arXiv subjects

Santiago A. Triana

Publications and source records attributed to Santiago A. Triana.

9 recordsLinked to original sources

arXiv2026

Effects of Magnetic Fields on Solar Inertial Modes in Global Nonlinear Simulations

Solar inertial modes are increasingly viewed as probes of the solar interior, with recent helioseismic evidence indicating that these modes may be sensitive to the internal magnetic field. We investigate the role of magnetic fields on inertial mode excitation and dynamics using global nonlinear MHD simulations with differential rotation forced toward a helioseismically derived profile. Initial poloidal fields of varying strengths and topologies self-consistently generate different toroidal field configurations through the $Ω$-effect. In all cases, baroclinic instability excites high-latitude inertial modes, with weak fields enhancing its growth rate above the hydrodynamic value. For stronger fields, the response depends on topology and depth. As a consequence, the polar vortex emerges earlier and modifies its relative amplitude, handedness, and hemispheric asymmetry. Only magnetic topologies exhibiting toroidal bands at the upper convection zone develop a second, magneto-shear ("edge") instability at the equatorward edges of these bands. Powered mainly by the toroidal field, this instability sets in earlier and grows faster for stronger initial fields. It excites equatorial modes absent in the hydrodynamic simulation, with fast magneto-Rossby signatures approaching the classical Rossby dispersion relation at tachocline depths. Alfvénic signatures and tentative slow magneto-Rossby branches also appear. Inertial-mode excitation and frequencies thus depend on magnetic topology and spatial distribution, not merely field strength.

astro-ph.SR↗

arXiv2026

Simulations of the onset and dynamical evolution of inertial waves in solar/stellar interior

Inertial modes have been recently detected in the Sun via helioseismology, yet their origin, evolution, and role in the dynamics of the solar plasma and magnetic field remain poorly understood. In this study, we employ global numerical simulations to investigate the excitation mechanisms and dynamical consequences of inertial modes in the Sun and stellar interiors. We validate first our numerical setup by analyzing the evolution of sectoral and tesseral perturbations imposed on a rigidly rotating sphere. The results confirm that a perturbation of a given mode can excite neighboring modes with both smaller and larger wavenumbers along the dispersion relation of Rossby waves. Subsequently, we use a physically motivated forcing to impose differential rotation with varying shear amplitudes, and examine the spontaneous onset and nonlinear evolution of inertial modes. The simulations reveal that the growth of velocity perturbations is primarily driven by baroclinic instability. It gives rise to high-latitude inertial modes in the form of retrograde polar vortices whose properties depend on the imposed shear. Equatorial Rossby modes are also excited, albeit with lower intensity than their high-latitude counterpart. Perturbations with arbitrary azimuthal wavenumbers lead to the excitation of Rossby modes for all available wave numbers, sustained by both direct and inverse energy cascades. In simulations with stronger shear, the high latitude modes produce Reynolds stresses able to modify the imposed differential rotation and accelerate the rotation of the poles.

astro-ph.SR↗

arXiv2025

The influence of a stably stratified layer on the hydromagnetic waves in the Earth's core and their electromagnetic torques

Evidence from seismic studies, mineral physics, thermal evolution models and geomagnetic observations is inconclusive about the presence of a stably stratified layer at the top of the Earth's fluid outer core. Such a convectively stable layer could have a strong influence on the internal fluid waves propagating underneath the core-mantle boundary (CMB) that are used to probe the outermost region of the core through the wave interaction with the geomagnetic field and the rotation of the mantle. Here, we numerically investigate the effect of a top stable layer on the outer core fluid waves by calculating the eigenmodes in a neutrally stratified sphere permeated by a magnetic field with and without a top stable layer. We use a numerical model, assuming a flow with an m-fold azimuthal symmetry, that allows for radial motions across the lower boundary of the stable layer and angular momentum exchanges across the CMB through viscous and electromagnetic coupling. On interannual time-scales, we find torsional Alfvén waves that are only marginally affected by weak to moderate stratification strength in the outer layer. At decadal time-scales similarly weak stable layers promote the appearance of waves that propagate primarily within the stable layer itself and resemble Magneto-Archimedes-Coriolis (MAC) waves, even though they interact with the adiabatic fluid core below. These waves can exert viscous and electromagnetic torques on the mantle that are several orders of magnitude larger than those in the neutrally stratified case.

astro-ph.EP↗

arXiv2024

Transition to turbulence in the wide-gap spherical Couette system

The spherical Couette system consists of two differentially rotating concentric spheres with a fluid filled in between. We study a regime where the outer sphere is rotating rapidly enough so that the Coriolis force is important and the inner sphere is rotating either slower or in the opposite direction with respect to the outer sphere. We numerically study the sudden transition to turbulence at a critical differential rotation seen in experiments at BTU Cottbus - Senftenberg, Germany and investigate its cause. We find that the source of turbulence is the boundary layer on the inner sphere, which becomes centrifugally unstable. We show that this instability leads to generation of small scale structures which lead to turbulence in the bulk, dominated by inertial waves, a change in the force balance near the inner boundary, the formation of a mean flow through Reynolds stresses, and consequently, to an efficient angular momentum transport. We compare our findings with axisymmetric simulations and show that there are significant similarities in the nature of the flow in the turbulent regimes of full 3D and axisymmetric simulations but differences in the evolution of the instability that leads to this transition. We find that a heuristic argument based on a Reynolds number defined using the thickness of the boundary layer as a length scale helps explain the scaling law of the variation of critical differential rotation for transition to turbulence with rotation rate observed in the experiments.

physics.flu-dyn↗

arXiv2023

Effects of the librationally induced flow in Mercury's fluid core with an outer stably stratified layer

Observational constraints on Mercury's thermal evolution and magnetic field indicate that the top part of the fluid core is stably stratified. Here we compute how a stable layer affects the core flow in response to Mercury's main 88-day longitudinal libration, assuming various degrees of stratification, and study whether the core flow can modify the libration amplitude through viscous and electromagnetic torques acting on the core-mantle boundary (CMB). We show that the core flow strongly depends on the strength of the stratification near the CMB but that the influence of core motions on libration is negligible with or without a stably stratified layer. A stably stratified layer at the top of the core can however prevent resonant behaviour with gravito-inertial modes by impeding radial motions and promote a strong horizontal flow near the CMB. The librationally driven flow is likely turbulent and might produce a non-axisymmetric induced magnetic field with a strength of the order of 1$\%$ of Mercury's dipolar field.

astro-ph.EP↗

arXiv2023

Electric interface condition for sliding and viscous contacts

First principles of electromagnetism impose that the tangential electric field must be continuous at the interface between two media. The definition of the electric field depends on the frame of reference leading to an ambiguity in the mathematical expression of the continuity condition when the two sides of the interface do not share the same rest frame. We briefly review the arguments supporting each choice of interface condition and illustrate how the most theoretically consistant choice leads to a paradox in induction experiments. We then present a model of sliding contact between two solids and between a fluid and a solid, and show how this paradox can be lifted by taking into account the shear induced by the differential motion in a thin intermediate viscous layer at the interface, thereby also lifting the ambiguity in the electric interface condition. We present some guidelines regarding the appropriate interface condition to employ in magnetohydrodynamics applications, in particular for numerical simulations where sliding contact is used as an approximation to the viscous interface between a conducting solid and a fluid of very low viscosity such as in planetary interior simulations.

physics.class-ph↗

arXiv2020

Inertial modes of a freely rotating ellipsoidal planet and their relation to nutations

We compute the inertial modes of a freely rotating two-layer planetary model with an ellipsoidal inviscid fluid core and a perfectly rigid mantle. We present a method to derive analytical formulae for the frequencies of the Free Core Nutation (FCN) and Chandler Wobble (CW) which are valid to all orders of the dynamical flattening of the core and mantle, and we show how the FCN and CW are the direct generalisation of the purely fluid Spin-Over mode (SO) and of the Eulerian Wobble (EW) to the case where the mantle can oscillate freely around a state of steady rotation. Through a numerical computation for an axisymmetric (oblate spheroidal) planet, we demonstrate that all other inertial modes of the steadily rotating fluid core are also free modes of the freely rotating two-layer planet.

astro-ph.EP↗

arXiv2017

A study of turbulence and interacting inertial modes in a differentially-rotating spherical shell experiment

We present a study of inertial modes in a differentially rotating spherical shell (spherical Couette flow) experiment with a radius ratio of $η= 1/3$. Inertial modes are Coriolis-restored linear wave modes which often arise in rapidly rotating fluids. Recent experimental work has shown that inertial modes exist in a spherical Couette flow for $Ω_{i}<Ω_{o}$, where $Ω_i$ and $Ω_o$ is the inner and outer sphere rotation rate. A finite number of particular inertial modes has previously been found. By scanning the Rossby number from $-2.5 < Ro = (Ω_{i}-Ω_{o})/Ω_{o} < 0$ at two fixed $Ω_{o}$, we report the existence of similar inertial modes. However, the behavior of the flow described here differs much from previous spherical Couette experiments. We show that the kinetic energy of the dominant inertial mode dramatically increases with decreasing Rossby number that eventually leads to a wave-breaking and an increase of small-scale structures at a critical Rossby number. Such a transition in a spherical Couette flow has not been described before. The critical Rossby number scales with the Ekman number as0 $E^{1/5}$. Additionally, the increase of small-scale features beyond the transition transfers energy to a massively enhanced mean flow around the tangent cylinder. In this context, we discuss an interaction between the dominant inertial modes with a geostrophic Rossby mode exciting secondary modes whose frequencies match the triadic resonance condition.

physics.flu-dyn↗

arXiv2015

The internal rotation profile of the B-type star KIC10526294 from frequency inversion of its dipole gravity modes and statistical model comparison

The internal angular momentum distribution of a star is key to determine its evolution. Fortunately, the stellar internal rotation can be probed through studies of rotationally-split non-radial oscillation modes. In particular, detection of non-radial gravity modes (g modes) in massive young stars has become feasible recently thanks to the Kepler space mission. Our aim is to derive the internal rotation profile of the Kepler B8V star KIC 10526294 through asteroseismology. We interpret the observed rotational splittings of its dipole g modes using four different approaches based on the best seismic models of the star and their rotational kernels. We show that these kernels can resolve differential rotation the radiative envelope if a smooth rotational profile is assumed and the observational errors are small. Based on Kepler data, we find that the rotation rate near the core-envelope boundary is well constrained to $163\pm89$ nHz. The seismic data are consistent with rigid rotation but a profile with counter-rotation within the envelope has a statistical advantage over constant rotation. Our study should be repeated for other massive stars with a variety of stellar parameters in order to deduce the physical conditions that determine the internal rotation profile of young massive stars, with the aim to improve the input physics of their models.

astro-ph.SR↗