arXiv subjects

P. A. Gilman

Publications and source records attributed to P. A. Gilman.

3 recordsLinked to original sources

arXiv2026

alpha_theta waves in the tachoclines of Sun-like stars

Vertical gradient of mean turbulent electromotive force (expressed by the dynamo coefficient, alpha, caused by the penetrating convection into the tachoclines of Sun-like stars leads to the non-propagating alpha-mode patterns, which result in the periodic variations of magnetic field horizontal components. The aim of the paper is to study the influence of the latitudinal variation of the dynamo coefficient on the linear dynamics of large-scale waves in the overshoot layers of the tachoclines in Sun-like stars. We use the linear magnetohydrodynamic (MHD) equations with the dynamo coefficient in a simple rectangular geometry. It is shown that the vertical gradient of the alpha coefficient has the same appearance in the induction equation as the Coriolis force in the momentum equation. Therefore, the latitudinal variation of alpha parameter excites new large-scale waves similar to the Rossby waves on a rotating sphere. The dispersion properties of the waves depend only on vertical and latitudinal gradients of the alpha parameter, therefore the modes are basically different from the ordinary dynamo waves. The alpha_theta waves may have either prograde or retrograde propagation depending on the signs of the gradients. The time scales of the waves may vary from hundreds of days to tens of years in various parameters of the solar tachocline. These waves are coupled with the Rossby waves on a rotating sphere in the existence of the large scale magnetic field, which may lead to the mutual transformation of convective and rotation energies.

astro-ph.SR↗

arXiv2025

Magnetohydrodynamic shallow water equations with the alpha effect: Rossby-dynamo waves in solar--stellar tachoclines

The activity of Sun-like stars is governed by the magnetic field, which is believed to be generated in a thin layer between convective and radiative envelopes. The dynamo layer, also called the tachocline, permits the existence of Rossby waves (r-modes) described by magnetohydrodynamic shallow water models, which may lead to short-term cycles in stellar activity. Convective cells penetrate into the layer creating an overshoot upper part, where they transport an additional energy for vigorous activity. The aim of this paper is to study the influence of overshooting convection on the dynamics of Rossby waves in the tachoclines of Sun-like stars. Here we write the magnetohydrodynamic shallow water equations with the effect of the penetrative convection and study the dynamics of wave modes in the layer. The formalism leads to the excitation of new oscillation modes connected with the dynamo coefficient, alpha, causing periodic modulations of all parameters in the tachocline. The modes are coupled with the Rossby waves resulting mutual exchange of convective and rotation energies. The timescales of Rossby-dynamo waves, for certain parameters, correspond to Schwabe (11 years) and Rieger (150-170 days) cycles as observed in solar activity. The waves provide a new paradigm for internal magnetism and may drive the dynamos of Sun-like stars. Theoretical properties of the waves and observations can be used for magneto-seismological sounding of stellar interiors.

astro-ph.SR↗

arXiv2002

Solar Multi-Scale Convection and Rotation Gradients Studied in Shallow Spherical Shells

The differential rotation of the sun, as deduced from helioseismology, exhibits a prominent radial shear layer near the top of the convection zone wherein negative radial gradients of angular velocity are evident in the low- and mid-latitude regions spanning the outer 5% of the solar radius. Supergranulation and related scales of turbulent convection are likely to play a significant role in the maintenance of such radial gradients, and may influence dynamics on a global scale in ways that are not yet understood. To investigate such dynamics, we have constructed a series of three-dimensional numerical simulations of turbulent compressible convection within spherical shells, dealing with shallow domains to make such modeling computationally tractable. These simulations are the first models of solar convection in a spherical geometry that can explicitly resolve both the largest dynamical scales of the system (of order the solar radius) as well as smaller-scale convective overturning motions comparable in size to solar supergranulation (20--40 Mm). We find that convection within these simulations spans a large range of horizontal scales, and that the radial angular velocity gradient in these models is typically negative, especially in low- and mid-latitude regions. Analyses of the angular momentum transport indicates that such gradients are maintained by Reynolds stresses associated with the convection, transporting angular momentum inward to balance the outward transport achieved by viscous diffusion and large-scale flows in the meridional plane. We suggest that similar mechanisms associated with smaller-scale convection in the sun may contribute to the maintenance of the observed radial shear layer located immediately below the solar photosphere.

astro-ph↗