Optical tomography of a sunspot. III. Velocity stratification and the Evershed effect

The stratification with optical depth of the line-of-sight (LOS) velocity o f a simple, isolated, round sunspot observed with the Advanced Stokes Polar imeter (ASP; Elmore et al.) presented here completes this series of papers that investigates the stratification in optical depths of such a typical su nspot. These results have been obtained through the use of the SIR techniqu e (Stokes Inversion based on Response functions of Ruiz Cobo & del Toro Ini esta). From these data we have confirmed that there are strong downflowing velocities at log tau (5) = 0 that coincide spatially with the places where the magnetic field points downward (Westendorp Plaza et al.). Further conf irmation is obtained by the application of the same method on a different s unspot, already analyzed with the Milne-Eddington inversion technique (Stan chDeld, Thomas, & Lites). These downflows reconcile observations that have detected Evershed velocities outside sunspots together with suggestions of the possible return of the flow within the penumbra. The Evershed flow seem s to be concentrated in elevated channels not thicker than 1 or 2 scale hei ghts that are mostly located in the space between magnetic spines, i.e., in places where the magnetic field is more inclined, weaker in the inner-midd le penumbra, and stronger in the outer penumbra and beyond the visible limi ts of the sunspot. This conclusion is based upon the tight correlation foun d between LOS velocities and the (reported in the second paper of this seri es) magnetic field strength and zenith angle. The upstreaming material is s een in the inner penumbra and the downstreaming in the outer penumbra. A st rong increase with optical depth has been obtained for the LOS velocities t hat provides indications of the superposition of Evershed channels along th e LOS. The differential opacity effect between the center-side and the limb -side penumbra, already reported in the second paper in this series, is als o seen in the velocity maps and has suggested the comparison of the vertica l mass flux through the upstreaming zones (mostly seen in the center side) and the downstreaming zones (mostly seen in the limb side), obtaining a fai rly good balance between the two.