Fine structures in the atmosphere above a sunspot umbra

¹ Max-Planck Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
e-mail: bharti@mps.mpg.de
² School of Space Research, Kyung Hee University, Yongin, 446-701 Gyeonggi, Korea

Received: 22 November 2012
Accepted: 12 February 2013

Abstract

We present simultaneous photospheric and chromospheric observations of the trailing sunspot in NOAA 10904, obtained with the Swedish Solar Telescope (SST) La Palma, Canary Islands. Time series of high resolution Ca ii H images show transient jet-like structures in sunspot umbrae are elongated, which we call umbral microjets. These jets are directed roughly parallel to nearby penumbral microjets, suggesting that they are aligned with the background magnetic field. In general, first a bright dot-like structure appears, from which a jet later emerges, although some jets appear without an associated chromospheric dot. Bright photospheric umbral dots are associated with umbral microjets arising in the outer umbra. Nevertheless, a one-to-one correspondence between jet-like events and underlying umbral dots is not seen. They are typically less than 1′′ long and less than 0$\stackrel{\mathrm{″}}{.}$3 wide. The typical lifetime of umbral microjets is around one minute. The brightness of these structures increases from the center of the umbra toward the umbra-penumbra boundary along with the brightness of the local background.