Properties of sunspot penumbral grains observed with Hinode

Abstract

Context. Penumbral grains (PGs) are small-scale (subarcsec) bright features found in the bright penumbral filament in the outer parts of sunspots. Aims. We aim to study properties of PGs at the blue continuum (4504 A) and the G-band (4305 A) by using simultaneous seeing free data obtained by Broadband Filter Imager (BFI) on board Hinode. Methods. We use an automatic identification and tracking algorithm to identify PGs, which were observed in a period of 170 min in the blue continuum and 88 min in the G-band. Results. Our results indicate that 776 PGs were identified in the blue continuum, and 413 PGs were identified in the G-band,. A statistical study reveals that about 55% of PGs move toward the umbra, about 13‐19% of PGs move toward the surrounding granulation, and the rest are relatively static. The inward moving PGs are mostly located in the inner penumbra (up to 0.6 of the distance from the umbra to the photosphere) and outward moving PGs are located in the outer penumbra. In the blue continuum (and G band), the average lifetime, speed, and brightness of inward moving PGs are 14.7 (13.5) min, 0.71 (0.70) kms −1 and 0.89 (0.88) of the quiet Sun. For outward moving PGs, the average lifetime, speed and brightness are 8.0 (7.0) min, 0.93 (0.65) kms −1 and 0.99 (0.98) of the quiet Sun. For PGs that are relatively static, the average lifetime and brightness are 7.6 (6.0) min and 0.94 (0.91) of the quiet Sun. Moreover, our observational results show that the correlation between the temporal evolutions of the speed and brightness is complex.