Using the infrared iron lines to probe solar subsurface convection

Context. Studying the properties of solar convection using high-resolution spectropolarimetry began in the early 1990s with the focus on observations in the visible wavelength regions. Its extension to the infrared (IR) remains largely unexplored. Aims. The IR iron lines around 15 600 Å, most commonly known for their high magnetic sensitivity, also have a non-zero response to line-of-sight (LOS) velocity below log(τ) = 0.0. In this paper we explore the possibility of using these lines to measure subsurface convective velocities. Methods. By assuming a snapshot of a three-dimensional magnetohydrodynamic simulation to represent the quiet Sun, we investigate how well the iron IR lines can reproduce the LOS velocity in the cube and to what depth. We use the recently developed spectropolarimetric inversion code SNAPI and discuss the optimal node placements for the retrieval of reliable results from these spectral lines. Results. We find that the IR iron lines can measure the convective velocities down to log(τ) = 0.5, below the photosphere, not only at the original resolution of the cube, but also when degraded with a reasonable spectral and spatial PSF and stray light. Instead, the commonly used Fe I 6300 Å line pair performs significantly worse. Conclusions. Our investigation reveals that the IR iron lines can probe the subsurface convection in the solar photosphere. This paper is a first step towards exploiting this diagnostic potential.