17",
pages = "1--10",
journal = "Physics of Fluids",
issn = "1070-6631",
publisher = "American Institute of Physics Publising LLC",
number = "7",

}

TY - JOUR

T1 - Dynamo action in a rotating spherical shell at high Rayleigh numbers

AU - Takahashi, Futoshi

AU - Matsushima, Masaki

PY - 2005/1/1

Y1 - 2005/1/1

N2 - We investigate convection-driven dynamos in a rotating spherical shell with the Rayleigh number Ra up to about 53 times the critical value Rac, emphasizing Rayleigh number dependence of the thermal convection and the magnetic field generated by dynamo action. The Rayleigh numbers used in calculations are chosen so as to be in a range which allows us to study the sequence of bifurcation. In the low-Ra-dynamo regime, the flow structure is characterized by columnar convection cells, which mainly generate the magnetic field that is predominantly dipolar. Force balance is essentially in a geostrophic state. Both the magnetic energy and the kinetic energy increase with increase in Ra. In the moderate-Ra-dynamo regime, convective motions appear inside the tangent cylinder (TC), where the azimuthal magnetic field is generated through the so-called ω effect. However, the magnetic energy shows saturation due to relatively inefficient magnetic field generation. In the high-Ra-dynamo regime, dominance of convection inside the TC is remarkable. The advection processes play important roles both in force balance and in magnetic field generation. The magnetic field is generated very inefficiently, leading to the reduced magnetic energy in spite of higher kinetic energy. These three dynamo regimes exhibit distinctive differences in the process of generating magnetic field and characteristic dissipation scales.

AB - We investigate convection-driven dynamos in a rotating spherical shell with the Rayleigh number Ra up to about 53 times the critical value Rac, emphasizing Rayleigh number dependence of the thermal convection and the magnetic field generated by dynamo action. The Rayleigh numbers used in calculations are chosen so as to be in a range which allows us to study the sequence of bifurcation. In the low-Ra-dynamo regime, the flow structure is characterized by columnar convection cells, which mainly generate the magnetic field that is predominantly dipolar. Force balance is essentially in a geostrophic state. Both the magnetic energy and the kinetic energy increase with increase in Ra. In the moderate-Ra-dynamo regime, convective motions appear inside the tangent cylinder (TC), where the azimuthal magnetic field is generated through the so-called ω effect. However, the magnetic energy shows saturation due to relatively inefficient magnetic field generation. In the high-Ra-dynamo regime, dominance of convection inside the TC is remarkable. The advection processes play important roles both in force balance and in magnetic field generation. The magnetic field is generated very inefficiently, leading to the reduced magnetic energy in spite of higher kinetic energy. These three dynamo regimes exhibit distinctive differences in the process of generating magnetic field and characteristic dissipation scales.

UR - http://www.scopus.com/inward/record.url?scp=23044473683&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=23044473683&partnerID=8YFLogxK

U2 - 10.1063/1.1972016

DO - 10.1063/1.1972016

M3 - Article

AN - SCOPUS:23044473683

VL - 17

SP - 1

EP - 10

JO - Physics of Fluids

JF - Physics of Fluids

SN - 1070-6631

IS - 7

M1 - 076601

ER -