Self-assembly of shallow magnetic spots through strongly stratified turbulence

TL;DR

This paper demonstrates that strongly stratified turbulence can spontaneously form magnetic spots, such as sunspots, through a large-scale instability driven by negative effective magnetic pressure, highlighting key ingredients for magnetic structure formation.

Contribution

It shows that vertical magnetic fields in stratified turbulence produce magnetic spots of equipartition strength, offering a minimal model for sunspot formation emphasizing turbulence and stratification.

Findings

Vertical magnetic fields form circular magnetic spots.

Magnetic spots reach equipartition field strengths.

Stratification and turbulence are critical for spot formation.

Abstract

Recent studies have demonstrated that in fully developed turbulence, the effective magnetic pressure of a large-scale field (non-turbulent plus turbulent contributions) can become negative. In the presence of strongly stratified turbulence, this was shown to lead to a large-scale instability that produces spontaneous magnetic flux concentrations. Furthermore, using a horizontal magnetic field, elongated flux concentrations with a strength of a few per cent of the equipartition value were found. Here we show that a uniform vertical magnetic field leads to circular magnetic spots of equipartition field strengths. This could represent a minimalistic model of sunspot formation and highlights the importance of two critical ingredients: turbulence and strong stratification. Radiation, ionization, and supergranulation may be important for realistic simulations, but are not critical at the level of a minimalistic model of magnetic spot formation.