A distinct magnetic property of the inner penumbral boundary Formation of a stable umbra-penumbra boundary in a sunspot

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.