Can overturning motions in penumbral filaments be detected?

TL;DR

This study assesses whether convective overturning motions in sunspot penumbral filaments can be observed with current instruments, highlighting challenges due to spatial smearing and dominant flows.

Contribution

It demonstrates how observational limitations affect the detection of overturning convection in penumbral filaments using synthetic spectral line profiles.

Findings

Detection is hindered by spatial smearing, especially for downflows.

Line-of-sight velocities are dominated by the Evershed flow unless observed near disk center.

Lines with low formation height are more suitable for detecting convective flows.

Abstract

Numerical simulations indicate that the filamentation of sunspot penumbrae and the associated systematic outflow (the Evershed effect) are due to convectively driven fluid motions constrained by the inclined magnetic field. We investigate whether these motions, in particular the upflows in the bright filaments and the downflows at their edges can be reliably observed with existing instrumentation. We use a snapshot from a sunspot simulation to calculate 2D maps of synthetic line profiles for the spectral lines Fe\sci 7090.4 \AA ~ and C\sci 5380.34 \AA. The maps are spatially and spectrally degraded according to typical instrument properties. Line-of-sight velocities are determined from line bisector shifts. We find that the detectability of the convective flows is strongly affected by spatial smearing, particularly so for the downflows. Furthermore, the line-of-sight velocities are dominated by the Evershed flow unless the observation is made very near to disk center. These problems may have compromised recent attempts to detect overturning penumbral convection. Lines with a low formation height are best suited to detect the convective flows.