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light bridges ( lbs ) are bright structures in the otherwise dark umbra that often exhibit a granular - like morphology @xcite . they represent a discontinuity or interruption in the regular umbral field @xcite . lbs are known to harbor weak and inclined fields @xcite , but their origin and magnetic nature is still a matter of debate @xcite . one of the reasons that make lbs interesting is their chromospheric activity , seen as surges in h@xmath1 @xcite , localized brightenings in h filtergrams @xcite , and brightness enhancements in the upper chromosphere and transition region @xcite . the stressed magnetic configuration associated with lbs is perceived to be responsible for this activity , but the precise relationship , if any , is yet to be established . unfortunately , there is a lack of simultaneous vector magnetic field measurements in the photosphere to diagnose these events .    here we analyze _ hinode _ spectropolarimetric observations and h filtergrams of a sunspot lb in an attempt to relate its chromospheric activity to the photospheric magnetic field . we discover patches of supersonic downflows in the photospheric layers of the lb and show that some of them are associated with strong h brightness enhancements . interestingly , the supersonic flows produce spectral signatures never seen before in lbs . on 2007 may 1 , the leading spot of noaa active region 10953 was observed with _ hinode _ @xcite at a heliocentric angle of 8@xmath2 ( @xmath3 ) . between 10:46 and 12:25 ut , the _ hinode _ spectropolarimeter @xcite recorded the four stokes profiles of the iron lines at 630 nm with a spectral sampling of 21.55 m , a pixel size of 016 , and an exposure time of 4.8  s per slit position ( normal map mode ) . the observations were corrected for dark current , flat field , thermal flexures , and instrumental polarization using routines included in the solarsoft package . simultaneously , the broadband filter imager of _ hinode _ took h filtergrams with a cadence of 1 minute to monitor the chromosphere of the lb . the effective pixel size of the ca images is 011 . [ sunspot_image ] the left panel of figure  [ sunspot_image ] shows a continuum map of the spot and the lb . we have inverted the observed stokes profiles using the sir code ( stokes inversion based on response functions ; * ? ? ? sir computes perturbations in the physical quantities at specific locations across the optical depth grid called _ nodes _ , and then carries out an interpolation to yield values at all grid points . to determine the global structure of the lb and the surroundings , we performed a one - component inversion setting the magnetic and dynamic parameters to be constant with depth . the temperature stratification was perturbed with two nodes . a total of 9 parameters were retrieved from the observed profiles , including height - independent micro- and macro - turbulent velocities and a stray - light factor . the three components of the vector magnetic field ( strength , inclination , and azimuth ) deduced from the inversion are shown in the second and third columns of figure  [ sunspot_image ] . all the angles are expressed in the local reference frame after a manual disambiguation of the line - of - sight ( los ) azimuths . as can be seen , the lb is characterized by weaker and more inclined fields than the umbra . this confirms earlier results by , e.g. , @xcite and @xcite . in the upper half of the lb , the magnetic field is parallel to the axis of the bridge . both photometrically and magnetically , the lb looks like an extension of the penumbra protruding into the umbra . @xcite detected a horizontal flow along the lb that starts in the adjacent penumbra , demonstrating that the two structures are also connected dynamically . at the lower end of the lb , where the lb fields pointing south encounter sunspot fields oriented toward the north , one observes an isolated region with relatively weak magnetic fields . in addition , there is a discontinuity in the field azimuth running parallel to the west edge of the lb . the los velocity map displayed in the third column of figure  [ sunspot_image ] reveals the existence of strong , localized downflows in the lb with velocities of up to 4 km  s@xmath0 . interestingly , the downflows occur close to the weak - field region and the azimuth discontinuity described above , i.e. , at positions where the magnetic field changes orientation very rapidly ( fourth column of figure  [ sunspot_image ] ) . some of the downflowing patches coincide with chromospheric h brightness enhancements , as can be seen in figure  [ calcium ] . the filtergram displayed there was taken during the polarimetric scan of the lb and shows a strong h line - core brightening at the position and time of the largest photospheric velocities ) . these structures show brightenings , but not as intense as those associated with the strongest downflows . ] . noaa ar 10953 produced many other long - lasting chromospheric plasma ejections on april 29 and 30 @xcite . the stokes @xmath4 profiles associated with the downflows have two peaks in the red lobe , i.e. , they exhibit a total of three peaks . hereafter they will be labelled as type 1 . in the lb one also finds anomalous linear polarization profiles with normal stokes @xmath4 signals which are designated as type 2 . type 3 profiles are essentially a combination of the other two classes . examples of these profiles are given in figure  [ combo_plots ] , together with their spatial distribution . milne - eddington - like atmospheres such as the ones used to determine the global structure of the lb can not reproduce the complex shapes of type 1 profiles . for this reason , the velocities given in section  [ sir ] are only approximate . here we obtain more reliable values with the help of two - component inversions . are shown in blue . the arrows indicate the transverse component of the field in the local reference frame for every alternate pixel , after solving the 180@xmath2 azimuth ambiguity . white and black contours represent fields weaker than 750 g and bright continuum structures , respectively . the white dashed lines mark thin chromospheric threads whose ends are located in or near the downflowing patches . , scaledwidth=44.0% ]                     we start with a simple two - component model in which the magnetic field and the los velocity do not vary with height . the results of this inversion are presented in figure 4 . as can be seen , one of the components has supersonic velocities of @xmath5 km  s@xmath0 and the other is essentially at rest . the model explains the observed profiles in a very satisfactory way , but the assumption of two magnetic components separated horizontally in the resolution element may not be realistic because of the relatively large sizes of the downwflowing patches ( 0.5 - 0.7 arcsec ) . an alternative scenario is that they are stacked in the vertical direction , one on top of the other .    to investigate this possibility we consider a different model atmosphere with two magnetic components . one of them has height - independent parameters and the other features a discontinuity in the stratifications at a certain optical depth . the amplitude and the location of the discontinuity are free parameters . these inversions have been carried out using sirjump , an extension of the sir code ( bellot rubio , in preparation ) . the results show that the fast component always has velocities exceeding 10 km  s@xmath0 . as an example , figure  [ inv_results2 ] displays the stratifications of one such model with the downflows occurring in the lower half of the photosphere . while the two - component models used in this section differ in complexity , all of them indicate supersonic velocities and magnetic fields of the same polarity . in fact , the existence of supersonic flows is obvious from the shapes of the observed stokes @xmath4 profiles , which are similar to those emerging from the outer part of the penumbra where the evershed flow exceeds the sound speed . the inclined red wings of stokes @xmath6 also demonstrate the occurrence of strong velocities . all these spectral signatures are well reproduced by the two - component inversions . to the best of our knowledge , this is the first time that supersonic downflows are found in lbs . subsonic speeds of @xmath71 km  s@xmath0 have been reported earlier by @xcite , @xcite , and @xcite .      in regions where the magnetic field is relatively horizontal , the supersonic flows are associated with type 3 profiles . they have similar stokes @xmath6 and @xmath4 spectra as type 1 profiles , but also complex linear polarization signals which can not be seen when the field is more vertical . we observe anomalous stokes @xmath8 profiles with up to five peaks and signatures of strong redshifts ( figure  [ combo_plots ] , left ) . the multiple peaks indicate that different magnetic components coexist in the resolution element at the position of the downflows . type 3 profiles are common near the azimuth discontinuity on the west side of the lb , where the lb fields meet the umbral fields ( figure  [ combo_plots ] , right ) . thus , it is likely that these are the magnetic components giving rise to the complex stokes @xmath8 signals . they may lie horizontally next to each other in the resolution element , or could be stacked one on top of the other . in both cases the large variations of field azimuth that occur on very small spatial scales should facilitate the reconnection of magnetic field lines from the lb and the adjacent umbra , even if they have the same polarity @xcite . the interaction is expected to produce bidirectional jets , and we speculate that the supersonic downflows observed near the azimuth discontinuity and the weak - field region are the signatures of reconnection jets directed toward the photosphere . this would agree with @xcite , who believe that the lb is essentially a horizontal , twisted flux tube that reconnects with the umbral field at a low altitude . however , our measurements can not confirm the reconnection scenario unambiguously because the upward jet components are not always visible in the h images . indeed , another possibility is that the supersonic downflows are produced by a mechanism similar to that driving the evershed flow at the outer end of penumbral filaments . we have determined the global magnetic and dynamic properties of a sunspot light bridge from the inversion of stokes profiles observed by _ a one - component model atmosphere with height - independent parameters ( except for the temperature ) has been used to that end . our analysis reveals patches of strong downflows exceeding 4  km  s@xmath0 in the lb , associated with complex stokes @xmath4 spectra having double red lobes . two - component inversions of the profiles indicate supersonic velocities of 10 km  s@xmath0 or more . a milne - eddington inversion by @xcite of the same lb on the previous day shows only a small downflow patch with velocities of 0.7 km  s@xmath0 . the supersonic downflows occur in regions where the magnetic field of the lb meets sunspot fields with rather different orientations . in those locations one observes anomalous linear polarization signals , but only when the field is relatively horizontal . moreover , the supersonic downflows are sometimes associated with transient chromospheric brightenings . the interaction of the lb field and the sunspot fields may create current sheets , leading to magnetic reconnection in the upper photosphere / lower chromosphere . the strong photospheric flows could represent the downward directed jets generated in this process . however , such an explanation remains speculative because the associated upward jets are not always seen as chromospheric brightenings . the discovery of supersonic downflows in lbs has been possible due to the high spatial resolution and stability of _ hinode _ , but their interpretation is still unclear . additional observations should be performed to investigate the chromosphere and photosphere of lbs simultaneously , in an attempt to detect small brightness enhancements that would confirm the reconnection scenario or favor alternative explanations . our sincere thanks to the _ hinode _ team for providing the data used in this letter , and to the referee for valuable suggestions . hinode is a japanese mission developed and launched by isas / jaxa , with naoj as domestic partner and nasa and stfc ( uk ) as international partners . it is operated by these agencies in co - operation with esa and nsc ( norway ) . we acknwoledge support by the spanish micinn through projects esp2006 - 13030-c06 - 02 and pci2006-a7 - 0624 , and by junta de andaluca through project p07-tep-2687 .
we report the discovery of supersonic downflows in a sunspot light bridge using measurements taken with the spectropolarimeter on board the _ hinode _ satellite . 
 the downflows occur in small patches close to regions where the vector magnetic field changes orientation rapidly , and are associated with anomalous circular polarization profiles . 
 an inversion of the observed stokes spectra reveals velocities of up to 10  km  s@xmath0 , making them the strongest photospheric flows ever measured in light bridges . 
 some ( but not all ) of the downflowing patches are cospatial and cotemporal with brightness enhancements in chromospheric h filtergrams . 
 we suggest that these flows are due to magnetic reconnection in the upper photosphere / lower chromosphere , although other mechanisms can not be ruled out .