A distinct magnetic property of the inner penumbral boundary

Abstract

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, $B_{\mathrm{ver}}^{\mathrm{stable}}$, was found along the umbra-penumbra boundary of developed sunspots. 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. The vertical component of the magnetic field, $B_{\mathrm{ver}}$, at the umbra-penumbra boundary increases during penumbra formation owing to the incursion of the penumbra into umbral areas. After 2.5 hours, the penumbra reaches a stable state as shown by the GFPI data. At this stable stage, the simultaneous Hinode/SP observations show a $B_{\mathrm{ver}}$ value comparable to that of umbra-penumbra boundaries of fully fledged sunspots. We confirm that the umbra-penumbra boundary, traditionally defined by an intensity threshold, is also characterised by a distinct canonical magnetic property, namely by $B_{\mathrm{ver}}^{\mathrm{stable}}$. During the penumbra formation process, the inner penumbra extends into regions where the umbra previously prevailed. Hence, in areas where $B_{\mathrm{ver}}<B_{\mathrm{ver}}^{\mathrm{stable}}$, the magneto-convection mode operating in the umbra turns into a penumbral mode. Eventually, the inner penumbra boundary settles at $B_{\mathrm{ver}}^{\mathrm{stable}}$, which hints toward the role of $B_{\mathrm{ver}}^{\mathrm{stable}}$ as inhibitor of the penumbral mode of magneto-convection.

Accepted as a Letter to A&A. Reproduced with permission from Astronomy & Astrophysics, ©ESO

References in corpus (2)

• The Solar Optical Telescope for the Hinode Mission: An Overview
• A 3-D sunspot model derived from an inversion of spectropolarimetric observations and its implications for the penumbral heating

Cited by in corpus (20)

• The magnetic nature of umbra-penumbra boundary in sunspots
• Formation of the penumbra and start of the Evershed flow
• Plasma Beta Stratification in the Solar Atmosphere: A Possible Explanation for the Penumbra Formation
• A distinct magnetic property of the inner penumbral boundary. II. Formation of a penumbra at the expense of a pore
• Normal and counter Evershed flows in the photospheric penumbra of a sunspot. SPINOR 2D inversions of Hinode-SOT/ SP observations
• Magnetic properties of a long-lived sunspot - Vertical magnetic field at the umbral boundary
• Properties of the inner penumbral boundary and temporal evolution of a decaying sunspot
• High-resolution imaging and near-infrared spectroscopy of penumbral decay
• Characterization of magneto-convection in sunspots -- The Gough & Tayler stability criterion in MURaM sunspot simulations
• A distinct magnetic property of the inner penumbral boundary III. Analysis of simulated sunspots
• Magnetic properties on the boundary of an evolving pore
• No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots
• Characterization of the umbra-penumbra boundary by the vertical component of the magnetic field -- Analysis of ground-based data from the GREGOR Infrared Spectrograph
• Onset of penumbra formation
• Solar and stellar flares: frequency, active regions and stellar dynamo
• The formation and decay of sunspot penumbra in Active Region NOAA 12673
• Probing sunspots with two-skip time-distance helioseismology
• Does H$\alpha$ Stokes~$V$ profiles probe the chromospheric magnetic field? An observational perspective
• Clarifying Physical Properties of Magnetic Fields in Sunspots
• The magnetic origin of the outer boundaries of sunspots