Spectrally selective instability of mixed poloidal-toroidal magnetic fields in radiative stellar interiors

¹ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
² Dipartimento di Fisica e Astronomia “Ettore Majorana”, Università di Catania, Via S. Sofia 64, 95123 Catania, Italy
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 3 February 2026
Accepted: 25 May 2026

Abstract

We present a global linear analysis of a spectrally selective magnetic instability of mixed poloidal–toroidal field configurations within radiative stellar interiors. This instability, previously investigated in cylindrical geometry in the context of fusion physics but largely unexplored in astrophysical applications, arises when the Suydam stability criterion is violated. It occurs only within a narrow range of wavevectors that satisfy k ⋅ B ≈ 0, that is, for perturbations with wavevectors nearly perpendicular to the magnetic field. This instability shares several dynamical properties with the classical Tayler (kink-type) instability of a pure toroidal field, most notably its partial resilience to suppression by stable stratification. Unstable modes with sufficiently short radial wavelengths are essentially unaffected by the stabilizing buoyancy and grow on nearly Alfvénic timescales. In contrast, large-scale modes are affected by stratification and their growth rates σ follow the scaling σ ∝ ω_A³/N², where ω_A is the Alfvén frequency of the background axisymmetric toroidal field B_ϕ and N is the Brunt-Väisälä frequency. Unlike classical Tayler modes, the instability studied here persists in the presence of a substantial poloidal magnetic field: increasing the axial field B_z reduces the growth rate but does not completely suppress the instability, following the scaling σ ∝ ω_AB_ϕ/B_z. Owing to its spectrally selective nature, the instability can develop with dominant azimuthal wavenumbers m > 1, leading to a broader mode spectrum than that of the classical Tayler instability. This instability may significantly influence the dynamics and transport processes in radiative stellar interiors, thereby contributing to stellar evolution.