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Astron. Astrophys. 361, L60-L62 (2000)

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3. Data analysis

The interferograms were processed using standard Fourier transform spectroscopic techniques, as described in Clark et al. (2000). The precise position of each limb measurement was determined from the symmetry of continuum intensity profiles from each limb of the Sun. Corresponding background spectra were subtracted from all limb and disk-centre spectra. Fig. 1 shows an average of three such difference spectra for the disk centre position along with a synthetic atmospheric transmission spectrum calculated for Mauna Kea using the spectral modelling program FASCOD (Anderson et al. 1996). The transmission spectrum has been convolved to a resolution of 0.005 cm-1 to match the measured resolution of the interferometer (Naylor et al. 2000).

[FIGURE] Fig. 1. The lower curve represents the average of three disk-centre spectra, after subtraction of the corresponding background spectra. A synthetic atmospheric transmission spectrum, calculated for Mauna Kea and convolved to the spectral resolution of the interferometer (0.005 cm-1), is superimposed for comparison (upper curve). The overall shape of the measured spectrum is controlled by a cooled bandpass filter, which is not included in the synthesis. The location of the HI n=22-21 feature is indicated.

The structure in the measured spectrum in Fig. 1 arises from several sources: line absorption by atmospheric O2 and O3, line and continuum absorption by atmospheric H2O, the transmission characteristic of the bandpass filter, and channel fringes generated by resonant optical cavities in the detector system. The combination of these effects, and their variability, makes identification of weak spectral features in the disk-centre solar spectrum difficult. We therefore searched for the presence of a limb-brightened feature due to the n=22-21 transition of HI using the method of Clark et al. (2000), with which we detected the n=20-19 transition by comparing limb and disk-centre spectra.

The emission feature was extracted from each background-corrected limb spectrum by subtracting from it a scaled, background-corrected disk-centre spectrum, where the scaling factor was chosen to compensate for overall variations in atmospheric transmission between limb and disk-centre spectra and for the partial filling of the telescope beam when viewing the solar limb. The small residual baseline was then removed by spline-fitting to neighbouring regions of the difference spectrum. The resulting spectra from different positions on the positive-azimuth side of the Sun are shown in Fig. 2. Standard spectroscopic fitting algorithms (GRAMS/32, Galactic Industries Corp.) were used with fully variable fitting parameters to extract positions, heights and widths of the detected emission features.

[FIGURE] Fig. 2. A sequence of 7 difference spectra between limb and corresponding disk-centre spectra is shown. Observing position for each limb spectrum, described by distance from disk-centre as a fraction of submillimetre solar radius, is shown to the right of each graph. The vertical scale of each spectrum is indicated at top left. The HI n=22-21 emission feature at 22.095 cm-1 is clearly evident.

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© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000
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