Sunspot simulations: penumbra formation and the fluting instability

TL;DR

This study uses numerical simulations to show that flux tube curvature influences fluting instability, which affects sunspot structure and penumbra formation, with fluting leading to sunspot breakup and lightbridge formation.

Contribution

It demonstrates how flux tube curvature affects fluting instability and penumbra formation in sunspots through detailed MURaM simulations.

Findings

Flux tube curvature determines fluting degree.

Fluted sunspots develop penumbral filaments.

Fluting leads to sunspot breakup and lightbridge formation.

Abstract

The fluting instability has been suggested as the driver of the subsurface structure of sunspot flux tubes. We conducted a series of numerical experiments where we used flux tubes with different initial curvatures to study the effect of the fluting instability on the subsurface structure of spots. We used the MURaM code, which has previously been used to simulate complete sunspots, to first compute four sunspots in the slab geometry and then two complete circular spots of opposite polarities. We find that the curvature of a flux tube indeed determines the degree of fluting the flux tube will undergo - the more curved a flux tube is, the more fluted it becomes. In addition, sunspots with strong curvature have strong horizontal fields at the surface and therefore readily form penumbral filaments. The fluted sunspots eventually break up from below, with lightbridges appearing at the surface several hours after fluting commences.