The influence of density stratification and multiple
nonlinearities on solar torsional oscillations

Covas, Eurico; Moss, D.; Tavakol, Reza

doi:10.1051/0004-6361:20034046

Cited by 36 publications

(35 citation statements)

References 17 publications

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“…This parallels our previous studies of solar torsional oscillations (see, e.g., Covas et al 2000;Tavakol et al 2002;Covas et al 2004, and references therein). We investigate the nature of variations in the convection zones of rapidly rotating stars, by considering three families of models with different depths of dynamo-active regions ("convective envelopes").…”

Section: Introductionsupporting

confidence: 91%

“…Throughout we take α r > 0 and R α < 0 and use a uniform density: our earlier work (Covas et al 2004) illustrates the effects of introducing a strongly radially dependent density -in brief, no qualitatively new effects are found, although the radial distribution of perturbations to the angular velocity can be altered. In particular, it tends to increase (decrease) the strength of the torsional oscillations at the top (bottom) of the convection zone.…”

Section: The Modelmentioning

confidence: 99%

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Dynamo models and differential rotation in late-type rapidly rotating stars

Covas

Moss

Tavakol

2004

A&A

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Abstract. Increasing evidence is becoming available about not only the surface differential rotation of rapidly rotating cool stars but, in a small number of cases, also about temporal variations, which possibly are analogous to the solar torsional oscillations. Given the present difficulties in resolving the precise nature of such variations, due to both the short length and poor resolution of the available data, theoretical input is vital to help assess the modes of behaviour that might be expected, and will facilitate interpretation of the observations. Here we take a first step in this direction by studying the variations in the convection zones of such stars, using a two dimensional axisymmetric mean field dynamo model operating in a spherical shell in which the only nonlinearity is the action of the azimuthal component of the Lorentz force of the dynamo generated magnetic field on the stellar angular velocity. We consider three families of models with different depths of dynamo-active regions. For moderately supercritical dynamo numbers we find torsional oscillations that penetrate all the way down to the bottom of the convection zones, similar to the case of the Sun. For larger dynamo numbers we find fragmentation in some cases and sometimes there are other dynamical modes of behaviour, including quasi-periodicity and chaos. We find that the largest deviations in the angular velocity distribution caused by the Lorentz force are of the order of few percent, implying that the original assumed "background" rotation field is not strongly distorted.

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Section: Introductionsupporting

confidence: 91%

Section: The Modelmentioning

confidence: 99%

Dynamo models and differential rotation in late-type rapidly rotating stars

Covas

Moss

Tavakol

2004

A&A

Self Cite

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“…The amplitude of the oscillations is up to 5% of the angular velocity, i.e., a few times larger than the observed ones. In a turbulent mean-field model, where the magnetic feedback is mediated solely by the Lorentz force, Covas et al (2004) found that the amplitude of the oscillations depends on the amplitude of the α-effect. It is possible to verify this finding in global simulations by varying the Rossby number.…”

Section: Understanding Torsional Oscillationsmentioning

confidence: 99%

“…It is puzzling that the equatorward branch starts before the beginning of the magnetic cycle. The magnetic feedback via the Lorentz force on the plasma flow is one possible explanation motivated by mean-field turbulent dynamo models (Yoshimura 1981;Kleeorin & Ruzmaikin 1981;Covas et al 2000Covas et al , 2004. In a flux-transport dynamo model, where the source of the poloidal field is non-local and depends on the buoyancy of magnetic flux tubes, Rempel (2007) explained the high latitude branch of the oscillations as a result of the magnetic forcing, while arguing that the equatorial branch has a thermal origin.…”

Section: Introductionmentioning

confidence: 99%

Understanding Solar Torsional Oscillations From Global Dynamo Models

Gómez

Smolarkiewicz

Pino

et al. 2016

ApJL

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The phenomenon of solar "torsional oscillations" (TO) represents migratory zonal flows associated with the solar cycle. These flows are observed on the solar surface and, according to helioseismology, extend through the convection zone. We study the origin of the TO using results from a global MHD simulation of -2 -the solar interior that reproduces several of the observed characteristics of the mean-flows and magnetic fields. Our results indicate that the magnetic tension (MT) in the tachocline region is a key factor for the periodic changes in the angular momentum transport that causes the TO. The torque induced by the MT at the base of the convection zone is positive at the poles and negative at the equator. A rising MT torque at higher latitudes causes the poles to speed-up, whereas a declining negative MT torque at the lower latitudes causes the equator to slow-down. These changes in the zonal flows propagate through the convection zone up to the surface. Additionally, our results suggest that it is the magnetic field at the tachocline that modulates the amplitude of the surface meridional flow rather than the opposite as assumed by flux-transport dynamo models of the solar cycle.

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“…Candidates for the driving mechanism are Lorentz force feedback (e.g., Schüssler 1981;Yoshimura 1981), thermal feedback (Spruit 2003), and magnetic quenching of turbulent angular momentum transport (Kitchatinov & Pipin 1998;Kitchatinov et al 1999). Most models of torsional oscillations to date are based on Lorentz force feedback, most of them use a simplified momentum equation only considering zonal flows (see, e.g., Covas et al 2000Covas et al , 2004Covas et al , 2005Chakraborty et al 2009aChakraborty et al , 2009b. While this approach allows for quite realistic torsional oscillations there are two major problems.…”

Section: Introductionmentioning

confidence: 99%

High-Latitude Solar Torsional Oscillations During Phases of Changing Magnetic Cycle Amplitude

Rempel

2012

ApJ

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Torsional oscillations are variations of the solar differential rotation that are strongly linked to the magnetic cycle of the Sun. Helioseismic inversions have revealed significant differences in the high-latitude branch of torsional oscillations between cycle 23 and cycle 24. Here we employ a non-kinematic flux-transport dynamo model that has been used previously to study torsional oscillations and simulate the response of the high-latitude branch to a change in the amplitude of the magnetic cycle. It is found that a reduction of the cycle amplitude leads to an increase in the amplitude of differential rotation that is mostly visible as a drop in the high-latitude rotation rate. Depending on the amplitude of this adjustment the high-latitude torsional oscillation signal can become temporarily hidden due to the unknown changing mean rotation rate that is required to properly define the torsional oscillation signal.

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The influence of density stratification and multiple nonlinearities on solar torsional oscillations

Cited by 36 publications

References 17 publications

Dynamo models and differential rotation in late-type rapidly rotating stars

Dynamo models and differential rotation in late-type rapidly rotating stars

Understanding Solar Torsional Oscillations From Global Dynamo Models

High-Latitude Solar Torsional Oscillations During Phases of Changing Magnetic Cycle Amplitude

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