How do Sunspots Form?

Sunspots are the most prominent sign of magnetism on the solar surface, but how do they form? A post written by Dr. Nazaret Bello González, from the Leibniz Institute for Solar Physics (Freiburg, Germany).



Formation of a sunspot in active region 11024 as observed at the German VTT on July 4, 2009. / Image: Schlichenmaier et al., 2010, A&A 512, 1


Sunspots do not appear suddenly at once. They are rather the result of small concentrations of magnetic field that pop up on the solar surface from the deeper layers of the Sun with sizes of less than about 700 km. We believe that these small concentrations result from a large magnetic rope segmented in strands by the buffeting of convection during its rise and emergence to the solar surface.

The magnetic strands gather together to form pores, and pores subsequently coalesce to form larger pores or proto-spots. A proto-spot is a pore or umbra that will eventually develop into a sunspot. An example can be seen in the image. If the proto-spot collects enough magnetic flux and conditions allow for the magnetic field to become inclined (parallel to the solar surface), then, the so-called penumbra will form.

The penumbra embraces the umbra in about 5 hours, so it is a relatively fast process. This makes the observation of the phenomenon very elusive. Indeed, only a few datasets of penumbra formation exist.

In the penumbra, the horizontal magnetic fields will shape the convective cells giving them their characteristic filamentary appearance. The penumbra is the element distinguishing sunspots from pores.

Sunspots can steadily live for days to weeks. When sunspots decay, they will first loose their penumbra to become again pores (or “naked” sunspots) and will get disrupted by the surrounding granulation into smaller pores until they eventually disappear as small magnetic concentrations or plages.

The European Solar Telescope shall allow us to investigate in detail the phenomenon of sunspot formation little explored so far. The few existing high-resolution observations hint towards a strong involvement of the various layers of the solar atmosphere during sunspot formation. EST’s high spatial resolution, polarimetric capabilities and multi-line spectral coverage shall provide us with the necessary information to disentangle the processes inhibiting the granular convection to form dark umbrae and the development of the magneto-convective filaments observed in the penumbra.


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