THE FRONTIER BETWEEN SMALL-SCALE BIPOLES AND EPHEMERAL REGIONS IN THE SOLAR PHOTOSPHERE: EMERGENCE AND DECAY OF AN INTERMEDIATE-SCALE BIPOLE OBSERVED WITH SUNRISE/IMaX

Abstract

We report on the photospheric evolution of an intermediate-scale (~4 Mm\nfootpoint separation) magnetic bipole, from emergence to decay, observed in the\nquiet Sun at high spatial 0".3 and temporal (33 s) resolution. The observations\nwere acquired by the IMaX imaging magnetograph during the first science flight\nof the Sunrise balloon-borne solar observatory. The bipole flux content is 6 x\n10^17 Mx, representing a structure bridging the gap between granular scale\nbipoles and the smaller ephemeral regions. Footpoints separate at a speed of\n3.5 km s-1 and reach a maximum distance of 4.5 Mm before the field dissolves.\nThe evolution of the bipole is revealed to be very dynamic: we found a proper\nmotion of the bipole axis and detected a change of the azimuth angle of\n90{\\deg} in 300 seconds. The overall morphology and behaviour are in agreement\nwith previous analyses of bipolar structures emerging at granular scale, but we\nalso found several similarities with emerging flux structures at larger scale.\nThe flux growth rate is 2.6 x 15 Mx s-1, while the mean decay rate is one order\nof magnitude smaller. We describe in some detail the decay phase of the bipole\nfootpoints which includes break up into smaller structures, interaction with\npre-existing fields leading to cancellation but appears to be dominated by an\nas-yet unidentified diffusive process that removes most of the flux with an\nexponential flux decay curve. The diffusion constant (8 x 10^2 km^2 s-1)\nassociated with this decay is similar to the values used to describe the large\nscale diffusion in flux transport models.\n