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

Abstract

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^stable_ver, 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^stable_ver, 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.