SOLARNET Conference: The Many Scales of the Magnetic Sun

It has been known for a long time that bright heads of penumbral filaments, penumbral grains (PGs), are moving. Their movements are oriented toward the umbra (inwards) in the inner penumbra and away from the umbra (outwards) in the outer penumbra. The inward motion was first explained by Schlichenmaier et al. (1998, A&A 337, 897) as an apparent motion of the intersection of a rising flux tube with the visible surface. In a more recent model by Tiwari et al. (2013, A&A 557, A25), penumbral filaments are magnetoconvective cells distributed everywhere in the penumbra and embedded in a background magnetic field whose inclination increases with the distance from the umbra. PGs are places where hot plasma emerges to the surface, but their motion is not considered in that model. Sobotka & Puschmann (2022, A&A 662, A13) have shown that PGs motions change their orientation from inwards to outwards in the middle penumbra and their outward speed gradually increases with distance in the outer penumbra. They suggested that the apparent motions of PGs may be affected by the inclination of surrounding magnetic field. In the present work we use spectropolarimetric observations of sunspot penumbrae to compare magnetic inclinations inside PGs with those in their surroundings. We show that inward-moving PGs mostly have magnetic inclination larger than that in the surroundings and the inclination in outward-moving PGs is usually smaller than the surrounding one. Consequently, rising hot plasma 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 the surrounding more horizontal magnetic field such that its crossing point with the surface moves outwards.