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Title: DYNAMO EFFECTS NEAR THE TRANSITION FROM SOLAR TO ANTI-SOLAR DIFFERENTIAL ROTATION

Abstract

Numerical MHD simulations play an increasingly important role for understanding the mechanisms of stellar magnetism. We present simulations of convection and dynamos in density-stratified rotating spherical fluid shells. We employ a new 3D simulation code for obtaining the solution of a physically consistent anelastic model of the process with a minimum number of parameters. The reported dynamo simulations extend into a “buoyancy-dominated” regime where the buoyancy forcing is dominant while the Coriolis force is no longer balanced by pressure gradients, and strong anti-solar differential rotation develops as a result. We find that the self-generated magnetic fields, despite being relatively weak, are able to reverse the direction of differential rotation from anti-solar to solar-like. We also find that convection flows in this regime are significantly stronger in the polar regions than in the equatorial region, leading to non-oscillatory dipole-dominated dynamo solutions, and to a concentration of magnetic field in the polar regions. We observe that convection has a different morphology in the inner and the outer part of the convection zone simultaneously such that organized geostrophic convection columns are hidden below a near-surface layer of well-mixed highly chaotic convection. While we focus our attention on the buoyancy-dominated regime, we also demonstratemore » that conical differential rotation profiles and persistent regular dynamo oscillations can be obtained in the parameter space of the rotation-dominated regime even within this minimal model.« less

Authors:
 [1];  [2];  [3]
  1. School of Mathematics and Statistics, University of Glasgow—Glasgow G12 8QW (United Kingdom)
  2. NASA Ames Research Center—Moffett Field, CA 94035 (United States)
  3. Department of Earth and Space Sciences, University of California, Los Angeles—Los Angeles, CA 90095 (United States)
Publication Date:
OSTI Identifier:
22525492
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 810; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CHAOS THEORY; COMPUTERIZED SIMULATION; CONCENTRATION RATIO; CONVECTION; CORIOLIS FORCE; DENSITY; DIPOLES; MAGNETIC FIELDS; MAGNETISM; MAGNETOHYDRODYNAMICS; MATHEMATICAL SOLUTIONS; OSCILLATIONS; PRESSURE GRADIENTS; ROTATION; SPACE; SUN

Citation Formats

Simitev, Radostin D., Kosovichev, Alexander G., and Busse, Friedrich H. DYNAMO EFFECTS NEAR THE TRANSITION FROM SOLAR TO ANTI-SOLAR DIFFERENTIAL ROTATION. United States: N. p., 2015. Web. doi:10.1088/0004-637X/810/1/80.
Simitev, Radostin D., Kosovichev, Alexander G., & Busse, Friedrich H. DYNAMO EFFECTS NEAR THE TRANSITION FROM SOLAR TO ANTI-SOLAR DIFFERENTIAL ROTATION. United States. doi:10.1088/0004-637X/810/1/80.
Simitev, Radostin D., Kosovichev, Alexander G., and Busse, Friedrich H. 2015. "DYNAMO EFFECTS NEAR THE TRANSITION FROM SOLAR TO ANTI-SOLAR DIFFERENTIAL ROTATION". United States. doi:10.1088/0004-637X/810/1/80.
@article{osti_22525492,
title = {DYNAMO EFFECTS NEAR THE TRANSITION FROM SOLAR TO ANTI-SOLAR DIFFERENTIAL ROTATION},
author = {Simitev, Radostin D. and Kosovichev, Alexander G. and Busse, Friedrich H.},
abstractNote = {Numerical MHD simulations play an increasingly important role for understanding the mechanisms of stellar magnetism. We present simulations of convection and dynamos in density-stratified rotating spherical fluid shells. We employ a new 3D simulation code for obtaining the solution of a physically consistent anelastic model of the process with a minimum number of parameters. The reported dynamo simulations extend into a “buoyancy-dominated” regime where the buoyancy forcing is dominant while the Coriolis force is no longer balanced by pressure gradients, and strong anti-solar differential rotation develops as a result. We find that the self-generated magnetic fields, despite being relatively weak, are able to reverse the direction of differential rotation from anti-solar to solar-like. We also find that convection flows in this regime are significantly stronger in the polar regions than in the equatorial region, leading to non-oscillatory dipole-dominated dynamo solutions, and to a concentration of magnetic field in the polar regions. We observe that convection has a different morphology in the inner and the outer part of the convection zone simultaneously such that organized geostrophic convection columns are hidden below a near-surface layer of well-mixed highly chaotic convection. While we focus our attention on the buoyancy-dominated regime, we also demonstrate that conical differential rotation profiles and persistent regular dynamo oscillations can be obtained in the parameter space of the rotation-dominated regime even within this minimal model.},
doi = {10.1088/0004-637X/810/1/80},
journal = {Astrophysical Journal},
number = 1,
volume = 810,
place = {United States},
year = 2015,
month = 9
}
  • We report the results of a magnetohydrodynamic (MHD) simulation of a convective dynamo in a model solar convective envelope driven by the solar radiative diffusive heat flux. The convective dynamo produces a large-scale mean magnetic field that exhibits irregular cyclic behavior with oscillation time scales ranging from about 5 to 15 yr and undergoes irregular polarity reversals. The mean axisymmetric toroidal magnetic field is of opposite signs in the two hemispheres and is concentrated at the bottom of the convection zone. The presence of the magnetic fields is found to play an important role in the self-consistent maintenance of amore » solar-like differential rotation in the convective dynamo model. Without the magnetic fields, the convective flows drive a differential rotation with a faster rotating polar region. In the midst of magneto-convection, we found the emergence of strong super-equipartition flux bundles at the surface, exhibiting properties that are similar to emerging solar active regions.« less
  • On the example of the model developed in the authors' previous reports it is shown that compatibility of the theory of the solar magnetic dynamo with observations requires that 1) the angular velocity of the convective shell have a radial gradient and 2) this gradient be negative.
  • Recently Yoshimura has evaluated the gradient of the Sun's angular velocity (d$Omega$/dr) necessary to give a good fit to the observed solar activity cycle. We estimate the meridional velocities in the convection zone, implied by this value of d$omega$/dr. These meridional velocities are in good agreement with those necessary to explain the Sun's surface differential rotation.
  • The nonlinear reaction of the magnetic field on the field-generating fluid motions in the dynamo process is formulated within the framework of the dynamo equation of the mean magnetohydrodynamics. If this nonlinear reaction mechanism is working in the process of the solar cycle, the differential-rotation--global-convection system operating the solar dynamo must be undergoing an oscillatory modulation associated with the solar cycle. As observable evidences of the operation of the dynamo process, the extended period of near absence of the surface activity of the Sun in the 17th to 18th centuries, the rapid resumption of the magnetic activity to the normalmore » level of the present solar cycle, and the observed variation of the differential rotation at the surface are discussed. To study the first two phenomena, the response of the nonlinear dynamo system to the temporal cessation of operation of the global convection is examined. The temporal cessation or drastic weakening of the global convection itself is assumed and is proposed to be an explanation of the extended period of absence of the surface activity of the Sun. It is suggested that the modulation of the global convection, which could carry a nonnegligible part of the whole convective flux, could bring about a solar-cycle modulation of the total energy output of the Sun from the surface, i.e., the solar constant, the convection zone becoming a temporal heat reservoir. This could give us a theoretical explanation of the solar-cycle--related climatic change of the Earth. Numerical solutions of the dynamo equation as an initial-boundary-value problem show that this nonlinear reaction mechanism works so efficiently that steady oscillations can be achieved quickly from arbitrary initial conditions of negligible field level even without the other possible nonlinear mechanism of the magnetic flux eruption from the system.« less