Two-dimensional spectroscopic observations of chromospheric oscillations

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 D-2 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 m Angstrom and a spectral resolution of about 200000. 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 (2x) 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." 4 x 50." 4 remained with 0." 2/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 D-2, among them being the non-detection of a chromospheric eigenmode. While an explanation for a conspicuous 70 degrees plateau in a small region of the phase spectra already exists, the suspected reason behind the decreasing phase difference from about -60 degrees for the f-mode down to similar to -120 degrees 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 D-2.