THE MAGNETIC-STRUCTURE OF PORES AND SUNSPOTS DERIVED FROM ADVANCED STOKES POLARIMETER DATA

We investigate the radial variation of the magnetic field structure ac ross sunspots, pores and azimuth centers (ACs). We define ACs as magne tic structures of about the same size as pores (all structures studied here are larger than 3 Mm diameter), but without a clear (at least 5% ) continuum decrease associated with them. We start from the full 3D v ector fields as observed with the Advanced Stokes Polarimeter (ASP), a nd perform a statistical study of the azimuthally averaged field compo nents in the local cylindrical reference frame centered on the structu res. Our statistical study comprises a sample of 16 sunspot observatio ns, a sample of 51 pores, and a sample of 22 ACs. For all structures, we derive mean radial profiles and their standard deviations. Due to t he relatively large sample of pores, we are able to investigate variat ions of this mean radial field structure with the size of the pores. O n the basis of our statistics, we identify systematic changes in the m agnetic field structure over a considerable size range. We suggest how this may be the natural consequence of a formation scenario for the l argest pores, by a lateral clustering of magnetic elements. Indeed, in this process, an AC may develop into a dark pore and gradually grow i n size through the incremental addition of magnetic flux. Several obse rvations where ACs turn into pores provide an estimate of about 4-5 x 10(19) Mr for the critical magnetic flux at which such transitions occ ur. We confirm the existence of a magnetic canopy for pores of all siz es, as their magnetic extent is virtually always larger than the assoc iated continuum darkening. We observe a relatively rapid change in the continuum appearance of a large pore in the sample. We identify the a ssociated changes in the field structure, and confront it with the det ermined mean field variation across sunspots. It appears that we have witnessed the formation of a partial penumbra.