Generation of magnetic structures in stratified turbulence.

Kemel Koen, koen@nordita.org, Nordita
Brandenburg Axel, Nordita
Kleeorin Nathan, Ben-Gurion University of the Negev
Mitra Dhrubaditya, Nordita
Rogachevskii Igor, Ben-Gurion University of the Negev

Abstract
The rising flux tube paradigm is an essential component in the majority of the models explaining the existence and evolution of the solar magnetic field. However, the deep creation and subsequent coherent rise of thin flux tubes in the vigourously turbulent solar convection zone, remain unsubstantiated. The present numerical work is a step in a search for a mechanism operating in the top of the convection zone which could lead to the observed surface magnetic activity, like active regions and sunspots. Turbulent pressure reduction in the presence of weak a magnetic field was found to be strong enough to lead to a negative effective contribution of the field to the total pressure. In a magnetised stratified plasma, the dependence of the pressure reduction on the ratio of the mean magnetic field to the equipartition field, creates a downwards effective magnetic pressure gradient. In this setup, a local enhancement of the field causes horizontal inflows and antibuoyancy. In the linear stage of this process, the horizontal field accumulation is dominating, resulting in a vertically static exponential growth of magnetic flux concentrations on spatial and time scales much larger than the turbulent scales.