COMPLEX MAGNETIC-FIELDS IN AN ACTIVE-REGION

High-resolution observations of the full Stokes vector in Fe I spectra l lines around 5250 Angstrom obtained at the Swedish Vacuum Solar Tele scope on La Palma with the ZIMPOL I Stokes polarimeter in a complex ac tive region reveal the presence of anomalously shaped Stokes profiles indicating the coexistence of at least two magnetic components within the same spatial resolution element. These Stokes profiles have been a nalyzed with an inversion code based on a 3-component atmospheric mode l with two magnetic and one field-free component. The fits to the obse rvations in a magnetic region that resembles a small penumbra reveal t he presence of a horizontal magnetic field component with an average f ield strength of (B) over bar = 840 G, a mean filling factor of <(alph a)over bar> = 0.12, and an average temperature (T) over bar = 5400 K a t log tau(5000) = -1.5 embedded in the main ''penumbral'' magnetic fie ld that has (B) over bar = 1500 G, <(alpha)over bar> = 0.56, and (T) o ver bar = 4900 K. The horizontal component exhibits a mean outflow of 2.7 km s(-1) which is mainly due to the Evershed flow. In a region whe re there are strong downflows up to 7 km s(-1), we infer the possible presence of a shock front whose height changes along the slit. The hei ght variation can be explained by a change of the gas pressure at the base of the photosphere below the shock front as proposed by Thomas & Montesinos (1991). Small plages with field strengths below 900 G have been observed in the vicinity of some pores. Finally, we present a puz zling field structure at the boundary between two adjacent pores. Ambi guous results suggest that although the inversion code is able to succ essfully invert even very complex Stokes profiles, we are far from a c omplete description of the field structure in complex magnetic regions . We warn that magnetograms and fits to data involving only a single m agnetic component may hide the true complexity of the magnetic structu re in at least some parts of active regions.