Multi-height probing of horizontal flows in the solar photosphere

TL;DR

This study demonstrates that high-resolution spectropolarimetric imaging in specific solar spectral lines can effectively infer horizontal plasma velocities at various heights in the solar photosphere, aiding in understanding solar atmospheric dynamics.

Contribution

We introduce a method combining spectropolarimetric imaging and FLCT to accurately determine height-dependent horizontal flows in the solar photosphere, validated with MURaM simulations.

Findings

Magnetic field tracking yields accurate horizontal velocities up to the temperature minimum.

Temperature-based tracking fails at mid-photosphere levels.

Spectropolarimetric imaging provides meaningful velocity information across different atmospheric heights.

Abstract

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. We first tested the feasibility of our method using Fourier local correlation tracking (FLCT) to track physical quantities at different optical depths ($\log\tau_{500}={-1,-2,-3,-4}$) 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. 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 $\tau_{500}$ surfaces, up to the temperature minimum ($\log\tau_{500}=-3$). When tracking the temperature at constant $\tau_{500}$ surfaces, this agreement already breaks down completely at the mid photosphere ($\log\tau_{500}=-2$). 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 ($\log\tau_{500}\approx-1$) and the temperature minimum ($\log\tau_{500}\approx-3$), respectively. 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.