A distinct magnetic property of the inner penumbral boundary
Formation of a stable umbra-penumbra boundary in a sunspot
Astronomical Institute of the Academy of Sciences,
Fričova 298 25165 Ondřejov Czech Republic
2 Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
Received: 11 December 2014
Accepted: 18 June 2015
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, Bstablever, was found along the umbra-penumbra boundary of developed sunspots.
Aims. We study the changing value of Bver 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, Bver, 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 Bver 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 Bverstable. During the penumbra formation process, the inner penumbra extends into regions where the umbra previously prevailed. Hence, in areas where Bver<Bstablever, the magneto-convection mode operating in the umbra turns into a penumbral mode. Eventually, the inner penumbra boundary settles at Bverstable, which hints toward the role of Bverstable as inhibitor of the penumbral mode of magneto-convection.
Key words: Sun: magnetic fields / Sun: photosphere / sunspots
© ESO, 2015