Role of partial stable stratification on the onset of rotating magnetoconvection with a uniform horizontal field

Abstract We investigate the linear onset of convection in a rotating plane layer permeated by a uniform horizontal magnetic field perpendicular to the rotation axis, a configuration relevant to dynamics within Earth’s tangent cylinder and complementing recent investigations [Barman and Sahoo 2025. Phys. Scr. 100 125018]. The primary objective is to examine how partial thermal stratification modifies convective initiation across three thermal states—fully unstable, weakly stable, and strongly stable—over rotation rates corresponding to Ekman numbers E = 10 −3 , 10 −4 , 10 −5 , thermal-to-magnetic diffusivity contrasts characterized by Roberts numbers q = 0.01, 1, 10, and magnetic field strengths spanning Elsasser numbers 0 ≤ Λ ≤ 10. Analysis of convective onset, supported by locally derived scaling relations, shows that introducing a stable segment raises the critical threshold for convection and promotes smaller-scale structures, particularly under rapid rotation. When magnetic forcing is weak, rotational effects dominate, strengthening the vertical alignment of convective motions and enhancing the stabilizing influence of stratification, thereby delaying convective onset. In contrast, strong magnetic forcing maintains broader convective rolls even at rapid rotation, although penetration into the stable layer remains limited. Magnetic damping is most pronounced at low to moderate diffusivity contrasts and weakens when magnetic diffusion becomes dominant, while the penetration depth decreases with increasing rotation and magnetic intensity in strongly stratified regimes. This study shows that partial stable stratification significantly modifies the onset of rotating magnetoconvection under a horizontal magnetic field by increasing the instability threshold, favoring smaller-scale structures, and limiting convective penetration into the stable layer, with the combined effects of buoyancy, rotation, and magnetic forces shaping the flow morphology. These results extend classical plane-layer magnetoconvection models and provide insight into planetary core dynamics—such as within Earth’s tangent cylinder—where stable stratification and magnetic feedback coexist, while also highlighting contrasts between weak-field planetary interiors and strong-field stellar environments.