Rotational effects on the negative magnetic pressure instability

TL;DR

This study investigates how rotation influences the negative magnetic pressure instability in the Sun's surface layers, revealing suppression effects, wave patterns, and potential explanations for observed magnetic tracer behaviors.

Contribution

It provides the first detailed analysis of rotation's impact on the negative magnetic pressure instability using mean-field magnetohydrodynamics in solar-like conditions.

Findings

Instability is suppressed at slow rotation rates (Coriolis number ~0.2).

Traveling wave solutions propagate prograde at the equator.

Magnetic pattern rotation may explain faster magnetic tracer rotation on the Sun.

Abstract

The surface layers of the Sun are strongly stratified. In the presence of turbulence with a weak mean magnetic field, a large-scale instability resulting in the formation of non-uniform magnetic structures, can be excited over the scale of many turbulent eddies or convection cells. This instability is caused by a negative contribution of turbulence to the effective (mean-field) magnetic pressure and has previously been discussed in connection with the formation of active regions and perhaps sunspots. We want to understand the effects of rotation on this instability in both two and three dimensions. We use mean-field magnetohydrodynamics in a parameter regime in which the properties of the negative effective magnetic pressure instability have previously been found to be in agreement with those of direct numerical simulations. We find that the instability is suppressed already for relatively slow rotation with Coriolis numbers (i.e. inverse Rossby numbers) around 0.2. The suppression is strongest at the equator. In the nonlinear regime, we find traveling wave solutions with propagation in the prograde direction at the equator with additional poleward migration away from the equator. The prograde rotation of the magnetic pattern near the equator is argued to be a possible explanation for the faster rotation speed of magnetic tracers found on the Sun. In the bulk of the domain, kinetic and current helicities are negative in the northern hemisphere and positive in the southern.