Sunspot Decay

The stability of sunspots is one of the long-standing unsolved puzzles in the field of solar magnetism and the solar cycle. The thermal and magnetic structure of the sunspot beneath the solar surface is not accessible through observations, thus processes in these regions that contribute to the decay of sunspots can only be studied through theoretical and numerical studies.

Map of magnetic field strength at a depth of 7 500 km beneath the solar surface. As time evolves, the corrugations due to fluting instability along the spot boundary (red contour) increase. These corrugations are the ultimate cause for the decay process.

New insights were gained taking advantage of a numerical simulation of a sunspot with a lateral extension of more than 98 000 km times 98 000 km extending almost 18 000 km beneath the solar surface. These simulations had been performed by Matthias Rempel at the High Altitude Observatory (HAO). A data set of 30 hours, showing a stable sunspot at the solar surface, was analysed with respect to sunspot decay as a PhD project at KIS, in collaboration with the HAO.

This simulation allows to study the non-local dynamics of fluting instability interacting with stabilising buoyance forces within in the magnetic flux tube that constitutes the spot. The simulation shows that the fluting instability causes a corrugation along the circumference of the flux tube (sunspot) which proceeds fastest at a depth around 7500 km beneath the solar surface. For a long time, the decay of the spot is suppressed by the stabilising effect close to the surface, which holds the bundle of magnetic field lines together. Yet, ultimately, large intrusions of field-free plasma below the surface form convective bubbles which might form light-bridges in the spot at the solar surface. This can finally disrupt the spot.

The analysis of the simulation also allow to assess the results of a linear stability analysis developed in the 1970s (Meyer, Schmidt, & Weiss 1977). As predicted back then, the magnetic field configuration of a sunspot in the uppermost few 1000 km is stable, mostly due to buoyancy forces. Deeper layers in the spot are unstable to fluting (interchange) instability as it was predicted.

PhD thesis, Hanna Strecker