Properties of the inner penumbral boundary and temporal evolution of a decaying sunspot

Abstract

It was empirically determined that the umbra-penumbra boundaries of stable sunspots are characterized by a constant value of the vertical magnetic field. We analyzed the evolution of the photospheric magnetic field properties of a decaying sunspot belonging to NOAA 11277 between August 28 - September 3, 2011. The observations were acquired with the spectropolarimeter on-board of the Hinode satellite. We aim to proof the validity of the constant vertical magnetic-field boundary between the umbra and penumbra in decaying sunspots. A spectral-line inversion technique was used to infer the magnetic field vector from the full-Stokes profiles. In total, eight maps were inverted and the variation of the magnetic properties in time were quantified using linear or quadratic fits. We found a linear decay of the umbral vertical magnetic field, magnetic flux, and area. The penumbra showed a linear increase of the vertical magnetic field and a sharp decay of the magnetic flux. In addition, the penumbral area quadratically decayed. The vertical component of the magnetic field is weaker on the umbra-penumbra boundary of the studied decaying sunspot compared to stable sunspots. Its value seem to be steadily decreasing during the decay phase. Moreover, at any time of the shown sunspot decay, the inner penumbra boundary does not match with a constant value of the vertical magnetic field, contrary to what was seen in stable sunspots. During the decaying phase of the studied sunspot, the umbra does not have a sufficiently strong vertical component of the magnetic field and is thus unstable and prone to be disintegrated by convection or magnetic diffusion. No constant value of the vertical magnetic field was found for the inner penumbral boundary.

arXiv commentAccepted for publication in Astronomy & Astrophysics, 6 pages, 7 figures

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• Study of the decay rates of the umbral area of sunspot groups by using a high resolution database
• Magnetic properties on the boundary of an evolving pore
• Penumbral decay observed in active region NOAA 12585
• 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
• On the (in)stability of sunspots
• The Decay of Two Adjacent Sunspots Associated with Moving Magnetic Features
• Clarifying Physical Properties of Magnetic Fields in Sunspots
• High-resolution observations of two pores with the integral field unit (IFU) of the GREGOR Infrared Spectrograph (GRIS)
• The Decay Process of an α-configuration Sunspot
• The dependence of the magnetism of a near-limb sunspot on height