Properties of sunspot umbrae observed in cycle 24

Kiepenheuer-Institut fu¨r Sonnenphysik, Scho¨neckstr. 6, D-7104 Freiburg, Germanye-mail:[kiess,rrezaei,wolfgang]@kis.uni-freiburg.dePreprint online version: March 4, 2014ABSTRACTAims.There is an ongoing debate whether the solar activity cycle is overlaid with a long-term decline that may lead to another grandminimum in the near future. We use the size, intensity, and magnetic field strength of sunspot umbrae to compare the presen t cycle 24with the previous one.Methods. We used data of the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory and selected all sunspotsbetween May 2010 and October 2012, using one image per day. We created two subsets of this data with a manual tracking algorithm,both without duplication. One is containing each sunspot (910 umbrae within 488 spots) and was used to analyze the distributionof umbral areas, selected with an automated thresholding method. The other one contains 205 fully evolved sunspots. We estimatedthe magnetic field and the total magnetic flux or those and disc uss the relations between umbral size, minimum continuum intensity,maximum field strength and total magnetic flux.Results. We find nonlinear relations between umbral minimum intensit y and size and between maximum magnetic field strength andsize. The field strength scales linear with the intensity and the umbral size scales roughly linear with the total magnetic flux, whilethe size and field strength level o ff with stronger flux. When separated in hemisphere and average d temporally, the southern umbraeshow a temporal increase in size and the northern umbrae stay constant. There is no temporal variation in the umbral mean intensitydetectable. The probability density function of the umbral area in the ascending phase of the current solar cycle is similar to that ofthe last solar cycle.Conclusions. From our investigation of umbral area, magnetic field, magne tic flux and umbral intensity of the sunspots of the risingphase of cycle 24, we do not find a significant di fference to the previous cycle, and hence no indication for a long-term decline ofsolar activity.Key words. sun: solar cycle – sun: sunspot – sun: umbra – sun: magnetic fie ld