Evershed effect: ion-neutral decoupling

Theory suggests that an ion-neutral velocity drift may exist in the solar atmosphere if the collisional momentum exchange between both species is not large enough. This work aims to offer a fresh perspective on this issue by investigating the drift velocity within the Evershed flow. In a previous analysis (Khomenko et al. 2015), the authors derived the Evershed velocities associated to Fe I and Fe II species in a sunspot using the lambda-meter method, estimating the formation heights of the observed spectral lines using semi-empirical models. The results showed that the flow measured using the Fe I lines was a few hundred ms-1 faster than that derived from the Fe II lines at all photospheric heights and radial distances to the sunspot centre. In this work, we have applied the SIR (Stokes Inversion based on Response functions) technique to different spectral regions, carefully selecting pairs of spectral lines from Fe I and Fe II. In addition to various atmospheric parameters, we consistently and separately obtained the ion and neutral velocity stratifications as a function of optical depth at all individual penumbral points around a sunspot. This contribution describes the main results obtained with this analysis. The influence of NLTE effects in the velocity and height determinations is also under study.