Abstract
Context. Bright heads of penumbral filaments, penumbral grains (PGs), show apparent horizontal motions inwards, towards the umbra, or outwards, away from the umbra.
Aims. We aim to prove statistically whether the direction of PGs' apparent motion is related to the inclination of the surrounding magnetic field.
Methods. We use spectropolarimetric observations of five sunspot penumbrae to compare magnetic inclinations inside PGs with those in their surroundings. The data are taken by three observatories: Hinode satellite, Swedish Solar Telescope, and GREGOR solar telescope. The direction of PGs' motion is determined by feature tracking. The atmospheric conditions in PGs and their surroundings, including the magnetic field information, are retrieved by means of height-stratified spectropolarimetric inversions.
Results. On a sample of 713 PGs we show that 43% of inward-moving PGs have magnetic inclination larger by 8 ° +- 4 ° than that in the surroundings and the inclination of 51% of outward-moving PGs is by 13 ° +- 7 ° smaller than the surrounding one. Only approximately one-fifth of PGs show an opposite relation of inclinations and this relation is undefined in the rest of PGs.
Conclusions. Rising hot plasma in PGs surrounded by a less inclined magnetic field may adapt its trajectory to be more vertical, causing the apparent motion inwards. Oppositely, it may be dragged by a more horizontal surrounding magnetic field such that its crossing point with the surface, i.e. PG, moves outwards.