Sunspot simulations: penumbra formation and the fluting instability

Kavli Affiliate: Robert Cameron

| First 5 Authors: Mayukh Panja, Robert Cameron, Sami K. Solanki, ,

| Summary:

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.

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