THE VECTOR MAGNETIC-FIELDS AND THERMODYNAMICS OF SUNSPOT LIGHT BRIDGES - THE CASE FOR FIELD-FREE DISRUPTIONS IN SUNSPOTS

We present observations with the Advanced Stokes Polarimeter of 11 lig ht bridges in sunspots of various ages and sizes, all very close to di sk center. Full vector spectropolarimetry and a nonlinear least-square s inversion algorithm allows us to determine not only the vector magne tic field in the bridges and host sunspots but also thermodynamic para meters such as continuum brightness, Doppler shifts, Doppler widths, o pacity ratio, and the source function parameters. We can also separate the magnetic and nonmagnetic components of the spectral signal within each resolution element. We find that there is a disruption of the ma gnetic fields in light bridges, relative both to neighboring umbrae an d to normal, undisturbed penumbrae. This change takes the form of lowe r intrinsic field strength and sparser, more horizontal fields in the bridges relative to umbrae. The magnetic fields in the bridges remain more vertically oriented, however, than those in undisturbed penumbra. There are systematic upflows observed in the bridge plasma relative t o the neighboring umbrae, and the evidence points toward a component t hat is heated and departs from radiative equilibrium. In four cases, w e follow a light bridge over several days and find that as the bridges age, they grow wider and brighter, the fields weaken and become spars er, and the heating increases. We also find some evidence that the mag netic field begins to reorganize itself to accommodate the (now) two a zimuth centers before there are strong signals of a light bridge in th e thermodynamic parameters. This paper presents the first systematic l ook at sunspot light bridges with full vector polarimetry and thermody namic determination. The results show that there is an intrusion of he ld-free, possibly convective material into an otherwise stable, magnet ic sunspot. The departure from stability is seen in the magnetic field orientation prior to its appearance in continuum intensity, and the e ffects of this disruption are evident beyond the immediate umbral intr usion. The results do not unambiguously determine the physical mechani sm that makes sunspots disappear. However, it strongly points toward a ropelike magnetic structure through which convection may penetrate wh en the magnetic fibrils separate or around which field-free plasma may flow. The appearance of field-free heated material is likely an effec t, not the cause, of the sunspot light bridges.