Astron. Astrophys. 364, 816-828 (2000)

3. The Mt. Wilson line profiles

The Na line profiles used to model the GOLF velocity sensitivity functions were observed at the Mt. Wilson 150-ft solar tower (e.g. Ulrich et al. 1991). The Na D₁ and D₂ profiles were observed at three disc positions: 0, 60 and 75 degrees. Additional sets of data are available at 45 degrees for D₁ and 30 degrees for D₂. These were well represented by linear interpolation between 0 and 60 degrees and since they corresponded to different center-to-limb angles it was determined to leave them out of the interpolation table and simply use linear interpolation between 0 and 60 degrees for both lines. Linear interpolation and extrapolation was also used for all angles greater than 60 degrees using the measurements at 60 and 75 degrees. The sampling of the solar surface can be carried out in a variety of ways as was described by Ulrich et al. (1991). For the scans of D₁, the entrance aperture used was plain slit (no Walraven Image Slicer). The sampled area in these cases was 1 arc-second by 20 arc-seconds. For the D₂ scans, the Walraven Image slicer was left in place and an aperture of 12 arc-seconds by 12 arc-seconds was used. Comparisons between profiles of the D₁ line profile made with these different configurations show that the changes due to solar conditions are larger than those caused by the optical configuration. The average line profiles obtained with this system are formed out of 180 separate scans each of which requires 30 seconds of time. As each scan is added to the sum, it is first shifted so that its line bisector at pm (10 picometersmÅ) coincides with those already in the sum. In this way, the smearing effect of the 5-minute oscillations is largely removed. The instrumental smearing due to spectral resolution is equivalent to convolving the true solar profile with a gaussian having a half-width of pm. This is larger than the smearing caused by the GOLF instrument itself (Boumier & Damé 1993) but is comparable to the smearing caused by short wavelength surface velocity fields.

The D₂ line is strongly affected by telluric water absorption at wavelengths of interest for modeling the GOLF response. The water vapor is distributed somewhat irregularly through the atmosphere and is time variable during the observing day so that the absorption components introduced by this species cannot be assumed to follow a secant of the zenith angle law. During the months of October to December of 1995, the D₂ line was measured at the Mt. Wilson 150-foot solar tower on a regular basis for about one hour near local noon. Due to the above noted variability of the column depth of water vapor in the earth's atmosphere, the telluric features vary considerably in strength from one observation to the next. Initially the data was obtained at disk center in order to learn the nature of the telluric features as observed by the Mt. Wilson system. A set of three days has been selected as representing a considerable range in these features. These are Oct. 22 with the weakest features, Oct. 14 with intermediate strength features and Oct. 16 with the strongest features. The ratio of the spectrum for Oct. 16 to that for Oct. 22 is shown in the left panel of Fig. 1. The right panel of Fig. 1 shows these three line profiles. The effect of the telluric absorption was removed by multiplying the observed intensity by . The telluric optical depth was modeled as the sum of four gaussians: where the width pm was adopted at wavelengths of -32.30, -1.13, 11.30 and pm relative to the D₂ line center. The apparent feature at pm was omitted as it appeared from other line pairs to be a result of a line profile change rather than telluric absorption. The relative amplitudes of the components were taken to be . The nominal line center wavelength and the overall scale factor A were adjusted to provide a smooth profile at wavelengths near the strongest feature at pm. When the profile in this wing was relatively smooth, the features at wavelengths of greater interest also largely vanish. The D₁ lines did not require a correction of this type. Each line profile is corrected for the effect of spectral resolution by deconvolving with an instrumental function derived from the ratio of the line profile measured when the sunlight traverses a hot sodium cell to the profile without the sodium absorption. The profiles are also corrected for the effect of spectrograph scattered light which introduces a constant offset equivalent to 2% of continuum. The resulting corrected line profiles are shown in Fig. 2.

Fig. 1. The Na D2 water vapor absorption spectrum (left) as observed from the Mt. Wilson 150-ft solar tower. The water vapor spectrum is derived from the ratio of the D2 profiles observed with the strongest and weakest absorption features shown in the right panel.

Fig. 2. Na D line profiles observed from the Mt. Wilson 150-ft solar tower compared to the Debouille et al. (1973) profiles (upper panels). In addition, the Na D profiles observed at disk center and = 60 (lower panels).

The line profile system described by Ulrich et al. (1991) operates by scanning a fiber-optic aperture in alternating directions over the spectral image at the focal point of the Littrow lens. The final scans are formed out of averages of 120 to 150 of the subscans. The scanning process is controlled by a precision motor which has a maximum translation speed. The requirement of finishing each subscan in less than 30 seconds in order to properly sample the 5-minute oscillations imposes a limit on the wavelength coverage of the profiles. Consequently, the Mt. Wilson profiles do not extend to the continuum. However, as long as the relative intensity over the full solar disk can be measured at the GOLF working points, there is no need to know the continuum intensity. During normal magnetograph operation at the Mt. Wilson 150-foot solar tower, the average intensity at points on opposite sides of the solar line is mapped over the full solar surface. This intensity corresponds to the line bisector intensity and the quantity we require is the limb darkening function for the bisector whose blue and red sampling points are separated in wavelength by an amount the same as the physical separation of the magnetograph blue and red line wing sampling optics. The shifted intensity resulting from a velocity difference between the bisector and an arbitrarily moving part of the solar surface adds a correction to this bisector velocity which can be calculated from knowledge of the line profiles as a function of position on the solar surface.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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