Evidence for Fine Structure in the Chromospheric Umbral Oscillation

Novel spectro-polarimetric observations of the \ion{He}{1} multiplet are usedto explore the dynamics of the chromospheric oscillation above sunspot umbrae.The results presented here provide strong evidence in support of thetwo-component model proposed by Socas-Navarro and co-authors. According to thismodel, the waves propagate only inside channels of sub-arcsecond width (the``active'' component), whereas the rest of the umbra remains nearly at rest(the ``quiet'' component). Although the observations support the fundamentalelements of that model, there is one particular aspect that is not compatiblewith our data. We find that, contrary to the scenario as originally proposed,the active component remains through the entire oscillation cycle and harborsboth the upflowing and the downflowing phase of the oscillation.Comment: To appear in the Astrophysical Journa