arXiv subjects

Lydia Korre

Publications and source records attributed to Lydia Korre.

12 recordsLinked to original sources

arXiv2026

The relationship between solar and stellar tachoclines, dynamos, and spin-down

The solar tachocline, a thin shear layer separating the differentially rotating convective zone (CZ) from the underlying rigidly rotating radiative zone (RZ), remains a dynamical mystery. Under the influence of "radiative spreading" (the process by which meridional circulation, baroclinicity, and differential rotation burrow through a stably stratified fluid), the tachocline should have spread significantly by the current age of the Sun. Some unknown torque must therefore rigidify the whole solar interior below the CZ and keep the tachocline confined to a thin layer. At the same time, the spin-down process (through which cool stars shed angular momentum via surface magnetic torques) must extract angular momentum from the solar RZ, presenting a second mystery: solar spin-down must be communicated from the near-surface layers to the deep interior, all the while leaving the tachocline intact. In this work, we explicitly analyze the dynamics of radiative spreading in two 3D, spherical-shell, fully nonlinear fluid simulations of a solar-like CZ--RZ system, one without a magnetic field (hydrodynamic---HD) and one with a small random seed magnetic field (magnetohydrodynamic---MHD). We find that radiative spreading is unmitigated in the HD case (as expected), but in the MHD case, a self-excited dynamo not only confines the tachocline, but \textit{also} extracts angular momentum from the RZ, thereby communicating the spin-down downward. We thus speculate that solar and stellar tachoclines may be intimately linked to both the spin-down process and global dynamo.

astro-ph.SR↗

arXiv2026

A Dynamo Confinement Scenario for the Solar Tachocline and its Implications for Spin-down in the Radiative Spreading Regime

At the base of the Sun's convective zone, a narrow shear layer called the tachocline separates strong latitudinal differential rotation above from nearly rigid rotation in the radiative zone below. The observed thinness of the tachocline is a long-standing dynamical puzzle because the tachocline should have spread significantly due to inward-burrowing meridional circulation, also called "radiative spreading." We recently presented the first pair of global simulations to reveal a statistically stationary tachocline confined against radiative spreading by the Maxwell stresses from the large-scale nonaxisymmetric modes of a dynamo, which penetrated into and below the tachocline through a novel magnetic skin effect. In the work presented here, we systematically examine how this "dynamo confinement scenario" works against radiative spreading in a suite of simulations as the governing parameters trend in the direction of the true solar regime. We find that as the stable stratification of the radiative zone is made progressively stronger, the dynamo cycles get longer, the magnetic field consequently penetrates deeper due to the skin effect, and the tachocline becomes more confined. Furthermore, these results have interesting consequences for solar spin-down. In all of our radiatively spreading simulations, the tachocline region spins down due to the burrowing circulation. Below the tachocline, the Maxwell stresses transmit this spin-down further to rigidify the deeper radiative zone. We thus speculate that, in addition to confining the tachocline, the dynamo may provide a pathway to communicate spin-down from the near-surface layers to the deep interior.

astro-ph.SR↗

arXiv2025

Dynamo Confinement of a Radiatively Spreading Solar Tachocline Revealed by Self-consistent Global Simulations

The helioseismically observed solar tachocline is a thin internal boundary layer of shear that separates the rigidly-rotating solar radiative zone from the differentially-rotating convective zone and is believed to play a central role in the 22-year solar dynamo cycle. The observed thinness of the tachocline has long been a mystery, given the expected tendency of such shear to undergo radiative spreading. Radiative spreading is the process by which the meridional circulation and angular velocity burrow into a stably-stratified fluid owing to the mitigating effect of radiative thermal diffusion. A confinement mechanism is thus required to keep the tachocline so thin. In previous work using global dynamo simulations, we achieved a statistically-stationary confined tachocline where the confinement mechanism was derived from the Maxwell stress arising from a dynamo-generated nonaxisymmetric poloidal magnetic field. However, the parameters chosen meant that the tachocline was confined against viscous spread instead of radiative spread. Here, we show that the previously identified dynamo confinement mechanism still succeeds in a simulation that lies in the more solar-like radiative spreading regime. In particular, a nonaxisymmetric, quasicyclic dynamo develops in the convective zone and overshoot layer, penetrates into the radiative zone via a novel type of skin effect, and creates a Maxwell stress that confines the tachocline over many magnetic cycles. To the best of our knowledge, this is the first fully self-consistent rendering of a confined tachocline in a global numerical simulation in the parameter regime appropriate to the Sun.

astro-ph.SR↗

arXiv2024

On the penetration of large-scale flows into stellar radiative zones

The propagation of meridional circulation below the base of the convection zone of low-mass stars may play a crucial role in the transport of angular momentum and also significantly contribute to the transport of chemical species and magnetic fields within their stable radiative zone. We systematically study these large-scale mean flows by performing three-dimensional (3D) global numerical simulations in a spherical shell that consists of a convection zone (CZ) overlying a stably stratified region. We find that the meridional flows can penetrate distances as large as $\sim 0.21r_o$ (where $r_o$ is the outer radius) below the base of the convection zone, provided that the Eddington-Sweet timescale $t_{ES}$ is much shorter than the viscous timescale $t_{\nu}$ as measured by the parameter $\sigma=(t_{ES}/t_{\nu})^{1/2}$. In the solar-like regime where $\sigma\lesssim 1$ in the upper radiative zone (RZ), we find that the angular momentum transport in the deep RZ is determined primarily by the action of the Coriolis force on meridional flows. In contrast, in models run in the $\sigma> 1$ regime, the meridional flows become weaker and the viscous effects dominate. We find that the penetration lengthscale $\delta_{MC}$ of these mean flows when $\sigma\lesssim 1$ is proportional to $\sigma^{-0.22}$. Our findings may provide a better understanding of the role of the meridional flows in the dynamics of the solar interior and inform future numerical studies that are focused on capturing solar-like dynamics self-consistently.

astro-ph.SR↗

arXiv2023

Assessing the Observability of Deep Meridional Flow Cells in the Solar Interior

Meridional circulation regulates the Sun's interior dynamics and magnetism. While it is well accepted that meridional flows are poleward at the Sun's surface, helioseismic observations have yet to provide a definitive answer for the depth at which those flows return to the equator, or the number of circulation cells in depth. Here, we explore the observability of multiple circulation cells stacked in radius. Specifically, we examine the seismic signature of several meridional flow profiles by convolving time-distance averaging kernels with mean flows obtained from a suite of 3D hydrodynamic simulations. At mid and high latitudes, we find that weak flow structures in the deep convection zone can be obscured by signals from the much stronger surface flows. This contamination of 1--2 m s$^{-1}$ is caused by extended side lobes in the averaging kernels, which produce a spurious equatorward signal with flow speeds that are one order of magnitude stronger than the original flow speeds in the simulations. At low latitudes, the flows in the deep layers of the simulations are stronger ($> 2$ m s$^{-1}$) and multiple cells across the convection zone can produce a sufficiently strong signal to survive the convolution process. Now that meridional flows can be measured over two decades of data, the uncertainties arising from convective noise have fallen to a level where they are comparable in magnitude to the systematic biases caused by non-local features in the averaging kernels. Hence, these systematic errors are beginning to influence current helioseismic deductions and need broader consideration.

astro-ph.SR↗

arXiv2023

The Puzzling Structure of Solar Convection: Window into the Dynamo

The operation of the solar dynamo, with all of its remarkable spatio-temporal ordering, remains an outstanding problem of modern solar physics. A number of mechanisms that might plausibly contribute to its operation have been proposed, but the relative role played by each remains unclear. This uncertainty stems from continuing questions concerning the speed and structure of deep-seated convective flows. Those flows are in-turn thought to sustain both the Sun's turbulent EMF and the large-scale flows of differential rotation and meridional circulation suspected of influencing the dynamo's organization and timing. Continued progress in this area is complicated by (i) inconsistencies between helioseismic measurements of convective and meridional flow made with different techniques and instruments, and (ii) a lack of high-latitude data for convection, differential rotation, and meridional flow. We suggest that the path forward to resolving these difficulties is twofold. First, the acquisition of long-term helioseismic and emissivity measurements obtained from a polar vantage point is vital to complete our picture of the Sun's outer convection zone. Second, sustained and expanded investment in theory-oriented and combined theory/observational research initiatives will be crucial to fully exploit these new observations and to resolve inconsistencies between existing measurements.

astro-ph.SR↗

arXiv2022

Can we reveal the core-chemical composition of ultra-massive white dwarfs through their magnetic fields?

Ultra-massive white dwarfs ($ 1.05 \rm M_\odot \lesssim M_{WD}$) are particularly interesting objects that allow us to study extreme astrophysical phenomena such as type Ia supernovae explosions and merger events. Traditionally, ultra-massive white dwarfs are thought to harbour oxygen-neon (ONe) cores. However, recent theoretical studies and new observations suggest that some ultra-massive white dwarfs could harbour carbon-oxygen (CO) cores. Although several studies have attempted to elucidate the core composition of ultra-massive white dwarfs, to date, it has not been possible to distinguish them through their observed properties. Here, we present a new method for revealing the core-chemical composition in ultra-massive white dwarfs that is based on the study of magnetic fields generated by convective mixing induced by the crystallization process. ONe white dwarfs crystallize at higher luminosities than their CO counterparts. Therefore, the study of magnetic ultra-massive white dwarfs in the particular domain where ONe cores have reached the crystallization conditions but CO cores have not, may provide valuable support to their ONe core-chemical composition, since ONe white dwarfs would display signs of magnetic fields and CO would not. We apply our method to eight white dwarfs with magnetic field measurements and we suggest that these stars are candidate ONe white dwarfs.

astro-ph.SR↗

arXiv2021

On the dynamics of overshooting convection in spherical shells: Effect of density stratification and rotation

Overshooting of turbulent motions from convective regions into adjacent stably stratified zones plays a significant role in stellar interior dynamics as this process may lead to mixing of chemical species, and contribute to the transport of angular momentum and magnetic fields. We present a series of fully non-linear, three-dimensional (3D) anelastic simulations of overshooting convection in a spherical shell which are focused on the dependence of the overshooting dynamics on the density stratification and the rotation, both key ingredients in stars which however have not been studied systematically together via global simulations. We demonstrate that the overshoot lengthscale is not simply a monotonic function of the density stratification in the convective region but instead, it depends on the ratio of the density stratifications in the two zones. Additionally, we find that the overshoot lengthscale decreases with decreasing Rossby number Ro and scales as Ro$^{0.23}$ while it also depends on latitude with higher Rossby cases leading to a weaker latitudinal variation. We examine the mean flows arising due to rotation and find that they extend beyond the base of the convection zone into the stable region. Our findings may provide a better understanding of the dynamical interaction between stellar convective and radiative regions, and motivate future studies particularly related to the solar tachocline and the implications of its overlapping with the overshoot region.

astro-ph.SR↗

arXiv2021

On the dynamical interaction between overshooting convection and an underlying dipole magnetic field -- I. The non-dynamo regime

Motivated by the dynamics in the deep interiors of many stars, we study the interaction between overshooting convection and the large-scale poloidal fields residing in radiative zones. We have run a suite of 3D Boussinesq numerical calculations in a spherical shell that consists of a convection zone with an underlying stable region that initially compactly contains a dipole field. By varying the strength of the convective driving, we find that, in the less turbulent regime, convection acts as turbulent diffusion that removes the field faster than solely molecular diffusion would do. However, in the more turbulent regime, turbulent pumping becomes more efficient and partially counteracts turbulent diffusion, leading to a local accumulation of the field below the overshoot region. These simulations suggest that dipole fields might be confined in underlying stable regions by highly turbulent convective motions at stellar parameters. The confinement is of large-scale field in an average sense and we show that it is reasonably modeled by mean-field ideas. Our findings are particularly interesting for certain models of the Sun, which require a large-scale, poloidal magnetic field to be confined in the solar radiative zone in order to explain simultaneously the uniform rotation of the latter and the thinness of the solar tachocline.

astro-ph.SR↗

arXiv2020

Convective dynamics with mixed temperature boundary conditions: why thermal relaxation matters and how to accelerate it

Astrophysical simulations of convection frequently impose different thermal boundary conditions at the top and the bottom of the domain in an effort to more accurately model natural systems. In this work, we study Rayleigh-Benard convection (RBC) under the Boussinesq approximation. We examine simulations with mixed temperature boundary conditions in which the flux is fixed at the bottom boundary and the temperature is fixed at the top ("FT"). We aim to understand how FT boundaries change the nature of the convective solution compared to the traditional choice of thermal boundaries, in which the temperature is fixed at the top and bottom of the domain ("TT"). We demonstrate that the timescale of thermal relaxation for FT simulations is dependent upon the initial conditions. "Classic" initial conditions which employ a hydrostatically -- and thermally -- balanced linear temperature profile exhibit a long thermal relaxation. This long relaxation is not seen in FT simulations which use a TT simulation's nonlinear state as initial conditions ("TT-to-FT"). In the thermally relaxed, statistically stationary state, the mean behavior of an FT simulation corresponds to an equivalent simulation with TT boundaries, and time- and volume-averaged flow statistics like the Nusselt number and the Peclet number are indistinguishable between FT and TT simulations. FT boundaries are fundamentally asymmetric, and we examine the asymmetries that these boundaries produce in the flow. We find that the fixed-flux boundary produces more extreme temperature events than the fixed-temperature boundary. However, these near-boundary asymmetries do not measurably break the symmetry in the convective interior. We briefly explore rotating RBC to demonstrate that our findings with respect to thermal relaxation carry over to this more complex case, and to show the power of TT-to-FT initial conditions.

physics.flu-dyn↗

arXiv2018

Convective overshooting and penetration in a Boussinesq spherical shell

We study the dynamics associated with the extension of turbulent convective motions from a convection zone (CZ) into a stable region (RZ) that lies below the latter. For that purpose, we have run a series of three-dimensional direct numerical simulations solving the Navier-Stokes equations under the Boussinesq approximation in a spherical shell geometry. We observe that the overshooting of the turbulent motions into the stably stratified region depends on three different parameters: the relative stability of the RZ, the transition width between the two, and the intensity of the turbulence. In the cases studied, these motions manage to partially alter the thermal stratification and induce thermal mixing, but not so efficiently as to extend the nominal CZ further down into the stable region. We find that the kinetic energy below the convection zone can be modeled by a half-Gaussian profile whose amplitude and width can be predicted a priori for all of our simulations. We examine different dynamical lengthscales related to the depth of the extension of the motions into the RZ, and we find that they all scale remarkably well with a lengthscale that stems from a simple energetic argument. We discuss the implications of our findings for 1D stellar evolution calculations.

astro-ph.SR↗

arXiv2017

Weakly non-Boussinesq convection in a gaseous spherical shell

We examine the dynamics associated with weakly compressible convection in a spherical shell by running 3D direct numerical simulations using the Boussinesq formalism [1]. Motivated by problems in astrophysics, we assume the existence of a finite adiabatic temperature gradient $\nabla T_{\rm{ad}}$ and use mixed boundary conditions for the temperature with fixed flux at the inner boundary and fixed temperature at the outer boundary. This setup is intrinsically more asymmetric than the more standard case of Rayleigh-Bénard convection in liquids between parallel plates with fixed temperature boundary conditions. Conditions where there is substantial asymmetry can cause a dramatic change in the nature of convection and we demonstrate that this is the case here. The flows can become pressure- rather than buoyancy- dominated leading to anomalous heat transport by upflows. Counter-intuitively, the background temperature gradient $\nabla\bar{T}$ can develop a subadiabatic layer (where $\boldsymbol{g}\cdot\nabla\bar{T}<\boldsymbol{g}\cdot\nabla T_{\rm{ad}}$, where $\boldsymbol{g}$ is gravity) although convection remains vigorous at every point across the shell. This indicates a high degree of non-locality.

physics.flu-dyn↗