Relation between magnetic field inclination and apparent motion of penumbral grains

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 444 inward- and 269 outward-moving PGs we show that 43% of the inward-moving PGs have magnetic inclination larger by $8^\circ \pm 4^\circ$ than the inclination in their surroundings and 51% of the outward-moving PGs have the inclination smaller by $13^\circ \pm 7^\circ$ than the surrounding one. The opposite relation of inclinations is observed at only one-fifth of the inward- and outward-moving PGs. Conclusions. Rising hot plasma in PGs surrounded by a less inclined magnetic field may adapt its trajectory to be more vertical, causing an inward apparent motion of PGs. Oppositely, it may be dragged by a more horizontal surrounding magnetic field such that an outward apparent motion is observed.Comment: 7 pages, 4 figures, 2 table