Magnetic properties of a long-lived sunspot - Vertical magnetic field at the umbral boundary

Abstract

Context. In a recent statistical study of sunspots in 79 active regions, the vertical magnetic field component $B_{\text{ver}}$ averaged along the umbral boundary is found to be independent of sunspot size. The authors of that study conclude that the absolute value of $B_{\text{ver}}$ at the umbral boundary is the same for all spots. Aims. We investigate the temporal evolution of $B_{\text{ver}}$ averaged along the umbral boundary of one long-lived sunspot during its stable phase. Methods. We analysed data from the HMI instrument on-board SDO. Contours of continuum intensity at $I_{\text{c}}=0.5I_{\text{qs}}$, whereby $I_{\text{qs}}$ refers to the average over the quiet sun areas, are used to extract the magnetic field along the umbral boundary. Projection effects due to different formation heights of the Fe I 617.3 nm line and continuum are taken into account. To avoid limb artefacts, the spot is only analysed for heliocentric angles smaller than $60^{\circ }$. Results. During the first disc passage, NOAA AR 11591, $B_{\text{ver}}$ remains constant at 1693 G with a root-mean-square deviation of 15 G, whereas the magnetic field strength varies substantially (mean 2171 G, rms of 48 G) and shows a long term variation. Compensating for formation height has little influence on the mean value along each contour, but reduces the variations along the contour when away from disc centre, yielding a better match between the contours of $B_{\text{ver}}=1693$ G and $I_{\text{c}}=0.5I_{\text{qs}}$. Conclusions. During the disc passage of a stable sunspot, its umbral boundary can equivalently be defined by using the continuum intensity $I_{\text{c}}$ or the vertical magnetic field component $B_{\text{ver}}$. Contours of fixed magnetic field strength fail to outline the umbral boundary.

arXiv commentaccepted for publication in A&A; v2 minor edit, correcting statement regarding one citation

References in corpus (7)

• The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Overview and Performance
• VFISV: Very Fast Inversion of the Stokes Vector for the Helioseismic and Magnetic Imager
• Spectral Line Selection for HMI: A Comparison of Fe I 6173 and Ni I 6768
• The magnetic nature of umbra-penumbra boundary in sunspots
• A distinct magnetic property of the inner penumbral boundary
• A distinct magnetic property of the inner penumbral boundary. II. Formation of a penumbra at the expense of a pore
• A Statistical Comparison between Photospheric Vector Magnetograms Obtained by SDO/HMI and Hinode/SP

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