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

Context. In a recent statistical study of sunspots in 79 active regions, the vertical magnetic field component B-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-ver at the umbral boundary is the same for all spots. Aims. We investigate the temporal evolution of B-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-c, = 0.54I(qs,) whereby I-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 FeI617.3 nm line and continuum are taken into account. To avoid limb artefacts, the spot is only analysed for heliocentric angles smaller than 60 degrees. Results. During the first disc passage, NOAA AR 11591, B-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-ver= 1693 G and I-c = 0.54(qs). Conclusions. During the disc passage of a stable sunspot, its umbral boundary can equivalently be defined by using the continuum intensity I-c or the vertical magnetic field component B-ver. Contours of fixed magnetic field strength fail to outline the umbral boundary.