Astron. Astrophys. 336, 743-752 (1998)

Two-dimensional spectroscopic observations of chromospheric oscillations

Nurol Al ¹, Cornelia Bendlin ² and Franz Kneer <sup>3

1</sup> Istanbul Üniversitesi, Üniversite Rasathanesi, 34452 Üniversite-Istanbul, Turkey
² Astronomisches Institut der Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
³ Universitäts-Sternwarte Göttingen, Geismarlandstrasse 11, D-37083 Göttingen, Germany

Received 23 December 1997 / Accepted 27 April 1998

Abstract

The oscillatory behaviour of the solar chromosphere was studied from observations of a quiet region at disk centre using various diagnostic tools. The two-dimensional spectrometer in the Vacuum Tower Telescope/Tenerife (Spain) served to obtain a spatially highly resolved time series of "white-light" images and narrow-band filtergrams in the Na D₂ line. With a tunable Fabry-Perot interferometer, this line was scanned taking 30 images (i.e. a "scan") around the line core with wavelength steps of 30 mÅ and a spectral resolution of about 200 000. From these images, line profiles were derived for every pixel in the field of view. With each such narrow-band scan, a scan of "white-light" images was taken strictly simultaneously. The whole time series comprises (2) 128 scans. Every 56 s, a new pair of scans was started with two CCDs, thus the observation covers nearly two hours. Finally, after correlation and other reduction procedures, a field size of 69:004 50:004 remained with 0:002/pixel on the CCD-chips. In the data reduction, new images were created representing the minimum intensity (I) of each line profile in the field of view, and also velocity (V) maps (derived from the Doppler shifts of the line profiles) for all 128 scans. From these images, power spectra and diagnostic diagrams were computed. In the subsequent analysis, a distinction between network and intra-network regions was made where this seemed appropriate. One- and two-dimensional (V-I) phase and coherence spectra were analysed with regard to oscillations and to the nature of the waves leaving their marks in these diagrams. Several noteworthy results also raised the question of the actual line formation height of Na D₂, among them being the non-detection of a chromospheric eigenmode. While an explanation for a conspicuous 70^o plateau in a small region of the phase spectra already exists, the suspected reason behind the decreasing phase difference from about -60^o for the f-mode down to 120^o for higher modes is still subject to some speculation. Moreover, the data gave evidence of gravity waves, probably discovered for the first time in a V-I phase spectrum of Na D₂.

Key words: Sun: chromosphere Sun: oscillations techniques: spectroscopic

Send offprint requests to: F. Kneer

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Contents

• 1. Introduction
• 2. Observations
  • 2.1. General overview
  • 2.2. Short introduction to the two-dimensional spectrometer
  • 2.3. Outline of the observations
• 3. Data reduction
  • 3.1. Preparatory steps
  • 3.2. White-light images
  • 3.3. Narrow-band filtergrams
  • 3.4. New images
  • 3.5. Diagnostic tools
• 4. Results and discussion
  • 4.1. One-dimensional power, coherence, and phase spectra
  • 4.2. Two-dimensional diagnostic tools
• 5. Summary and outlook
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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