Dynamics of small features in the solar chromosphere

¹ Istanbul Üniversitesi, Üniversite Rasathanesi, 34452 Üniversite–Istanbul, Turkey
² Universitäts-Sternwarte, Geismarlandstr. 11, 37083 Göttingen, Germany

Corresponding author: F. Kneer, kneer@uni-sw.gwdg.de

Received: 16 May 2001
Accepted: 27 November 2001

Abstract

A spatially highly resolved time series of a quiet region at disk centre taken in the Na D₂ line with the “Göttingen” spectrometer in the Vacuum Tower Telescope/Tenerife was used to study the dynamic behaviour of different chromospheric features. Two CCDs were employed for simultaneous exposures of narrow-band and “white-light” images forming 128 repetitive “scans” of 30 images each in nearly two hours. The data set yielded line profiles over the two-dimensional field of view which served to create various maps indicating, e.g., vertical velocities or minimum line intensities. The analysis of the features of interest was further based on maps displaying the horizontal velocity field derived from white-light images. The occurrence of various bright points was monitored, showing quite different properties. Some persistent intra-network bright points follow the horizontal flows of the underlying photosphere. During the time series, a “K grain” is seen to occur several times at the same location. The power spectrum of the fluctuation of the minimum intensity of its corresponding Na D₂ mean line profile exhibits a pronounced absolute maximum for a period of about 24 min. The behaviour of several peculiar intra-network bright points still lacks an unequivocal interpretation. Possibly, the signature of gravity waves has been detected. Further investigation revealed that the power spectra of the fluctuation of Doppler shift and minimum intensity calculated for one of these bright points both show an absolute maximum for a period of 4.3 min. In the course of these fluctuations, time spans of regular oscillations were noticed for all such points lasting from about 15 to 25 min. Power spectra calculated only over these times exhibit pronounced absolute maxima in a rather narrow frequency range.