Instituto de Astrofísica de Andalucía (IAA-CSIC)

Lateral downflows in sunspot penumbral filaments and their temporal evolution                                                        

For the first time, the temporal evolution of the lateral downflows is observed and characterized. The spatial and temporal evolution of the small-scale velocity field shows a pattern similar to that of the quiet Sun, the main difference being the existence of a strong Evershed flow


We study the temporal evolution of downflows observed at the lateral edges of penumbral filaments in a sunspot located very close to the disk center. Our analysis is based on a sequence of nearly diffraction-limited spectropolarimetric scans of the Fe I 6173 Å line taken with the CRISP instrument at the Swedish 1-m Solar Telescope. We compute Dopplergrams from the observed intensity profiles using line bisectors and filter the resulting velocity maps for subsonic oscillations. Lateral downflows appear everywhere in the center-side penumbra as small, weak patches of redshifts next to or along the edges of blueshifted flow channels. These patches have an intermittent life and undergo mergings and fragmentations quite frequently. The lateral downflows move together with the hosting filaments and react to their shape variations, very much resembling the evolution of the granular convection in the quiet Sun. There is a good relation between the brightness and velocity of the flow structures in the center-side penumbra, with downflows being darker than upflows on average, which is again reminiscent of convection in the quiet Sun. These results point to the existence of overturning convection in sunspot penumbrae, with elongated cells in the radial direction where the flow is upward but very inclined, and weak lateral downward flows. In general, the circular polarization profiles emerging from the lateral downflows do not show sign reversals, although sometimes we detect three-lobed profiles which are suggestive of opposite magnetic polarities in the pixel.

Figure caption: Temporal evolution of lateral downflows at the edges of a filament located in the middle penumbra near the symmetry line. The intensity panels on top have been corrected for a stray light contamination of 40%. The velocity maps in the second row display the bisector velocities at the 70% intensity level. Red contours outline redshifts greater than 100 m s-1. The frames are not contiguous in time, as indicated in the upper right corner of each panel. The white arrow plotted in the first intensity panel points to the disk center. Axes are labeled in arcsec.

Reference: Esteban Pozuelo, S., Bellot Rubio, L. R. & de la Cruz Rodríguez, J., 2015, Accepted  for publication in ApJ, arXiv: 1502.02981
Lateral downflows in sunspot penumbral filaments and their temporal evolution