Journal
ASTRONOMY & ASTROPHYSICS
Volume 580, Issue -, Pages -Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/201425501
Keywords
Sun: magnetic fields; Sun: photosphere; sunspots
Categories
Funding
- GA CR [P209/12/0287, 14-04338S]
- RVO:67985815
- Senatsausschuss of the Leibniz-Gemeinschaft [SAW-2012-KIS-5]
- DFG [RE 3282/1-1]
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Context. A sunspot emanates from a growing pore or protospot. In order to trigger the formation of a penumbra, large inclinations at the outskirts of the protospot are necessary. The penumbra develops and establishes by colonising both umbral areas and granulation. Evidence for a unique stable boundary value for the vertical component of the magnetic field strength, B-ver(stable), was found along the umbra-penumbra boundary of developed sunspots. Aims. We study the changing value of B-ver as the penumbra forms and as it reaches a stable state. We compare this with the corresponding value in fully developed penumbrae. Methods. We use broadband G-band images and spectropolarimetric GFPI/VTT data to study the evolution of and the vertical component of the magnetic field on a forming umbra-penumbra boundary. For comparison with stable sunspots, we also analyse the two maps observed by Hinode/SP on the same spot after the penumbra formed. Results. The vertical component of the magnetic field, B-ver, at the umbra-penumbra boundary increases during penumbra formation owing to the incursion of the penumbra into umbral areas. After 2.5 h, the penumbra reaches a stable state as shown by the GFPI data. At this stable stage, the simultaneous Hinode/SP observations show a B-ver value comparable to that of umbra-penumbra boundaries of fully fledged sunspots. Conclusions. We confirm that the umbra-penumbra boundary, traditionally defined by an intensity threshold, is also characterised by a distinct canonical magnetic property, namely by B-ver(stable). During the penumbra formation process, the inner penumbra extends into regions where the umbra previously prevailed. Hence, in areas where B-ver < B-ver(stable), the magneto-convection mode operating in the umbra turns into a penumbral mode. Eventually, the inner penumbra boundary settles at B-ver(stable), which hints toward the role of B-ver(stable) as inhibitor of the penumbral mode of magneto-convection.
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