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

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4. Results and discussion

A narrow and distinct emission feature was evident in all limb spectra taken at than 950" from disk centre on either side of the Sun during this run. The detections were unambiguous in data from the early part of the run but were less clear in later spectra due to deterioration of the weather above the telescope, as evidenced by a marked increase in the amount and variability of atmospheric water vapour. The mean position of the detected features is 29.096[FORMULA]0.003 cm-1, which agrees well with the HI n=22-21 transition at 22.095 cm-1.

Fig. 3 shows the limb-brightening curve of the peak intensity of the HI feature. Intense limb brightening is evident very close to the limb. Each of these measurements represents a convolution of line emission over the solar disk with the beam pattern of the telescope, centred at the appropriate limb position. Data points which are apparently off the Sun actually represent samples of the extreme solar limb by the wide telescope beam, the central core of which was measured in the present experiment to be 19" (FWHM). This limb brightening curve agrees well with that observed for the n=20-19 transition of HI (Clark et al. 2000), showing a sharper rise at the limb which reflects the narrower beam pattern of the present observations.

[FIGURE] Fig. 3. Peak height of the HI n=22-21 emission feature as a function of the position of the centre of the telescope beam, represented as a fraction of the submillimetre solar radius (979"). Measurements on the positive-azimuth and negative-azimuth sides of the Sun are denoted by squares and triangles, respectively. The points which lie beyond the limb represent the sampling of the extreme limb by a portion of the wide telescope beam (width indicated).

As previously discussed (Clark et al. 2000), the line fitting procedure adopted in this analysis is sensitive to the assumed baseline and conceals any wide Lorentzian wings which may be present. The detected feature is then only the central core of the emission line, and was best fitted by a Gaussian function. The average width of the line core was found to be 18[FORMULA]4 mK, and no significant variation with limb position was seen. This is significantly smaller than the 45 mK value predicted by Hoang-Binh for ion-broadened Lorentzian lines in his original paper (1982), and even further below his more refined prediction of 50 mK in a later paper (1987). It is interesting to note that the widths of this and the n=20-19 transition (24[FORMULA]7 mK; Clark et al. 2000) are in direct proportion to their frequencies, as would be expected for a Doppler-broadened line, even though these widths are both an order of magnitude larger than the theoretical Doppler width at the temperature of the emitting region ([FORMULA]7000 K).

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© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000