Multi-height probing of horizontal flows in the solar photosphere

Context. Optical flow methods aim to infer horizontal (transverse, in the general case) velocities in the solar atmosphere from the temporal changes in maps of physical quantities, such as intensity or magnetic field. So far, these methods have mostly been tested and applied to the continuum intensity and line-of-sight (LOS) magnetic field in the low to mid-photosphere. Aims. We tested whether simultaneous spectropolarimetric imaging in two magnetically sensitive optical spectral lines, which probe two different layers of the solar atmosphere (the photosphere and the temperature minimum), can help constrain the depth variation of horizontal flows. Methods. We first tested the feasibility of our method using Fourier local correlation tracking (FLCT) to track physical quantities at different optical depths (log τ ₅₀₀ = −1, −2, −3, −4) in an atmosphere simulated with the MURaM code. We then inferred the horizontal distribution of the LOS magnetic field component from synthetic spectropolarimetric observations of Fe I 525.0 nm and Mg I b2 spectral lines, applied FLCT to the time sequence of these synthetic magnetograms, and compared our findings with the original height-dependent horizontal velocities. Results. Tracking the LOS magnetic field component (which coincides with the vertical component at the disk center) yields horizontal velocities that, after appropriate temporal and spatial averaging, agree excellently with the horizontal component of the simulated velocities, both calculated at constant τ ₅₀₀ surfaces, up to the temperature minimum (log τ ₅₀₀ = −3). When tracking the temperature at constant τ ₅₀₀ surfaces, this agreement already breaks down completely at the mid photosphere (log τ ₅₀₀ = −2). Tracking the vertical component of the magnetic field inferred from synthetic observations of the Fe I 525.0 nm and the Mg I b2 spectral lines yields a satisfactory inference of the horizontal velocities in the mid-photosphere (log τ ₅₀₀ ≈ −1) and the temperature minimum (log τ ₅₀₀ ≈ −3), respectively. Conclusions. Our results indicate that high-spatial-resolution spectropolarimetric imaging in solar spectral lines can provide meaningful information about the horizontal plasma velocities over a range of heights.