arXiv subjects

Suprabha Mukhopadhyay

Publications and source records attributed to Suprabha Mukhopadhyay.

3 recordsLinked to original sources

arXiv2026

Effects of the radiative interior on solar inertial modes

Solar inertial modes are believed to play important diagnostic and dynamical roles in the Sun's differentially rotating convection zone. However, the coupling of these modes to the radiative interior has not yet been discussed. We aim to understand the dependence of the modes on the uniformly rotating sub-adiabatic region below the convection zone and determine whether this leads to measurable changes at the surface. We used the Dedalus code to compute linear eigenmodes in the inertial frequency range in a setup that includes both the convection zone and the radiative interior down to $0.5R_\odot$. We imposed free-surface boundary conditions at both radial boundaries. For comparison, we also computed the eigenmodes in a setup restricted to the convection zone. We find that including the radiative zone only slightly modifies the frequencies and surface eigenfunctions, except for some modes with significant radial motion (high-frequency retrograde and prograde columnar modes). On the other hand, most modes penetrate significantly into the overshooting layer below the convection zone. This reduces their growth rates and distorts their eigenfunctions near the base of the convection zone. Furthermore, the uniformly rotating sub-adiabatic radiative zone supports oscillations due to Rossby modes of all possible spherical harmonics and radial nodes. In particular, when the nearest inertial mode in frequency space lies within around 10 nHz and shares the same north-south symmetry, these Rossby modes evolve into mixed modes characterized by significant motions within both the radiative and convection zones. However, such mixed modes have a high mode mass in the radiative interior and thus will be difficult to excite stochastically via convection.

astro-ph.SR↗

arXiv2025

On the origin of long-term modulation in the Sun's magnetic activity cycle

One of the most striking manifestations of orderly behavior emerging out of complex interactions in any astrophysical system is the 11-year cycle of sunspots. However, direct sunspot observations and reconstructions of long-term solar activity clearly exhibit amplitude fluctuations beyond the decadal timescale -- which may be termed as supradecadal modulation. Whether this long-term modulation in the Sun's magnetic activity results from nonlinear mechanisms or stochastic perturbations remains controversial and a matter of active debate. Utilizing multi-millennial scale kinematic dynamo simulations based on the Babcock-Leighton paradigm -- in the likely (near-critical) regime of operation of the solar dynamo -- we demonstrate that this supradecadal modulation in solar activity cannot be explained by nonlinear mechanisms alone; stochastic forcing is essential for the manifestation of observed long-term fluctuations in the near-critical dynamo regime. Our findings substantiate some independent observational and theoretical investigations, and provide additional insights into temporal dynamics associated with a plethora of natural phenomena in astronomy and planetary systems arising from weakly nonlinear, non-deterministic processes.

astro-ph.SR↗

arXiv2025

Assessing the validity of the anelastic and Boussinesq approximations to model solar inertial modes

Global-scale inertial modes of oscillations have been recently observed on the Sun. They might play an important dynamic and diagnostic role for the Sun. This work aims to assess the validity of simplifying assumptions in the equation of continuity, which have often been used in the linear models of solar inertial modes. We compute the linear eigenmodes of the Sun's convection zone in the inertial frequency range using the Dedalus code. This single framework enables us to compare the sensitivity of the modes to different model setups, such as the compressible setup and the Boussinesq and anelastic approximations. We consider both the cases of uniform rotation and solar differential rotation (as given by helioseismology). We find that the compressible and anelastic models have almost identical eigenmodes under uniform rotation and solar differential rotation. On the other hand, the absence of density stratification in the Boussinesq model results in significantly different eigenmodes under these setups. The differences are most prominent for the non-toroidal modes with significant radial motions mainly due to the absence of the compressional $β$ effect. The anelastic approximation simplifies the calculations and reduces the numerical cost without affecting the solar inertial modes. The Boussinesq or incompressible approximations cannot be used to model the solar inertial modes accurately. Given the strong influence of differential rotation on the eigenmodes, an acceptable setup is to use the anelastic approximation together with the solar differential rotation.

astro-ph.SR↗