Fast events and waves in an active region of the Sun observed in H with high spatial resolution

¹ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
² Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: [bruno;nazaret;blanco;kneer]@astro.physik.uni-goettingen.de
³ Instituto de Astrofísica de Canarias, C/ vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain e-mail: kgp@iac.es

Received: 11 December 2007
Accepted: 28 April 2008

Abstract

Context. We study the chromosphere of an active region of the Sun in the Hα line.

Aims. The development of new instrumentation and new methods of data analysis allows to scrutinize the dynamics of the solar chromosphere with high spatial, spectral, and temporal resolution. The observations we present shed light on some magneto-dynamic processes occurring above an active region in the chromosphere.

Methods. We took a time series of 55 min in Hα from AR 10875 at . We used the “Göttingen” Fabry-Perot spectrometer at the Vacuum Tower Telescope, Observatorio del Teide/Tenerife, to obtain two-dimensional spectrograms in Hα. Adaptive optics and image reconstruction yielded a spatial resolution better than 05 throughout the time sequence. From the wealth of structures, we selected areas of interest to study further, in detail, some ongoing processes.

Results. A small straight surge developed aside of a pore with upward phase speed of 100 km s^-1 and line-of-sight (LOS) velocity of 15 km s^-1. The surge retreated rapidly with LOS velocity of 45 km s^-1 at its mouth. It underwent a rebound and fell back again. Two sympathetic mini-flares were observed that lasted only approximately 40 s, but showed strong Hα emission. We found magnetoacoustic waves in long fibrils as mainly short wave trains, short packets or pulses, i.e., solitary waves consisting of small (1´´-2´´) blobs. They start at either end of the fibrils and travel with phase speeds of 12-14 km s^-1, i.e., close to the tube speed and approximately the sound velocity for sufficiently large magnetic field strengths. Some waves speed up to reach velocities of the order of 30 km s^-1. This is much lower than the expected Alfvén velocity of ≥200 km s^-1 for reasonable magnetic field strengths and mass densities. We suggest that slow waves are not purely longitudinal, but possess gas velocities perpendicular to the direction of propagation of few km s^-1. Also, fast waves travel along sinuous lines suggesting entangled magnetic fields. They spread out along the direction of propagation in the course of their evolution and often vanish. We discuss the implications.